Human contact currents induced by electrical field in Swedish 400 … · Kravet på kunskap om elektromagnetiska fält i arbetslivet har ökat efter att ett nytt EU direktiv, 2004/40EG,

Human contact currents induced by electrical field in Swedish 400 kV substations

Master of Science Thesis

JONAS CEDERGREN

Department of Signals and Systems CHALMERS UNIVERSITY OF TECHNOLOGY Göteborg, Sweden, 2006 Report No. EX070/2006

Thesis for the degree of Master of Science

Human contact currents induced by electrical field in Swedish 400kV substations Jonas Cedergren, Gothenburg, Sweden 2006

In Corporation with

The Swedish National Electrical Grid

Chalmers University of Technology Department of Signals and Systems

SE – 412 96 Gothenburg

Examinator at Chalmers: Prof. Yngve Hamnerius

Supervisor at Svenska Kraftnät:

Kjell-Åke Persson

August 2006

Abstract Due to a new European directive 2004/40/EC of the European Parliament and of the Council, the demand of knowledge about exposure to electromagnetic fields for workers has increased. This thesis focus on the contact current exposure levels in Swedish 400 kV substations. Earlier studies have been performed in the substation in Stenkullen (Fracke & Åke 2004) and (Pettersson & Österlund 2005). The outcome of those studies showed the magnetic fields to be within the restrictions (500 µT), but the electrical field strengths were exceeded in several positions according to the action value of 10 kV/m stated in the directive. The directive is directly based on ICNIRP’s guidelines from 1998. ICNIRP’s guidelines give the opportunity to increase the action value to 20 kV/m under some conditions. Such conditions are when adverse indirect effects from contact with charged conductors can be excluded. The action value for contact current is set to maximum 1 mA in the directive. The main purpose of this thesis is to study the contact current for different work tasks, to see for which situations this action value is fulfilled. If 1 mA not is exceeded, the possibility of increasing the field strength to 20 kV/m can be used. The highest contact current value obtained in this study was 180 µA when a person touches a single grounded object. To touch a van exposed to an electric field and a grounded object at the same time gave a current of 1.17 mA. Spark discharges at pre contact with a grounded object was found to cause the most annoyance by their stimulation of muscle nerves, but such discharges are not limited in the regulations. Levels of 1.75 A in a fraction of a second where obtained. Induced open circuit voltage potentials at a person were measured and found to fluctuate between 200 and 2400 V depending on position, soil and weather condition during work. The effect of available protective clothing were studied and found to protect well from spark discharges.

Sammanfattning Kravet på kunskap om elektromagnetiska fält i arbetslivet har ökat efter att ett nytt EU direktiv, 2004/40EG, antagits. Denna studie fokuserar på kontaktströmmar i svenska 400 kV ställverk. Tidigare studier har genomförts i ett ställverk i Stenkullen (Fracke och Åke 2004) & (Pettersson och Österlund 2005). Resultatet från dessa studier visade på att det magnetiska fältet låg inom insatssvärdet (500 µT), men det elektriska fältet överskred det föreskrivna värdet på 10 kV/m. EU-direktivet är baserat på ICNIRP’s riktlinjer från 1998, vilka ger en möjlighet att öka insatsvärdet hos det elektriska fältet till 20 kV/m om ett annat insatsvärde, kontaktström, ej överskrider 1 mA. Syftet med detta examensarbete är att undersöka nivån på kontaktströmmar som en arbetare utsätts för. Om det inte överskrider maxinivån kan möjligheten att öka den elektriska fältstyrkan till 20 kV/m tillämpas. Det högsta värde på kontaktström mellan en arbetare och ett jordat föremål uppmättes till 180 µA. Vid samtidig beröring av en skåpbil exponerad i ett elektriskt fält och en jordat föremål uppmättes en ström på 1.17 mA Gnisturladdningar precis innan kontakt etablerats med ett jordat föremål upplevs som det mest besvärande för arbetare. Dessa förorsakar ofrivilliga muskelrörelser genom stimulering av nerver men är ej reglerade i direktivet. Strömstyrkor på 1.75 A uppmättes under en bråkdel av en sekund. Inducerade spänningar i en person mättes under olika arbetspositioner. Resultatet visade på fluktuationer mellan 200 och 2400 V beroende på position, underlag och väder. Skyddskläders skärmande verkan uppmättes också och befanns skydda väl från gnisturladdningar.

Acknowledgements I would like to express my gratitude towards all the persons who have made this master thesis possible by all their support and guidance. First of all we would like to thank my supervisors, Kjell Åke Persson at Svenska Kraftnät for the opportunity to perform this thesis and Yngve Hamnerius at Chalmers for excellent guidance and support. I owe the kind people in Stenkullen and Lindome substations a great deal. Specially Kjell-Åke Elfving and JB Mårtensson for always helping me out when I needed support for my measurements. Kjell Hertzberg, Thorbjörn Karlsson and Peter Karlsson have also helped me a lot. At Chalmers, I specially want to show my gratitude towards Yuriy Serdyuk for patiently and skilfully answered all my questions. At the same institution as Yuriy, Jörgen Blennow and Björn Sonerud have also been very helpful. Thanks to Wilgot Bokhede for guidance and help with the measuring equipment, Ralf Berntsson for support and help with my room at Chalmers. Mikael Persson for reviewing the report. Two companies have been kind to support be with equipment; Caparol (Lars Bengtsson) for their paint “electro shield” and Shopservice in Stenkullen for giving me a great discount of a dummy. Also, I wish to thank; Göran Olsson at STRI and Anette Larsson, Vattenfall for helpful discussion and support with literature. Kari Jokela and Jarmo Elovaara at STUK in Finland for guidance and interpretation of EU and ICNIRP regulations. Kjell Hansson Mild at the “National Institute for Working Life” for the idea of voltage measurement. Rasmus Anthin for help with signal treatment and Matlab difficulties. Last but not least; thanks to my lovely girlfriend Suzanne always being there for me and my family for their support. Thank you all! Gothenburg, Chalmers August 2006.

Table of Contents

1 Introduction .................................................................................................................... - 1 - 2 Background .................................................................................................................... - 2 - 2.1 Regulations.................................................................................................................. - 2 - 2.1.2 Current density ......................................................................................................... - 4 - 2.1.3 Explanation of contact current .................................................................................. - 4 - 2.1.4 Contact current in the 2004/40/EC directive.............................................................. - 5 - 2.2 The Body impedance ................................................................................................... - 6 - 2.3 The impedance versus contact area .............................................................................. - 7 - 2.4 Impedance for transients or higher frequency current................................................... - 8 - 2.5 Earlier studies and calculations based on empirical data............................................... - 9 - 2.6 Contact current from large objects ............................................................................. - 10 - 2.7 Spark discharges from humans................................................................................... - 12 - 2.8 Perception levels and current effects on humans ........................................................ - 12 - 3 Methodology ................................................................................................................ - 15 - 4 Basic theory and technology ......................................................................................... - 17 - 4.1 Electromagnetic fields ............................................................................................... - 17 - 4.2 The wave ................................................................................................................... - 17 - 4.3 Electric fields............................................................................................................. - 18 - 4.4 Magnetic fields .......................................................................................................... - 19 - 4.5 The substation............................................................................................................ - 20 - 5 Work performed in a substation .................................................................................... - 22 - 5.1 Inspection and small work ......................................................................................... - 22 - 5.2 Maintenance, mounting and service at breakers ......................................................... - 24 - 5.3 Work with disconnectors ........................................................................................... - 25 - 6 Measuring equipment ................................................................................................... - 27 - 6.1 Electric field measurement......................................................................................... - 27 - 6.2 Stationary contact current measurement..................................................................... - 29 - 6.3 Contact current flowing through the body from a vehicle to a grounded object. ......... - 30 - 6.4 Transient contact current measurement ...................................................................... - 31 - 6.5 Using a phantom........................................................................................................ - 33 - 6.7 Protective clothing..................................................................................................... - 33 - 7 Measuring positions...................................................................................................... - 35 - 7.1 Stenkullen.................................................................................................................. - 35 - 7.2 Lindome .................................................................................................................... - 37 - 8 Measurement results ..................................................................................................... - 39 - 8.1 Measurements in Stenkullen ...................................................................................... - 39 - 8.1.1 Contact currents and open circuit voltage................................................................ - 39 - 8.1.2 Measurements with protective clothing ................................................................... - 43 - 8.2 Measurements in Lindome......................................................................................... - 44 - 8.3 Measurements in Strömma......................................................................................... - 45 - 8.4 Transient analysis – spark discharges......................................................................... - 45 - 8.5 Weather dependence.................................................................................................. - 49 - 9 Discussion and Conclusion ........................................................................................... - 50 - 9.1 Contact current .......................................................................................................... - 50 - 9.1.1 Uncertainty of the contact current measurements. ................................................... - 51 - 9.2 Voltage measurement ................................................................................................ - 51 - 9.3 Electric field measurements ....................................................................................... - 52 -

9.4 Current discharge measurements................................................................................ - 53 - 9.5 Protective clothing..................................................................................................... - 53 - 9.6 General conclusions................................................................................................... - 54 - References....................................................................................................................... - 55 -

Chalmers University of Technology

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1

Introduction Workers in a high voltage substation can be exposed to high electric and magnetic field strength, due to the high voltage and high current. During the years, there has been a raising concern about the health effect when being exposed to high electromagnetic fields. The issue of whether electromagnetic fields affect general health is still today a debate for scientists throughout the world. There are several recommendations, standards and directives that restrict the exposure levels of electromagnetic fields. Often, there are different regulations for exposure of the general public and for occupational exposure. This difference is due to better controllability for occupational workers. A new directive, 2004/40/EC, initiated by the European Parliament and of the Council of 29, April 2004 has been adopted for countries in the European Union (EU 2004). The directive is titled “minimum health and safety requirements regarding the exposure of workers to the risks arising from physical agents (electromagnetic fields)”. The member countries have, according to the directive, at most four years to implement the directive in their national legalisation. Thus, the levels stated in the directive shall be regulated at latest 29 April 2008. The limits in the directive are based on the International Commission on Non-Ionizing Radiation Protection (ICNIRP) guidelines (ICNIRP 1998). ICNIRP’s main function is to investigate the hazards with exposure of non-ionizing radiation and make recommendations and guidelines based on present science. The directive of 2004/40/EC is only directed towards short time EMF (electromagnetic field) exposure that causes health effects in the human body. Since little is yet known about long time exposure and no unambiguous scientific evidence has been shown about long time exposure hazards, such restrictions are not included. The directive includes contact currents, induced currents and SAR-limitations (energy absorption rate at high frequencies). Two earlier studies, performed in a 400 kV substation in Stenkullen, showed that the electric field strength exceeded the “action values” (10 kV/m) stated in the directive (Fracke & Åke 2004) and (Pettersson & Österlund 2005). This gave raise to a concern; will the workers be able to continue to work without violating the limitations in the directive in 400 kV high voltage substations? The main focus in this thesis will be on contact currents, one of the restraints in the directive that has not been well investigated in Swedish substations. The action values in the directive are set, not to exceed the human current density limitation prescribed. If the contact currents are shown to be low, it might be possible to enhance the allowance of the electric field strength to the double as ICNIRP suggest.


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2

Background This chapter tries to sort out the concept and give a background to the limitations in the directive. A human body exposed to a time varying electrical field obtain charges at the surface of the body. The distribution of surface charges depends of the strength of the field as well as size and shape of the body and its location within the field. Currents caused by the surface charges flows in the body. For a human standing in an electrical field from a overhead power line, the field induces a current flowing from the head down the body and out through the feet.

2.1 Regulations There are limitations for allowed body currents which are established by ICNIRP and the European Union. These limitations are given as current density (mA/m2) within the body. The limitations distinguish between occupational exposure and exposure for general public. This is because workers generally are considered more aware of their exposure and they act in accordance with this. Also workers are considered better observed than the general public. Table 2.1 shows the current density limits for 50 Hz electromagnetic field exposure and the action values appurtenant. Category of people Action values; electric

field exposure [kV/m] Action values; magnetic field exposure [µT]

Limitation; current density for head and trunk [mA/m2]

Occupational workers 10 500 10 General public 5 100 2

Table 2.1 Limits and action values for different category of people. Source: ICNIRP’s guidelines (1998), recommendations adopted by the European Council (1999). In directive 2004/40/EC of the European parliament and of the council of 29 April 2004, the occupational exposure for current density given in table 2.1 was established. The directive forces the member countries to institute national laws containing the above limitations. The directive is based on ICNIRP’s recommendations from 1998. Table 2.2 shows exposure limit values in the directive for occupational exposure. Data are exactly the same as the basic restrictions from ICNIRP.


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Table 2.2 Exposure limit values from directive 2004/40/EC of the European parliament and of the council of 29 April 2004. Note that this is the limits and shall under no circumstances be exceeded. Yellow marks indicate limits at 50 Hz. The action values in table 2.3 shows exposure values established if the limits in table 2.2 not shall be exceeded. They are set because there is no easy way to directly measure the current density within the head and trunk. Instead other measurable units are used. They are the same as ICNIRP’s reference values but are referred as “action values” in the directive.

Table 2.3 Action values from the directive 2004/40/EC for occupational exposure. Values are given as rms for unperturbed field. Yellow marks indicates limits at 50 Hz. The maximum contact currents for 50 Hz is 1 mA according to table 2.3 In ICNIRP (1998), reference levels are explained as follows:

Reference levels of exposure are provided for comparison with measured levels of

physical quantities; compliance with all reference levels given in these guidelines will


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ensure compliance with the basic restrictions. If measured values are higher than the

reference levels, it does not necessarily follow that the basic restrictions have been

exceeded, but a more detailed analysis is necessary to assess compliance with the basic

restrictions.

ICNIRP further writes:

The reference levels are intended to be spatially averaged values over the entire body of

the exposed individual, but with the important proviso that the basic restrictions on

localized exposure are not exceeded.

2.1.2 Current density

For low frequencies, induced current density is the only limitation that is restricted as a “limit value”. Current density, denoted (J), is defined as “the current flowing through a unit cross section perpendicular to its direction in a volume conductor such as the human body or part of it, expressed in amperes per square metre (A/m2)” (EU 2004). The following quotations from ICNIRP’s guidelines explain how the action values, or by terms of ICNIRP; the reference levels, are set.

In the frequency range up to 1 kHz, the general public reference levels for electric

fields are one-half of the values set for occupational exposure. The value of 10

kV/m for a 50-Hz or 8.3 kV/m for a 60-Hz occupational exposure includes a

sufficient safety margin to prevent stimulation effects from contact currents under

all possible conditions. Half of this value was chosen for the general public

reference levels, i.e., 5 kV/m for 50 Hz or 4.2 kV/m for 60 Hz, to prevent adverse

indirect effects for more than 90% of exposed individuals. Further down in the guidelines, one can read:

For the specific case of occupational exposures at frequencies up to 100 kHz, the

derived electric fields can be increased by a factor of 2 under conditions in which

adverse indirect effects from contact with electrically charged conductors can be

excluded. This means if one can state that contact currents do not have adverse indirect effects, i.e. if the action value of contact current does not exceed the maximum value, the maximum electric field strength for occupational exposure may be doubled from 10 kV/m to 20 kV/m at 50 Hz. Current density limits are given in the basic restrictions in ICNIRP. They are in the frequency range 1 Hz to 10 MHz set to prevent effects on nervous system functions (ICNIRP 1998).

2.1.3 Explanation of contact current

Contact current occurs when a human touches or grasps two objects with different potentials, resulting in a current flowing through the body. If the person is grounded, contacts current occur when touching a potential that is different from the ground. A third alternative is when the person is being exposed to an electrical field, and touches a grounded object. Here the


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induction object resulting in a potential would be the person herself (Reilly 1998 p 343). It is important to distinguish between transient current and steady state current when dealing with contact currents. A person exposed to a high electrical sinusoidal field carries a potential induced by the field which vary along with the period. If the potential is high enough, a spark will fly from either the person or the grounded object, just before contact has been established, resulting in a transient discharge current. This will happen every time the voltage is high enough to overcome the gap.

2.1.4 Contact current in the 2004/40/EC directive

The question whether the action values for contact currents should include pre contact i.e. the spark discharges or only the stationary contact came to an issue. Kari Jokela1 at “STUK”, (Finnish “Radiation and Nuclear Safety Authority”) answers in a mail;

The current limit values in the directive can in my opinion be applied only for continuous

current when the contact has been established.

He continues:

However, also the spark discharge currents are important because they cause discomfort

and even maybe some cellular damage in the finger. Therefore it is important to reduce

them in the working environment. It is a good idea to try to measure both of them by the

same time.

In a second mail, when asking how the current limitations in ICNIRP 1998 were set, he declares:

I should know something of the setting the ICNIRP 1998 limits because I was involved in

that work. The current limit 1 mA was simply based on classical data on the perception

threshold of electric current. However, according to the recent study of Leitgeb et

al.(2005) some people may sense considerably lower currents even below 0,1 mA. This

does not look to me a big health problem.

Jarmo Elovaara2, leading specialist at STUK adds:

We have carried out contact current measurements in places where 400 kV line cross

over public roads and where people have contacted us because of nuisance they have

experienced. Even people moving with bicycles have made complaints due to the

transient contact currents. We have not tried to find a correlation between the 50 Hz

contact current amplitudes and the transient current amplitudes but we wanted show the

effectiveness of the shield wires installed along the road at the crossing area.

Unfortunately the measurement report is written only in Finnish. We have not either

carried out these kinds of measurements in substations. In few lines I can tell that the

field strength 7 kV/m for sure causes unpleasant sensations within some individuals. In a

400 kV substation this level is regularly exceeded. In the described conditions the peak

value of the transient current (has approximately a double exponential form) can be

1 Kari Jokela, research professor at STUK, (Radiation and Nuclear Safety Authority, Finland), mail contact January 16 and 17, 2006. 2Jarmo Elovaara, leading specialist at STUK, (Radiation and Nuclear Safety Authority, Finland), mail contact January 16 and 17, 2006.


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about three orders of magnitudes higher than the peak value of the continuous current.

Our experience is also, that the measurement of the transients is not completely simple

task.

In principle when a person touches the in field a grounded object he might be exposed to

several current pulses. However, the pulses can cause so much pain or unpleasantness,

that the person draws e.g. his hand away in "safe area" or takes a grip from the charged

object causing that current pulses are transformed to a continuous current. My own

experience is, that nobody is willing to sense repeatedly transient contact currents, they

can be so unpleasant. In practice the workers ground themselves with a piece of copper

wire when they have to work in conditions where the electric field strength is high. Of

course, conductive protective clothing might be a solution but one should be able to earth

it.

Concluding the information above, one can state that the action values in the 2004/40/EC

directive can only be applied for stationary contact current, but it is of great importance to protect the workers from spark discharges as well.

2.2 The Body impedance

When a current flows through the human body, the impedance of the body itself is depending of the current path within the body, the applied voltage and the frequency of the current. For DC or low frequency steady state current, the impedance is dependent of the resistive part of the body. Figure 2.1 shows this relationship.

Figure 2.1 The hand to hand body resistance as a function of applied voltage. Source: IEE Proc. Gener. Transm. Distrib. (1999). Data from IEC 479-1 (1984). The diagram above can be applied for hand to hand voltage at 50 Hz, the grid frequency. The three different curves in the picture indicate body resistance which is not exceeded by five


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percent, 50 percent and 95 percent of the population respectively. If the current path differs from one hand to the other, figure 2.2 can be used.

Figure 2.2 Distribution of internal impedance of the human body. The numbers indicate percentage of the total internal impedance for hand to hand contacts. Numbers within brackets are the impedance from the specific place to both hands. Data from IEC 479-1 (1984). Deducting of information from figure 2.1 and 2.2, one can make a rough estimation of the lowest impedance from hand to hand or hand to foot. At a voltage level above 200 Volt, the impedance would be slightly below 1000 ohm. The impedance for upper body current to one hand would be approximately 500 ohm. According to Reilly (1998 p 21), the skin impedance is the primary factor that limits the current through the body if the applied voltage is below 200 V.

2.3 The impedance versus contact area

A large grasping area (i.e. contact area), generally implies better contact with increased current to the body as a result. However, the importance of contact area is decreasing with applied increasing voltage. Biegelmeier (1985) has made experiments of the influence of the contact area size. Figure 2.3 shows the total hand to hand body impedance for dry skin with a various set of electrode sizes.


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Figure 2.3 Body impedance as a function of applied voltage for different size of electrode areas. Source: Biegelmeier (1985). For low voltage levels, the impedance is almost inversely proportional to the area. The curves converge towards each other as voltage is increased. At a voltage level of 225 V, the total body impedance vary only about 4:1 for a dramatic change in electrode area of 8200:1 according to Reilly’s interpretation of the figure above (Reilly 1998 p 35). For low frequency and DC, experiment shows that wet or dry skin plays a significant role to the body impedance at low voltages, but the importance is decreasing with a raise in applied voltage (Freiberger 1934).

2.4 Impedance for transients or higher frequency current

For higher frequencies, body impedance plays an important role when spark discharges occur to the skin. A spark has a very short duration and has high frequency components. When grasping or touching a conducting device with a potential different from the body, a spark will ignite if the differences in voltage is over 500 V (peak voltage) according to Reilly (1998 p 54-56). This can be explained by the dielectric strength of the skins outermost layer, the corneum. This layer consists of dead body cells. The spark lowers the impedance of the skin for the fraction of a second it will last. At higher voltages, a spark will ignite before contact is mad. Thus, the spark will overcome both the gap in the air and the skin. However, breakdown of skin impedance has been reported for voltages as low as between 100 and 200 V (Reilly 1998 p 56-57). In those experiments, a small contact area was used. Breakdown of skin impedance can not be directly compared to spark discharges, but shows its complex nonlinear impedance behaviour. Reilly mentions that breakdown of skin and spark


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discharges can be related to skin having different “plateau voltages”. Those are approximately 100 V and 500 V. For capacitors discharges to the skin, the 100 V contact plateau suggests a level were the impedance becomes very high, implying that a capacitor cannot completely discharge through the skin. The 500 V plateau on the other hand states a level above which a spark discharge can occur. Translating this into terms of the environment in a switchyard means that one will not experience any spark discharges if the initial voltage difference is below 500 V. In further chapters, the initial voltage is presented as open circuit voltage, but is then given in RMS (root mean square) values, see equation 2.1.

dttUT

U

T

eff ∫=0

2 )(1

Equation 2.1 The rms value of a voltage U.

2.5 Earlier studies and calculations based on empirical data The American Electric Power Research Institute (EPRI) has in its “Transmission Line Reference book” performed a large number of experiments on voluntary people regarding their sensitivity for contact currents (EPRI 1983). EPRI has also carried out studies of the current from vehicles to ground, human to ground and human to vehicle. All objects were exposed to a high electric field from power lines. The obtained data from those studies resulted in empirical formulas how to calculate the current from different objects. A well studied case is the current to ground from a person with height h, standing upright in a vertical and homogenous electric field. The field strength is denoted E and the frequency is f. The geometry of the human within the field is with the arms close to the body. Then, the induced body current flowing to perfect ground through electrodes fitted under the feet can be calculated as: Equation 2.2 Ic = 9.010-11 h2 f E

Where: Ic = short circuit current (A)

h = body height (head to feet) (m) f = frequency (Hz) E = electrical field strength (V/m)

For instance, the author with a body height of 1.83 m standing in a vertical electric field at 50 Hz with a strength of 10 kV/m yields a current Ic =150.7 µA. If one is wearing shoes which are not perfectly isolated, i.e. normal working shoes, a smaller part of the body current flows through the soles. The soles of the shoes have both capacitive and resistive impedance. The capacitance human to ground is depending of the thickness of the soles. According to EPRI (1983), the current flowing across the soles equals to:


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Equation 2.3 Is = 26.110-22 h3 f E / C Where C is the capacitance over the soles. Equation 2.3 implies that shoes with a thickness of 1 cm (C = 180 pF) reduce the short circuit current with approximately one third. Fracke and Åke (2004) have in the switchyard in Stenkullen measured the unperturbed electric field strength at ground level. At one point, the maximum field strength was 17.94 kV/m. According to a latter work performed by Pettersson and Österlund (2005), using more adequate measurements by reducing the geometry influence of the E-field meter with a grounded plate, the electric field strength at this point was 13.6 kV/m. Fracke and Åke measured a body current to ground of 147.8 µA in this point. The girl who performed the measurement has a height of 1.60 m. Using EPRI’s formula, one can calculate; Ic = 9.010-11

1.625013.4 ≈ 154 µA which corresponds well to the measured value.

2.6 Contact current from large objects Large objects like cars or trucks in a substation have larger areas exposed to the electric field. This, in contrast to a person, results in more charges at the surface, and gives a much larger currents when short circuit to ground. In the former part, the factor 9.0 10-11 in equation 2.2 is suitable for an erect human and implies the size of the exposed charge obtaining surface. For short circuit current in larger objects, placed close to ground, one can use equation 2.4 reproduced from Transmission Line Reference Book (EPRI 1983). Equation 2.4 Ic = j ω ε E S Where εεεε ≈ 8.84 10-12

S = the equivalent charge obtaining surface ωωωω = 2πf

j indicates the current is capacitive coupled

S can be calculated when simple geometry is present; otherwise it has to be estimated. Figure 2.4 shows a diagram from EPRI how to calculate the equivalent surface having a simple geometry form. Using the diagram in figure 2.4, it is easy to calculate the maximum short circuit current from, for instance, a large van, which is used in the substation in Stenkullen. The size of the van is 5.25 m length (A), 1.90 m width (B) and has a height (H) of 1.95 m. Then: A / B = 2.76 H / B = 1.03


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Figure 2.4 Diagram for calculating the equivalent surface S of a simple object. Source: EPRI (1983). The curves in the diagram gives S / (A x B) ≈ 5. S = 5 x 5.25 x 1.90 ≈ 50 m2. Assume the van is exposed to a field strength of 10 kV /m. Then, the current can be calculated applying equation 2.4, which gives Ic ≈1.4 mA This is the highest short circuit current that will flow through the van when exposed to an electric field with a strength of 10 kV/m. In reality, resistive losses through the tyres make a part of the current flowing through them. The ground resistance (type of soil and weather conditions) plays a significant role according to EPRI (1983). In a study of the weather and soil dependence by EPRI (1983), three types of soil were chosen; dirt, gravel, and black top (asphalt). Different types of vehicles were placed under an overhead line and the current to a human standing on the same soil were measured twice a month during a year. The result showed a dramatic difference in the current levels with a spread of almost none to 100 % of the theoretical maximum current (dirt). Soil of asphalt and gravel showed a smaller current spread where no current level exceeded 30 % of the maximum. Calculations of short circuit current according to equations 2.2 - 2.4 assume the electric field to be homogenous over the complete equivalent area exposed. Such large areas of homogenous fields are rarely to be found in a substation and average values is necessary to use.


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2.7 Spark discharges from humans A person standing on an insulating ground exposed to an electric field is charged by the field. At touch contact with a grounded object, discharges occur by the means of sparks. If contact remains, the current will turn in to steady state current (short circuit). The open circuit voltage (Uc) a person will obtain, is a function of the electric field strength E, where the person is situated in relation to ground and potential as well as the impedance between the person and ground (level of insulation). Assuming perfect insulation to ground, the maximum voltage is determined by the human body’s capacitance to ground (EPRI 1983). The voltage (Uc) can then be calculated using the impedance formula for a capacitance and applying ohms law, which yields

Equation 2.5 Uc = Ic / ω C Where Ic is the short circuit current to ground (equation 2.4). In general, the capacitance for a person to ground in equation 2.5 can be estimated to 100 - 150 pF, Reilly (1998 p 343). EPRI (1983) has performed more detailed studies of the capacitance as a function of the soles height above ground. At 2 cm, the capacitance is approximately 150 pF, 180 pF at 1 cm and 210 pF at 0.5 cm. If the author (1.83 m) is exposed to 10 kV/m and has 2 cm thick soles (C = 150 pF), then the maximum open voltage will be Uc = Ic / ω C = 9.010-11 h2 f E / ω x C which equals to Uc = 3.2 kV. Resistive losses through the soles and (humid) air reduce this voltage and only in exceptional cases are the levels above reached (EPRI 1983). Their study showed that in reality, the voltage vary between 0 and Uc = 0.3 times the electric field strength. That means if a field strength of 10 kV/m is present, the highest obtained voltage potential is 3 kV for a human within the field. The study was performed in varying weather conditions and with a variation of test persons.

2.8 Perception levels and current effects on humans The restraints for contact currents given in ICNIRP (1998) and the European Union's directive 2004/40/EC (EU 2004) are based on perception levels. The International Labour Organization ILO (1994) has in a publication “Protection of workers from power frequency electric and magnetic fields” compiled information from several investigations regarding perception levels for current and indirectly the electric field. A summary of the result is shown in table 2.1 and table 2.2.


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Contact current (mA) Effects of currents (50/60 Hz) passing through the human

body. Experimental data for 50 % of men, woman and

children. (Touch area ~ 1cm2, Grasp area ~ 15 cm

2)

Men Woman Children

Touch perception 0,36 0,24 0,18 Grip perception 1,1 0,7 0,55* Shock, not painful (grasp contact) 1,8 1,2 0,9* Pain, finger contact 1,8x 1,2* 0,9* Shock, painful, muscle control (let go threshold for 0.5 percent of the population)

9 6 4,5

Painful shock, let go threshold 16 10,5 8* Severe shock, breathing difficulty 23 15 12*

Table 2.1 Source: International Labour Organization (1994). (*) indicates calculated values assuming thresholds for women two thirds of those of men. Children’s thresholds are assumed to be one half of men’s. (x) indicates values calculated from other frequency. Table 2.2 gives a summary of the different effects a person may experience when exposed to an electric field ranging from 0 to 50 kV/m. Table 2.2 Different effects on humans exposed to 50 Hz electric fields. The persons and objects charged in the fields are insulated from ground. The percentages indicate part of the group affected. Source: IEEE (1978), Zaffanella and Deno (1978), UNEP/WHO/IRPA (1984), table compiled by the International Labour Organization (1994).


- 14 -

In a recent study; “Electric current perception of the general population including children and the elderly” by Leitgeb et. al. (2005), even lower perception levels among the general public have been established. The study included a large number of persons, well representing the population. Table 2.3 shows the result of this study. Table 2.3 Perception levels for 50 Hz current (Iw) for different perception probabilities (p) including men, woman, children and the general population overall. Source: Leitgeb et. al. (2005). ICNIRP’s (1998) states that the ambition is to protect 90 % of the exposed individuals from adverse indirect effects (establishing the reference level for electric field exposure to 5 kV/m for the general public). The reference level for contact current level is as earlier discussed set to 1 mA for occupational exposure and half of that for general public (0.5 mA). As seen in table 2.3, 90 percent of the population will notice a current level of 0.553 mA and 10 % even as low as 0.111 mA. Conclusion to be drawn is that the existing regulations of current levels are set to high to protect from current perception. On the other hand, those low current levels are not considered harmful according to Kari Jokela3.

3Kari Jokela, research professor at STUK, (Radiation and Nuclear Safety Authority, Finland), mail contact January 16 and 17, 2006.


- 15 -

3

Methodology This chapter gives a short description how the work presented in this report was performed. A 400 kV switchyard in Stenkullen was chosen for the study, since earlier studies of electric fields have been performed here, showing the electric field being above the action value of 10 kV/m (Pettersson & Österlund 2005). The switchyard is old and will soon be out of service. The new switchyard is already under construction in the vicinity of the present location. The work began with the author taking part in the daily business to a substation in order to observe what the workers are touching and what contact current they are exposed to. Since the European directive 2004/40/EC (EU 2004) only deals with stationary contact current; i.e. when contact has been established, focus has been laid on a method of measuring those currents. Several measurements were performed in order to validate the measured values. The current flowing from a person when touching a large number of objects at different places in the 400 kV substation in Stenkullen has been measured. A vehicle often used when driving in the substation was placed near grounded poles right under the phase lines. The contact current when touching the vehicle was measured.. These situations occasionally occur according to Kjell-Åke Elfving4, service technician at Vattenfall. Touching the car and the grounded pole at the same time induces a current flow through the body, larger then only touching the grounded pole, since the vehicle has greater surface and can carry a lot more charges induced by the electrical field. In some points, where the contact current was high, the electrical field were measured with an electric field instrument The same kind of switchyard, like the one being constructed in Stenkullen, is operational in a substation in Lindome, south of Gothenburg. Here, one half of a cubicle was mapped with nine measuring points for the electrical field. The stationary currents were measured from an upright standing person touching the control units of the combined breakers and disconnectors. As a reference, some measurements were conducted in the 400 kV switchyards of Kilanda and Strömma. Protective clothing, manufactured by former SwedPower, (now Vattenfall Power Consultant) has earlier been used as protection against spark discharges at work in high voltage structures. According to J-B Mårtensson5, there are only a few dresses left together with semi conducting

4 Kjell-Åke Elfving, service technician at Vattenfall Syd, personal contact spring 2006. 5 J-B Mårtensson, service technician at Vattenfall Syd, personal contact, spring 2006.


- 16 -

boots. The workers seem to never use them, due to lack of effort and knowledge about them. The shielding effect on contact current with such an outfit was tested. Finally, an attempt was made to measure spark discharges from the authors hand to a grounded pole to observe the magnitude and shape of such discharges.


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4

Basic theory and technology Below is a short introduction to the theory of electric and magnetic fields. In order to give the reader a better understanding of the construction of a substation, such a description is also included.

4.1 Electromagnetic fields Electromagnetic waves consist of both an electric and a magnetic component. At high frequencies they are referred to as electromagnetic radiation. This is valid when the distance from the source is more than one wavelength. In figure 3.1, the fundamentals of a wave are outlined. At lower frequency, the wavelength is greater and we are often in the near field region, within a wavelength from the source. Here, the electric and magnetic field has a very loose connection, almost independent of each other. It is necessary to measure both the electric and the magnetic component.

4.2 The wave Electric and magnetic fields are waves that can be characterized by their strength (amplitude) and their number of oscillations (maxima or minima) per second in one point, the frequency, denoted hertz (Hz). There is a distinct relation between frequency, velocity and wavelength given by equation 3.1. The velocity c in vacuum or air is equal to 3*108 m/s

Equation 4.1 λ

cf =

Figure 4.1 shows the characteristic of a wave. It can be seen from equation 4.1 that high frequencies correspond to short wavelengths and low frequencies correspond to long ones. The wavelength for a 50 Hz wave is for instance 6000 km long.


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Figure 4.1 The fundamentals of a propagating wave.

4.3 Electric fields A system with electrical charges, such as a high voltage overhead line produces an electrical field throughout the space. The general term is E, implying its vector quantities. E is characterized by its magnitude and its direction. The field’s strength is measured in Volts per meter [V/m]. It arises from voltage, even if no current is present. It is relatively easy to shield an electric field using grounded net or wires since the electric field bends toward a conducting object with lower potential. An adverse effect of an electric field is that it also bends toward a grounded person, amplifying the strength as shown in figure 4.2 Figure 4.2 An upright person exposed to an electric field. The field strength close to the person is amplified due to his grounded geometry.


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Between two metal plates with different potential, an electric field arise as shown in figure 4.3 Separating the plates with a distance of 1 m and applying a voltage of 1000 V at one plate and 0 V at the other, a field strength of 1000 V/m arise. The electric field in between the plates far from the edges are homogenous; it does not matter of what height above the zero potential plate one measure.

Figure 4.3 Electric field arises between two metal plates with different potential. The arrows indicate the electric field lines and the colour scale demonstrates the electric potential. Figure is produced using Femlab.

4.4 Magnetic fields While electric fields arise from voltages (charge particles), the magnetic fields arise from currents (charged particles in motion). The unit of the magnetic field strength, denoted H, is ampere per meter, [A/m]. A more commonly used unit is the magnetic flux density B, measured in Tesla [T]. The relation between them can be written B=µ*H where µ is the magnetic permeability of the material that carries the field. Since Tesla is a large unit, the more practical micro tesla (µT) is generally used. As shown in figure 4.3, the magnetic field lines form closed paths around the currents that give rise to them. If one ampere is flowing in the conductor in figure 4.3, a magnetic flux density of 0.2 µT one metre from the centre of the conductor can be measured.


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Figure 4.3 The magnetic field around a straight conductor where current is moving out of the picture. Arrows is indicating the direction of the field (anti-clockwise). Colours indicate the magnetic field strength. Figure is produced using Femlab. The magnetic flux density from a current flow in a conductor can be calculated using equation 4.2. The material surrounding the conductor has permeability µ0 (permeability in space, air and most other material),

Equation 4.2 r

IB

πµ

2*0=

Where I is the current within the conductor and r is the distance from its centre. Magnetic fields are much more difficult to reduce than electric fields. One way is to encapsulate the source within a material that has a µ that deviates a lot from µ0, (magnetic material like iron). Another more realistic approach is to have parallel conductors with currents flowing in opposite directions, cancelling out the fields.

4.5 The substation A switchyard is a part of a substation where electrical energy is gathered and distributed. They usually consist of two or three bus-bars, called A, B and C (where C often is used as an


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auxiliary bus-bar when service is needed or when failure occur at the other). Cubicles are situated perpendicular to the bus-bars and distribute the energy to incoming or outgoing lines. Each cubicle consists of three phases, R, S and T. Planned connections, automatic fault clearance, measurement of voltage, current and effect as well as communication with the central radio office are the main functions in a switchyard. In many cases, the switchyard is part of a larger transforming station (which transforms voltage to different levels and balances active and reactive effect). Two common designs of a 400 kV switchyard are the ABC type and the two breaker design. Figure 4.3 shows a schematic sketch of the types. L1, L2 and L3 indicate the incoming lines, each consisting of tree phases.

(a) ( b) Figure 4.3. Two usual kinds of switchyard design. The first (a) switchyard is of ABC-type and the second (b) of two breaker design. Source: Elkraftteknisk handbok 3. p 192.


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5

Work performed in a substation In this chapter, the reader will be presented to the daily kind of work that is performed in a substation. Every kind of task will give exposure to different levels of electric and magnetic field as well as contact currents. According to J-B Mårtensson6 and Kjell-Åke Elfving7, service technicians at Vattenfall Syd, who has long time experiences working in substations, the major part of exposure levels and time will occur for those people that work with maintenance of breakers. This work requires special skill and most of their time, they are working with breakers. Peter Karlsson8, a technician who works with service at breakers, estimates his time in a 400 kV switchyard to 25-30 % of total time at work.

5.1 Inspection and small work Inspections are carried out in the substation at regular basis to check the function of every control unit and check gas pressure of the breakers in order to gain high reliability. The control boxes of the disconnectors are checked as well. The works involve a lot of touching of control units of circuit breakers and disconnectors as well as other grounded metallic objects exposed to high electrical fields. When inspections of the 400 kV switchyard in Stenkullen are performed, the checklist includes a visit to a big three-phase transformer used for reactive and inductive power distribution, see figure 5.1. This implies exposure for high magnetic fields, sometimes over 100 µT due to high currents in big reactance coils. According to Kjell-Åke Elfving, complete inspections are not carried out that often, maximum four times a year. Smaller inspections are conducted more often when there is a need. Elfving states that he sometimes needs to bring a grounded wire attached to his hand when climbing up a ladder to get within reach to a broken light bulb or device high above ground level, to avoid painful spark discharges.

6 J-B Mårtensson, service technician at Vattenfall Syd, personal contact, spring 2006. 7 Kjell-Åke Elfving, service technician at Vattenfall Syd, personal contact spring 2006 8 Peter Karlsson, technician at Vattenfall, phone contact June 12, 2006.


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Figure 5.1 Kjell-Åke Elfving checks that everything is right with the cooling system for a three phase

transformer in Stenkullen. In the background one can see the reactive power compensation. Figure 5.2 shows the control unit of a breaker in Stenkullen. When reaching to touch the handle, the person is discharged through repeatedly occurring sparks.

Figure 5.2 Kjell-Åke Elfving is just about to open the door to the control unit of a breaker. He feels pain right before contact is established due to spark discharges. The stationary contact current through his hand was measured to around 120 µA.


- 24 -

The situation where workers are exposed to spark discharges might be a hazard, not by the sparks directly, but indirect when working high above ground. In figure 5.3, a control unit to a breaker in Strömma are placed high above ground. Repeatedly sparks between the grounded ladder and the worker makes it a very unpleasant and hazardous place working at.

Figure 5.3 A grounded ladder in Strömma is necessary to use during inspection of the breakers. It is hard not to be a subject of spark discharges.

5.2 Maintenance, mounting and service at breakers Breaker maintenance is the most exposed kind of work from the electric field point of view. It often involves work of long duration. The breakers are sometimes in operation during work with high voltages a few meters above ones head. A working platform is often used to get within reach. It is a dangerous situation since it is easy to fall down from the platform if one may lean towards the pole and inevitable getting a number of spark discharges.


- 25 -

Figure 5.4 Thorbjörn Karlsson serves a 400 kV breaker in Kilanda with isolative SF6-gas. He is standing on a working platform with the insulating porcelain starting a few centimetres above his head. Even though wet and snowy weather conditions, he here experiences a contact current of 160 µA through his hand when touching the grounded pole.

5.3 Work with disconnectors When service and maintenance at high voltage disconnecting switches are performed, the power is disconnected and the device well grounded. For security reason, a ground wire is always attached to the incoming phase wire during work. The electrical field is low since the sources are from surrounding wires and bus bars. Often, a sky lift is used; elevating the worker closer to the higher overhead lines, but the distance to line voltage is still large.


- 26 -

Figure 5.4 A closed disconnecting switch in Stenkullen. When the switch is open and voltage applied, noisy coronas may occur in rainy weather.


- 27 -

6

Measuring equipment This chapter describes the different equipment used for acquiring the data.

6.1 Electric field measurement The instrument used for electric field measurement is an EMM-4 from EnviroMentor AB in Gothenburg. It consists of a circular plate of glass fibre laminate with a thin circular surface of conducting copper. The electrical field coming towards the surface induces a current flowing from the surface to a virtual ground produced by an operation amplifier. Virtual ground is also feasible to connect to ground in order to obtain an accurate result. The current flowing from the plate is converted to a voltage using an integrated current / voltage converter (the operation amplifier circuit). This voltage is then amplified and filtered. Two outputs are available, one RMS-signal output and one direct output. Both outputs have voltages that are linear proportional to the incoming electrical field. The output is then attached to a voltmeter for readout of the electric field strength. The instrument is shown in figure 6.1. The instrument was originally constructed to intercept the field a human being is exposed to, for instance the field from a CRT screen towards a human. The instrument has later been rebuilt to extend its range; it now handles fields up to 20 kV/m. To measure the undisturbed field at ground level in accordance to ICNIRP restrictions, a large grounded metal sheet with a hole for the instrument is necessary to achieve a correct result (Pettersson and Österlund 2005). This is because the instruments geometry intensifies the electric field strength. What the sheet does is simply to raise ground level to the same height as the measuring surface of the instrument. In this way, the electric field lines do not bend towards the instrument, enhancing the field. The size of the sheet was approximately two and a half times larger then the instrument.


- 28 -

(a) (b)

Figure 6.1 (a) the working principle of the electrical field instrument; (b) the instrument itself.

Figure 6.2 The electrical field measurement with a large grounded metal sheet in the same height as the instrument. The output is connected to a voltmeter with large display for distance readout using binoculars (the field is being disturbed if a human is standing close to the measurement setup)


- 29 -

6.2 Stationary contact current measurement For stationary contact current, i.e. when contact has been established, the current was measured using a very sensitive true RMS ampere meter, Metrahit 29S. The ampere meter was then attached to a short fishing rod (with adjustable length to match the length of one’s arm). A small copper electrode placed on top of the rod act as a contact device when pushing it toward a grounded object. A larger copper electrode (~15 cm2) transfers the body current from the hand through the instrument. The open circuit voltage (the potential difference between ground and a person being isolated from ground within an electric field) was measured using a resistance of 90 Mohm in conjunction with a sensitive voltmeter, Meterman 38X. This instrument has an impedance of 10 Mohm. Together with the resistance, it forms a 10:1 ratio voltage divider and an impedance of 100 Mohm is reached. This implies smaller load to the circuit and better accuracy. This equipment was attached to the fishing rod in the same way as the Metrahit. Figure 6.3 shows the two types of measurement. Figure 6.4 shows two photos of the Metrahit instrument attached to a short fishing rod.

(a)

(a) (b)

Figure 6.3 The measurement circuits: (a) open circuit voltage measurement, the resistance represents the voltage divider’s resistance. (b) short circuit current measurement.


- 30 -

Figure 6.4 Photo (a) and (b) shows the measuring instrument for stationary contact current.

6.3 Contact current flowing through the body from a vehicle to a grounded object. In this kind of measurement, the Metrahits ampere meter was used. Due to really high current peaks when establishing contact, a human was excluded and only the short circuit current to the vehicle was measured. In figure 6.5, the short circuit current from a van is measured.


- 31 -

Figure 6.5 Kjell -Åke Elfving is measuring the current floing from his working car parked near a grounded pole, right under a phase line, his body is not included in the circuit. Measurement performed in the 400 kV switchyard in Strömma.

6.4 Transient contact current measurement Spark discharge current was measured using a 100 ohm metal film shunt resistor, which has very good high frequency characteristics. A 100 MHz 10:1 Tektronix P6105 probe with an internal impedance of 10 Mohm in parallel with a 13 pF capacitance was attached to the resistance. A copper electrode leads the discharge current to the resistor. Everything was encapsulated in a metallic box to prevent external EMF noise to disturb the measurement, see schematic picture in figure 6.6. The oscilloscope, a Tektronix TDS 220 was also shielded in a grounded conducting box (covered with Electroshield, a conductive paint) to avoid EMF disturbance.


- 32 -

Figure 6.6 The equipment for acquiring spark discharges from a human’s hand to ground. The current is induced to the body by a high electric field. Figure 6.7 shows the setup in use at a pole of a 400 kV breaker (BA400). The field is about 10 kV/m and touching the pole or the electrode means painful sparks to the hand.


- 33 -

(a) (b) Figure 6.7 The pictures show how the spark discharge measurement was performed; (a) picture of the measuring equipment. (b) a mechanic from the construction of a new switchyard in Stenkullen holds his hand on the discharge electrode. He is charged from the electric field (10 kV/m) from a suspended phase line above his head.

6.5 Using a phantom A dummy named Lennart serves as a phantom model for contact current that is capacitively induced by low frequency electric fields. It is based on a dummy body used for showing dresses in a cloth store. The body is of size L (or 50) with a height of 185 cm. He is covered partly in aluminium foil to enhance the conductivity and totally covered with a conductive paint, “Electroshield” from Caparol. The paint is generally used to shield walls and office rooms from electric noise, and has very good conducting characteristics (Hamnerius 2005). According to Yngve Hamnerius at Chalmers9, the phantom mimics the influence of the human body in low frequency electric fields. The idea of using him is to get a good reference when performing measurement, not only using the author’s body. The pictures in figure 6.8 shows the phantom at different measuring points in Stenkullen. Protective clothing is worn by the phantom in 6.8 (b).

6.7 Protective clothing The spark protecting dress is an overall of cotton with conductive strips stitched together with the fabric. The strips, shown in figure 6.8 (b), shields the electric field when they are connected to ground. The strips are located at both sides of the dress and connect to each other at the back. Ground connection can be done by an alligator clip on a wire attached to the

9 Yngve Hamnerius, professor at Chalmers, personal contact spring 2006.


- 34 -

strips. The strips are also possible to connect to a protective (metal) helmet via a wire and to the boots (metal snap fastener). The boots are semi conductive (0.5 – 5 Mohm) and supposed to lead induced charges towards ground.

(a) (b) Picture 6.8; (a) The phantom standing close to a disconnector in Stenkullen. The setup is for contact current measurement. The yellow boots shown are semi conducting and used for protection of sparks. (b) Phantom is wearing dress for protective clothing. The yellow string at the dress is conductive and possible to connect to ground for shielding of the electric field.


- 35 -

7

Measuring positions The positions where the measurements have been performed are discussed in this chapter. In short, every place where a grounded control unit, breaker, switch etc is situated, the stationary contact current from a human to ground has been measured. This is valid for Stenkullen. In the recently built 400 kV switchyard in Lindome, one cubicle and a few other places has been investigated. In the switchyards in Strömma and Stenkullen only a few measuring points were chosen.

7.1 Stenkullen The switchyard in Stenkullen has three outgoing, three incoming and two connecting cubicles, where the latter are used for connecting the bus bars. There are three suspended overhead wires for each cubicle at a height of 19 m above ground. Each phase in a cubicle has its own circuit breakers and switches. The ground in the area around the circuit breakers is covered with asphalt. Diagonally to each breaker cubicle, there is a line with three switches, one for each phase in the cubicle. A road separates the breakers from the switches and bus bars area.

(a) (b) Figure 7.1 (a) shows a satellite photo of the 400 kV switchyard in Stenkullen (Google earth), (b) shows a drawing of the lines and connections. The disconnecting switches are placed over an area covered with grass.


- 36 -

At each grounded unit along the southwest side of the road (close to the long yellow line in figure 7.1(a)), the contact current to a human was measured several times. Using the equipment attached to the fishing rod, simulating the reach of ones arm, the highest obtained value for contact current and open circuit voltage was noted. In the area with disconnectors, the same procedure was conducted. The transient discharge measurement was done behind the most northern breaker in the northern cubicle BA 400. The breakers rest on large iron girders. A low suspended high voltage wire goes overhead to a current transformer which makes it a more exposed area although it is mainly visited during inspections. The place can be observed in figure 7.2. Figure 7.2 Photo of the place where the transient current measurement was performed, right behind a circuit breaker. The field is here approximately 10 kV/m even close to the grounded pole (presumably due to the inclination of the high voltage wire). The point where the highest electric field was measured according to a former study, performed by Fracke and Åke (2004), was right below the R-phase of bus bar A and between R and S phase of incoming cubicle number three. The name of the switchgears in this cubicle is T4-A400.


- 37 -

Fracke and Åke (2004) measured the electric field strength to 17, 5 kV/m in this area, not using any equipment cancelling out the field enhancement caused by the instruments geometry. Pettersson and Östlund (2005) later obtained a value of 13, 6 kV/m using a grounded metal sheet. As a reference, the place was measured again (07-05-2006) with a new metal sheet, gaining a maximum value of 13.4 kV/m. This value is within the interval of uncertainty for the instrument EMM-4. The maximum value was obtained several meters away from any grounded unit. Closer to the grounded disconnector poles (within arm's length) the field decreased to 7-8 kV/m.

7.2 Lindome The 400 kV switchyard in Lindome is recently built and has a compact design, using breakers and switches in the same unit. The distances to the phase lines are about the same as in Stenkullen, with suspended wires (duplex) between the switchgears. The design is of the so called two breaker type. The eastern part of the most southern cubicle was measured both regarding contact current to a person and the electric field in some points. Here, the high voltage lines from Strömma go overhead. The names of the switchgears are FL2 S4. Contact current from a person to the R S and T units were noted (poles of breakers and current measuring transformers). The distance between the phase lines is 5.5 meters and 8.5 meter between the breakers and the current measuring transformers, see figure 7.3. Six places for contact current and the electric field in nine points were measured.

(a)


- 38 -

(b)

Picture 7.3 (a) shows the measuring points in the switchyard in Lindome, (b) measuring points seen from above. The older part of the switchyard was measured in three places. The electric fields close to the circuit breakers connected to a transformer noted T2 were measured. Also measured was the contact current from a person to the control units as shown in figure 7.4.

Figure 7.4 The older part of the switchyard in Lindome. Measured breaker units are shown.


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8

Measurement results In this chapter, measurement results from the various locations are presented.

8.1 Measurements in Stenkullen The measurements in the switchyard in Stenkullen were performed during several occasions from January to June 2006.

8.1.1 Contact currents and open circuit voltage

The results from contact current measurement performed in Stenkullen are given below. The person conducting the measurement is the author with body size 52 and a height of 183 cm. A plastic protection helmet is always worn due to the safety regulations in a substation. The same shoes (Ecco working shoes with 1.5 cm thick sole) are also worn throughout all the measurements in order not to affect the outcome. Since the phase to phase voltage at the lines has very small fluctuations, no effort was spent to note those levels. Line voltage, phase to phase rms, fluctuate between 405 and 410 kV according to J-B Mårtensson10 As mentioned in the previous chapter, each breaker unit was measured several times. The result from contact current and open circuit voltage for a person touching the breakers are shown in diagram 8.1. The numbers indicate that the breaker units along the road, starting from north. Note that the continuous curves only are used for easier readout of the information.

10 J-B Mårtensson, service technician at Vattenfall Syd, personal communication, spring 2006.


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Diagram 8.1 All current measurements from a person to a breaker unit are here shown.

Diagram 8.2 The measured voltage levels. As a reference, the measured currents from May 7 are included

Short circuit current (rms)

0

20

40

60

80

100

120

140

160

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24

Breaker unit

[uA

]Current Januari 18

Current March 14

Current May 3

Current May 7

Open circuit voltage (rms)

0

500

1000

1500

2000

2500

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24

Breaker unit

[V]

an

d [

uA

]

Voltage June 10

Current May 7


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(square marks). To see if there are any differences between the types of breakers, table 8.1 presents an average value for the contact current and voltage in each group of breakers (three breakers). All measurements are included and an average has been calculated. Note that the breakers in group ABC 400 S are not in operation, and only the surrounding electric field give raise to the measured values.

Table 8.1 The averaged value for each group of circuit breakers. Values calculated by taking an average value for R, S and T phase within each type/group of breakers. The asterisk (*) indicates that this cubicle was not in use as one can see from the value of the current. The diagram 8.2 below shows the contact current and voltage measured when standing in front of the control unit to each disconnector. The numbers indicate the measuring path with the disconnectors. The path starts from north at bus bar B, diagonally along each group of disconnectors to the southern part of the switchyard, and then continues along the disconnectors at bus bar A back to the northern part.

Group (3

breaker units

in each)

Number Average

voltage [V]

Average

current [uA]

BA – 400 - J 1-3 1860

94

ABC 400 S 4-6 * 580

25

FL 15 S5 - S 7-9 1410

88

T2 – 400 - S 10-12 1667 92

FL5 S4 - S 13-15 1583 101

T3 – 400 - S 16-18 1953 113

FL 18 - S 19-21 1613 97

T400 - S 22-24 1267 84


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Diagram 8.2 Contact current and voltage standing in front of the control unit to a disconnector. The values indicated with triangles are from an additional measurement series using a voltage divider with higher impedance. First, open circuit voltages were measured with the MetraHits 5 Mohm internal impedance but found to be inaccurate. Supplementary measuring series using equipment with 100 Mohm internal impedance was performed in June 10. In a few places with high obtained current values, the electric field was measured at ground level. The EMM-4 and the grounded sheet were placed at the position of the person’s footprint from the former measurements. Vehicles (workers vans) were also placed within reach to the grounded pole of the breakers. Then, short circuit current was measured. Placing the vans under the phase lines, right below the bushing insulators of the breakers, gives the result shown in table 8.3.

Short circuit current and open circuit voltage (rms)

0

200

400

600

800

1000

1200

1400

1600

1800

2000

1 3 5 7 9 11 13 15 17 19 21 23 25 27 29 31 33 35 37 39 41 43 45 47

Disconnector unit

Voltage [V] 5 Mohm

Voltage [V] 100 Mohm

Current [uA]


- 43 -

Table 8.2 The measured open circuit voltage, short circuit current and electric field strength.. The “s” within the brackets indicates the most southern disconnector and “n” indicates the most northern breaker in the group.

Object Soil place Current

[µA]

Large van (1.95x1.90x5.25)

asphalt Placed under bushing insulator of breaker T3 400-S (unit 18)

725

Large van (1.95x1.90x5.25)


1079

Large van (1.95x1.90x5.25)


791

Large van (1.95x1.90x5.25)

asphalt Placed under bushing insulator of breaker BA400 (unit 1)

1169

Large van (1.95x1.90x5.25)


636

Large van (1.95x1.90x5.25)


980

Small van asphalt Placed under the bushing insulator of breaker T3 400-S (unit 18)

410

Table 8.3 Short circuit current from a small and a large van.

8.1.2 Measurements with protective clothing

Shown here are the results of the measurements using the protective clothing from Swedpower. The phantom was used, standing on the grounded girder behind breaker BA400 with and without an insulating rubber carpet (1 cm thick) placed under the semi-conducting boots. Each boot has a resistance of 0.5-5 Mohm (according to the labelling on the boots).

Object Soil place Current

[µA]

Voltage

[V]

Electric field

[kV/m]

Human 1,83m size 52

Dry grass Disconnector FL 18 BF (s) 161 950 8.2

Human 1,83m size 52

Wooden well cover Disconnector ABC 400 BF (n)

110 2060 5.6

Phantom 1,85m size 50

Wooden well cover Disconnector ABC 400 BF (n)

117 1870 5.6

Human 1,83m size 52

Rubber carpet (1cm thick) on steel girder

Behind breaker BA400 (unit 1)

166 3290 9.74

Phantom 1,85m size 50

Rubber carpet (1 cm thick) on steel girder

Behind breaker BA400 (unit 1)

150 3160 9.74


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Table 8.4 The effect of protective clothing. The helmet is of plastic but covered with conductive paint to simulate the old shielding helmets (appurtenant to the dress). The helmet is connected to the metal strips in the dress when noted as “grounded”. Between the rubber carpet and the boots were two foot-shaped metal plates put for easy grounding ability. The metal plates were grounded by a wire to the pole of the breaker when boots were noted as grounded in table 8.4. When clothes were grounded, an alligator clip attached to the shielding metal strips within the dress was connected to the breaker’s pole (ground).

8.2 Measurements in Lindome In this part, the results from the electric field and contact current measurement in Lindome (a modern 400 kV switchyard) are presented. Current and voltage are measured from a person to grounded objects shown in table 8.4

Measuring point Current

[µA]

Voltage

[V]

Electric field

[kV/m]

1 70 1160 3.97 2 54 970 2.83 3 66 1290 4.51 4 57 810 3.95 5 44 605 4.38 6 60 870 4.31 a - - 7.00 b - - 5.90 c - - 6.79 Old breaker T2400S (L1) 90 1420 4.95 Old breaker T2400S (L2) 65 970 3.30 Old breaker T2400S (L3) 95 1620 4.50 Staircase along phase down to transformer 97 1690 - Highest value under overhead line L1 from Strömma (open yard)

7.5

Table 8.4 Measured values of short circuit current, open circuit voltage and electric field strength in Lindome.

Action taken Short circuit

current [µA]

Open circuit

voltage [V]

Electric field strength

[kV/m]

Totally insulated on carpet (with clothes not grounded)

163 2720 9.74

Grounded boots 157 310 9.74 Grounded clothes 131 760 9.74 Grounded clothes and boots 128 223 9.74 Grounded clothes and helmet

92 514 9.74

Grounded clothes, boots and helmet

90 100 9.74


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8.3 Measurements in Strömma Contact currents in a few places were measured here. Those are shown in table 8.5. Object Short circuit current [µA] Car shown in figure 5.5 to breaker pole 610 Human 1.83 m on ladder to breaker, figure 4.3 180 Human 1.70 m on ladder to breaker, figure 4.3 170 Table 8.5 Contact currents measured in Strömma

8.4 Transient analysis – spark discharges In this part, the results from the spark discharges are presented. All measurements are performed at the same place behind a breaker. The person (1.83 m height) is exposed to an electric field with strength of 9.7 kV/m. The initial open voltage was measured to about 2.5 kV. Body discharges from fingertip, hand and arm was performed. In figure 8.1, the current through a 100 ohm metal film resistance is shown. The sparks are captured with a digital oscilloscope acquiring the voltage over the resistance. The stored signals are then after treated with Mathworks Matlab.

Figure 8.1 Repeatedly spark discharges from a fingertip. The spark gap is enlarged to a maximum, right before the discharges stop. The last discharge has a current peak over 1 A. Figure 8.2 shows a single discharge in higher time resolution.


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Figure 8.2 A single discharge with a time base of 1 us / div at the oscilloscope. The signal in figure 8.2 was transformed using Matlabs’s fast Fourier transform in order to get the frequency-response characteristics. The result is shown in figure 8.3, plotted in dB scale.

Figure 8.3 Frequency spectrum of the spark discharge in figure 8.2. The current is plotted in dB scale. As shown in figure 8.3, the signal has significant frequency components up to about 10 MHz.


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When the spark gap distance is lowered to almost contact, a larger number of sparks overcomes the gap at a lower potential. This can be observed in figure 8.4. Figure 8.4 A larger number of sparks occur when the distance is lowered. Eventually, when placing the hand on the electrode, the stationary contact current is achieved. Still partial discharges occur, as shown in figure 8.5.


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Figure 8.5 The stationary current when tha hand is placed on the electrode can be seen. Calculating the RMS value gives a current of about 176 µA which correspond well to the one obtained with the ampere meter. Partial discharges occur when voltages are high.

In an attempt to reach highest possible current peaks, the authors arm was placed closed to the discharge electrode. This resulted in high current discharges as observed in figure 8.6 and 8.7.

Figure 8.6 High current peaks from the authors arm, peaks of 1.75 A is reached.


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Figure 8.7 One of the peaks in figure 8.6 at higher time resolution. Signal is filtered in Matlab to improve clarity and suppress digitizing errors.

8.5 Weather dependence The workers experiences different levels of annoyance from spark discharges throughout a day. To observe the influence of wet and dry soil and shoes, the phantom was used and 10 litres of water poured at the soil below the shoes. Finally, the shoes were soaked as well. The result is given in table 8.6.

Soil Short circuit

current [µA]

Open circuit voltage

[V]

Dry grass 110 1060 Wet grass 110 340 Wet shoes and grass

109 180

Table 8.6 Simulated influence of weather. The position is in front of the most northern disconnector BA 400-BF.


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9

Discussion and Conclusion In this chapter, the results are discussed and conclusions are drawn. Sources of error are also treated.

9.1 Contact current The maximum measured contact currents from a person to a grounded object was 180 µA. This is a low value in comparison with the action value of 1 mA stated in the directive. Regarding the measurements in Stenkullen, touching a breaker gave lower contact current than touching a disconnector. Presumably, this is due to higher electric field strength in the area with disconnectors. The average contact current, human to breaker was 86 µA and human to disconnector gave an average of 116 µA. Other 400 kV switchyards of older design (Strömma and Kilanda) showed similar values regarding contact current. In Lindome, the new switchyard has a very compact design. This implies reduction of the electric field strength due to all grounded objects within the area. Contact current measurements showed significant lower values (about 44 - 70 µA) in this switchyard. The old part of the yard gave values similar to the ones in Stenkullen. The contact current measured from hand to grounded objects doesn’t vary much from one measuring occasion to another. The weather dependence plays a small role; resistive ways of losses through shoes have high resistance compared to impedance between hand and grounded object. Thickness of the soles plays some role due to capacitive coupling to ground. Thin soles reduce the measured current flowing through the hand. This is because some of the current induced in the body instead takes the path through the shoes. The shoes used in the measurements have thick soles (~1.5 cm). During the measurements, noticeable differences in the measured current (~10 µA higher at one foot) were shown if one was standing on either one or both feet. All measurements were performed standing on both feet. The contact current from a van to a grounded person did exceed the action value of 1 mA at some positions. This situation is on the other hand easy to avoid, keeping in mind to park the car out of reach to grounded objects when high field strength is present. Very often, those situations occur at maintenance. Extra high currents (and voltages) are reached at maintenance when the service car is placed close to the grounded object and the service doors are opened upwards, extending the charge collecting area of the car. JB Mårtensson11 tells that this is a standard procedure to protect instruments in rainy weather conditions. The difference in measured current gave an increase of up to 20 % when one service door was opened (lifted upwards).

11 J-B Mårtensson, service technician at Vattenfall Syd, personal communication, spring 2006


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Studies at the former NRPB, (National Radiological Protection Board), now part of the Health protection agency in Great Britain, has studied the relation between electric field strength and current density in the human body The computerized anatomical models “Norman” (Dimbylow 2000) and “Naomi” (Dimbylow 2005) have been used to calculate current density within the CNS (Central Nervous System) when exposed to 50 Hz electric field. In an electric field strength of 10 kV/m, the models showed a current density of approximately 1.84 mA/m2 in the CNS and about 2.17 mA/m2 in the retina in the eyes. The values were the maximum taken after forming mean values over an area of 1 cm2. Those studies, together with the measured contact current of 170 µA in approximately 10 kV/m, show a significant safety margin between action values and limitations. Enhancing the field strength with a factor 4 will exceed neither the contact current of 1 mA nor the current density of 10 mA/m2 in CSN. This is valid when the human acts as a charge collecting object and discharge herself to ground. If one is using the formula in equation 2.2 from EPRI (1983), the short circuit current from a human with a height of 1.80 m is about 15 µA/kV/m, which also shows a large margin to the 1 mA action value. Regarding ICNIRP’s suggestion of increasing the electric field strength to 20 kV/m for occupational exposure, this could be applied also in the European directive since it only deals with contact current after contact has been established. A field strength of 20 kV/m would on the other hand induce higher potential at the body and spark discharges will be of greater magnitude. Discharges to ground in a field strength of 10 kV/m is pretty much the level the author could handle without unintentional muscle movements.

9.1.1 Uncertainty of the contact current measurements.

The contact current measurements have high accuracy using the instrument MetraHit 29s which has a low uncertainty range of 0.5 % for AC current measurement. The measured current does change with different positions and type of soil. But it is still the current that a worker is exposed to. The relation of 15 µA/kV/m is not valid over all measured positions since the field is not homogenous close to conducting objects. In a switchyard, many high voltage sources contribute to the field and make the distribution of the field vary in a complex way. The field measured at ground level doesn’t necessary correspond to the field 1 m above ground where the major part of the charge collecting area of the body is. A more accurate correlation might be given if the electric field measurements were performed at a higher level above ground.

9.2 Voltage measurement The voltages measured fluctuated a lot depending on soil. Soil of grass gave raise to the largest fluctuations. Also higher humidity in the air plays some role; surface charges are being transported away from the body. All kind of resistive losses to ground lowers the initial voltage before contact


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is made. In general, voltage levels fluctuate from about 200 to 2400 volt (rms) in Stenkullen. Only in extraordinary conditions, those levels were exceeded. All measurements were performed in similar, dry, weather conditions. Lindome switchyard had significant lower voltage levels due to better shielding of the electric field as discussed in 9.1 above. To touch most of these switchgear units means insignificant annoyance from spark discharges. A voltage level exceeding 350 V (rms) implies the possibility for spark discharges through the skin. An ever higher voltage level means that sparks will ignite over the air gap found at pre contact. First, measurements were performed using the MetraHit 29s but found not to be accurate at high voltage levels since the paths of losses to ground are in the same order of impedance magnitude as the instruments internal 5 Mohm. The capacitive coupling from the overhead line to the body will also discharge considerably through 5 Mohm. Additional measurements were performed using a 10:1 voltage divider in conjunction with a multipurpose instrument; Meterman 38 XR which has an impedance of 10 Mohm and an uncertainty range of 1.2 %. With this equipment, the impedance was extended to 100 Mohm. An attempt using a voltage divider with 100:1 ratio (1 Gohm) gave result similar to the 100:1 divider but was found sensitive for high field strengths (induced charges). High impedance in the equipment means that the capacitive coupling from the power lines to the measuring wires has greater influence. The instrument was shielded with grounded metal foil, but cables to the instrument and the voltage divider could still be a subject of induced charges from the electrical field. This will give a higher readout of the potential than the real one. The highest obtained potential was 3290 Volt in a field strength of 10 kV/m. This was achieved by standing on an insulating rubber carpet. The voltages given are, besides instrument uncertainty, to be considered very approximately. To lean the head means several hundred volts difference in a measurement. Wet or dry soil and shoes imply considerably changes in the voltage levels. A straw of grass at ones leg during a measurement lowers the voltage significant. The workers experience spark discharges different one day to another or even within the same day at the same position. The explanation to this is differences in the potential due to wet shoes (transpiration or water) and the soil. Some working shoes have very thin soles with a protective steel mantel. When such shoes get wet, the resistance is significant lowered. The resistive impedance, feet to ground, is in those cases considerable dependent on the socks and transpiration. Clean and dry socks imply high resistance but are lowered to almost none when becoming wet. To simulate the weather dependence, 10 litres of water were poured out on the soil under the phantoms feet. The result showed a dramatically lowering in voltage, but no change in the short circuit current. This wetness of soil and shoes gave an outcome of no possible sensing level (< 350 V rms) of spark discharges due to low initial voltage.

9.3 Electric field measurements All measurements were performed at ground level using a metal sheet to avoid field enhancements. Measurements in open yard are easier to measure accurate. Close to grounded


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conducting objects (see figure 3.2); the field strength is very inhomogeneous. By a small displacement of the instrument, large differences in the result were shown. At ground level, the field strength might also be very different compared to a height in level with ones head. In this study, the attempt was to place the instrument right at the footprint from the former measurement. Other measuring equipment, capable of measure the field higher above ground might be better to use.

9.4 Current discharge measurements The current discharges measured showed very high current peaks. The highest peak had a magnitude of 1.75 A, but very short duration (~ 2 µs). This measurement was performed in an electric field strength of about 10 kV/m, with the same shoes worn as in the other measurements. Standing on an insulating rubber carpet gave even greater discharges due to higher potential, but the pain was too unendurable to acquire those. The major part of the discharge current lies below 10 MHz according to the frequency spectrum shown in figure 7.3. The equipment used was very simple using measurements over a resistance. A lot of external noise from the surroundings and from the digitizing process in the oscilloscope was present. Equipment with better shielding and better resolution might show higher accuracy. Another aspect is the non linear behaviour of the resistance at higher frequencies (above 100 MHz). In an article “Measurements of Body Impedance for ESD” (Zhancheng Wu et. al. 2003), current peaks from electrostatic discharges (charged capacitance) to the human body were measured. The result showed current peaks ranging from 2 to 10 A with time constants of 0.25 to 1.25 µs at applied voltages of 1000-5000 V. A contact electrode was used to transfer the charge to the body and not through a spark. The discharge capacitance in those experiments was 500 pF. The sparks from ESD is found to be very similar to the ones obtained in an alternating high voltage field. According to Reilly (p 57-58 1998), a typical carpet spark (a person obtain negative charges walking over a carpet) would be in the order of 2-3 kV with an energy of 0.2 to 0.45 mJ. A better way of measure, not disturbing the path of the current, would be with a current transformer surrounding the finger or cable where the discharge flows. Such a transformer or probe suitable would be the Pearson Electronics current monitor model 2877. This one is capable of acquire a signal with a bandwidth of 200 MHz and has a BNC output.

9.5 Protective clothing The old dresses from Swedpower used for protection of spark discharges works well if they are correctly grounded. With a conducting helmet attached to the grounded strips in the dress, as well as the alligator clip connected to ground, the contact current is reduced to almost one half. More important, when the semi conducting boots have good contact to ground, the voltage is dramatically reduced. This eliminates the risk of painful spark discharges. Unfortunately, the clothes are not used today due to the inconvenience to put them on and lack of knowledge of their ability to protect. More modern and comfortable clothing with better shielding effect (thin metal filament within the fabric) are available today according to


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Yngve Hamnerius12. One company distributing such material in Sweden is RTK (RTK 2006). Their fabric “Picasso” is suitable for fabrication of electric field protective clothing. At such places like the ladder in Strömma, shown in figure 5.3 or working at a platform, like the picture in figure 5.4, it would be compulsory to use protective clothing. There are great risks when performing tasks high above ground. The indirect effects of spark discharges can be fatally in such conditions. Luckily, there will be some reconstructions of hazardous positions in several substations, including Strömma according to Lars Wallin13.

9.6 General conclusions The action value for the electric field in the EU 2004/40/EC directive based on ICNIRP is set with a margin in relation to the current density limitation for the direct field induced current. To exceed the current density limit of 10 mA/m2 in CNS, body exposure of over 40 kV/m is necessary. This margin set is to prevent effects when touching charges conductors in electric fields. The contact current action value of 1 mA is based on classical data of perception. Even though newer studies of perception levels among the population show a considerably higher level of sensitivity (78 µA for 5 % of the population), this contact current is still not a big health problem14. Only at contact with larger objects exposed to electric field strengths in the order of the action value and ground at the same time, 1 mA is exceeded in the switchyards studied in this report. Avoiding those situations imply the possibility to increase the electric field strength to 20 kV/m, as suggested by ICNIRP. A human body exposed to 20 kV/m will not experience contact currents above 1 mA when touching a grounded object. Simultaneous contact with a large charge collecting object such as a van and ground will on the other hand likely exceed 1 mA at 20 kV/m. ICNIRP does not mention contact with grounded objects where the human body is exposed to an electric field, only “effects from contact with electrically charged conductors”. At contact with a charged conductor, such as a van exposed to an electric field, the human body must be well grounded in order to reach contact current levels above 1 mA. The biggest problem experienced is however not the contact current, but spark discharges at pre contact. Those are (yet) regulated by neither ICNIRP nor the directive 2004/40/EC. It seems very strange not to include them in the regulations. Current peeks of 1.75 A (although short durations) as shown in this study do trigger muscle nerves much more intense than current at established contact in the same field strength. More knowledge is necessary to bring out to those who work in high voltage areas. A change in the attitude towards protective clothing and information about the hazards in the work is of great importance. In order to facilitate protection, better and easier protective clothing could be used.

12 Yngve Hamnerius, professor at Chalmers, personal contact spring 2006. 13 Lars Wallin, Swedish national grid company (SvK) personal contact May 29. 14 Kari Jokela, research professor at STUK, (Radiation and Nuclear Safety Authority, Finland), mail contact January 16 and 17, 2006


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References Biegelmeier, G, 1985. New experiments with regard to basic safety measurements for electrical equipment and installations. In J.E. Bridges, G.L. Ford, I.A. Sherman, and M. Vainberg, Electric Shock Safety Criteria, Pergamon, New York. pp 161-171. Dimbylow, Peter 2000. Current densities in a 2 mm resolution anatomically realistic model of the body induced by low frequency electric fields. Physics in Medicine and Biology. Vol 45. 2000. pp 1013-1022. Dimbylow, Peter 2005. Development of the female voxel phantom, NAOMI, and its applications of induced current densities and electric fields from applied low frequency magnetic and electric fields. Physics in Medicine and Biology. Vol 50. 2005. pp 1047-1070. Deno, D. W. 1978. Electrostatic and electromagnetic effects of ulta-high voltage transmission lines. Final report EPRI EL-802. Palo Alto, California, Electric Power Research Institute. Elkraftteknisk handbook 3, Elanläggningar. Esselte Studium AB, 1984. Enviromentor (1998) “Bruksanvisning Electric Field Meter EMM-4”. Gothenburg: Enviromentor AB. EPRI 1982, Transmission Line Reference Book. 345 kV and Above. Second Edition. Prepared by Project UHV. 1982. EU, 1999: European Union's recommendation The limitation of exposure of the general public to electromagnetic fields (0 Hz to 300 GHz) (1999/519/EC), The Official Journal, Brussels, July 12th 1999. EU, 2004: European Union's Directive 2004/40/EC of the European Parliament and of the Council on the minimum health and safety requirements regarding the exposure of workers to the risks arising from physical agents (electromagnetic fields)(18th Individual directive within the meaning of article 16(1) of directive 89/391/EEC, The Official Journal, Brussels, April 29th 2004 Fracke, J and Åke, J: Magnetiska och elektriska fält i ställverksmiljö: Kartläggning av en äldre anläggning. Examensarbete 2004:17. Chalmers Lindholmen University Collegie. Freibeger, R, 1934. The electrical resistance of the human body to commercial direct and alternating currents. Translated from German by Allen Translation Service, Maplewood, NJ, Item no. 9005. Published by Bell Laboratories, Maplewood, NJ.


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Hamnerius,Yngve ”Uppmätning av skärmningsegenskaper hos väggar målade med Caparols färg ElectroShield” 2005-11-06. Electronic document. www.caparol.se (Accessed: 2006-03-29). ICNIRP, International Commission on the limitation of Non-Ionizing Protection, Guidelinesfor limiting exposure to time varying electric, magnetic, and electromagnetic fields (up to 300 GHz). Health Physics April 1998, Volume 74, No 4. IEEE (Institute of Electrical and Electronics Engineers) Committee Report. 1978. “Electric and magnetic filed coupling from high voltage power transmission lines – Classification of short-term effects on people”, in IEEE Transactions on Power Apparatus and System PAS-97, pp. 2243-2252. IEE Proc. Gener. Transm. Distrib., Vol. 146, No. 5, pp. 595-596, Sept. 1999. International Labour Organization 1994. Protection of workers from power frequency electric and magnetic fields. Occupational Safety and Health Series No 69. Leitgeb, N, Schroetiner, J, Cech R 2005. “Electric current perception of the general population including children and the elderly”. Journal of Medical Engineering & Technology, Vol. 29, No. 5, September/ October 2005, pp 215-218. Pettersson, A and Österlund, C: Reducering av elektriska fält i ställverksmiljö, Examensarbete 2005, Chalmers University of Technology. Reilly J. Patrick, 1998. “Applied Bioelectricity; From Electrical Stimulation to Electropathology”. 1998 Springer-Verlag New York Inc. RTK, 2006: “Avskärmningsväv Picasso”. Electronic document. www.rtk.se/Downloads/Picasso.pdf. (Accessed : 2006-08-29). Zaffanella, L. E. 1985. Answers to the CIGRE questionnaire “Survey of the situation concerning effects of electric and magnetic fields”, doc. 36-85 (SC) 21 IWD. Paris, CIGRE.

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