LIGHTNING PROTECTION IN BUILDINGS 1. … Lec08...... ground flashes are less common than cloud ... any pair of conducting objects in the building. If direct . ELEC9713: Lightning

ELEC9713 Industrial and Commercial Power Systems

LIGHTNING PROTECTION IN BUILDINGS

1. Introduction The purpose of lightning protection is to protect people, buildings and their contents from the devastating effects of lightnings. Lightning is a very common event. Worldwide, some 30 lightning flashes occur in every second on average. 2. Physics of lightning Thunderstorms are natural weather phenomena. In a thundercloud, turbulence from updrafts causes collisions among ice crystals and water droplets. For reasons not well understood, the smaller particles become positively charged and the larger ones negatively charged. Winds and gravity separate the charged particles resulting in regions with net negative charge mainly in the lower part of the thundercloud, and regions with net positive charge mainly in the upper part. Lightning is a sudden discharge of electricity between differently charged regions. This happens when the voltage difference caused by the charge build-up is high enough to cause the air to breakdown. This can occur within a cloud or from cloud to cloud (cloud flash).

ELEC9713: Lightning protection in buildings page 1/46

Furthermore, the negative charge building up at the cloud base in turn can cause a build-up of positive charge on the ground. The ground surface can be as little as 1km away from the cloud base. Thus lightning can also result from discharges between the cloud base and ground (ground flash). In comparison, ground flashes are less common than cloud flashes. They account for only ~20% of total lightning strikes.

Source: www.erico.com

The sequence of events occurring during a ground flash is as follows. Firstly, high electric field initiates ionisation of the air at cloud end and produces the downward leader (stepped leader). The ionised channel advances in a series of steps towards the ground. This then causes initiation from the ground of a number of positively charged ionised channels, called upward leaders. These originate from trees or pointy parts of structures on the ground, attracted to the downward


leader, and attempt to intercept it. The one that intercepts completes the ionised channel between the cloud and ground. This high conductivity channel enables charge equalisation between the cloud base and ground, called the main return stroke. The process involves current flow down the ionised channel, through any intervening structure and into the ground.


Lightning protection systems are designed to ensure that lightning terminates on an air terminal (lightning rod) instead of on some other parts of the building. A complete ground flash usually consists of a sequence of one or more high-amplitude short-duration current impulses (strokes). The currents are uni-directional, and usually negative (negative charge injected into the struck object). For analysis, we consider the stroke as generated from a current


source, i.e. the current waveshape and magnitude are not affected by the characteristics of the ground termination. The Table below gives statistical distributions of some characteristics of ground flashes.

Characteristics of ground flashes [Table B1, AS/NZS1768:2003]

The principal effects of a lightning discharge to an object are electrical, thermal, and mechanical. The action integral is equivalent to the energy deposited in a 1Ω resistor by the passage of the entire current for the duration of the flash. Thus, the amount of energy deposited in any object carrying lightning current is the product of action integral and object resistance.

2i dt∫



Because of the large peak current, the mechanical force ( )F B i l= × × acting on the conductor is substantial. It is necessary that lightning conductors are safely secured to the building they are intended to protect. Otherwise, explosive reaction may occur, dislodging materials or hurling large pieces of masonry/wood many metres.

Potentials during a lightning flash to earthed conductor.

Lightning may cause death or serious injury in various ways:

Direct strikes to the person causing heart failure, brain damage, suspension of breathing or paralysis burns, and many other medical effects.

Asphyxiation or injury due to fires or structural damage Side flashes Step and touch voltages

Modes of entry of lightning impulses [Fig 5.1, AS1768:2003]

3. Elements of a lightning protection system

(a) Air terminals Its function is to absorb the lightning discharge that might otherwise strike a vulnerable part of the object under protection. A network of air terminals may be required to


shield a large area and they are placed so as to achieve a high probability of intercepting the lightnings. (b) Down conductors Its function is to provide a low impedance path to convey the lightning current from the air terminal to earth, without the development of excessive large voltages which can cause side flashing. The lightning conductors can have a potential of up to 1 MV with respect to true earth. The non-uniform field breakdown voltage for air is about 500kV/m so (bare) downconductors should be kept more than 2m away from adjacent structures. To reduce the possibility of side flashing, the downconductor route should be as direct as possible with no sharp bends. (c) Earth termination network This consists of one or more earthing electrodes and the conductors that interconnect them. Its function is to deliver the lightning current into the earth mass. Test links may be required between downconductors and earthing electrodes to facilitate testing of protection system. (d) Equipotential bonding This is used to reduce hazardous potential differences between any pair of conducting objects in the building. If direct


connection between dissimilar metals may create corrosion problems, common bonding may be affected by using SPDs. (e) Over-voltage protection Its function is to prevent excessive voltages being applied to equipment while allowing correct operating voltages to exist. This is achieved through the use of various surge protection devices (SPD) such as spark gaps, gas-filled surge arrestors,and metal oxide varistors (MOV). The air terminal, downconductor and earth electrode all contribute to the total impedance of the earth connection. In particular, the inductances of the air terminal and downconductor are very significant. A typical inductance value for a down conductor would be ~1μH/m. Thus if the di dt is 50kA/μs then the voltage drop for 10m of downconductor would be 500kV. Hence, it is very important that downconductors are kept as short as possible, straight and vertical to minimise the potential voltage rise.


4. Standards on lightning protection The most important standard is AS/NZS 1768:2003 Lightning Protection. It provides guidelines for the protection of:

People, both outdoors (direct effect of lightning strike) and indoors (indirect risk due to lightning currents conducted into the building)

Buildings and structures Sensitive electronic equipment from overvoltages resulting from a lightning

The standard is applicable to conventional lightning protection systems (LPS) and surge protective devices (SPD). 4.1 Assessment and management of lightning risk

Table 2.1 AS/NZS1768:2003

The risk management approach is used to determine whether protection is needed and if so the selection of adequate


protection measures to reduce the risk to below a tolerable level. The risk R is defined as the probability of loss occurring over a one-year period. The table below lists some probabilities of death associated with everyday living. In comparison, the probability of lightning striking a person is insignificant. For a particular building, the types of risk due to lightning and acceptable limits are:

Risk Type of loss Tolerable level R1 R2 R3 R4

Human life Service to the public

Cultural heritage Economic value

10-5 10-3 10-3

varying (based on cost/benefit consideration)

The extent of damage is influenced by the proximity of the lightning strike:

C1 – direct strike to the structure C2 – strike to the ground near the structure C3 – direct strike to a conductive electrical service line C4 – strike to ground near electrical service line

The number of lightning strikes depends on:

Dimensions and characteristics of the building Dimensions and characteristics of incoming service lines Environment around the building



Density of lightning strikes in the region where the building is located.

The type of damage caused by lightning depends on:

Construction type of the building Its contents and application Incoming conductive electrical service lines Measures taken for limiting the risk

Three types of damage are considered:

D1 – injury to people D2 – fire, explosion, mechanical destruction etc D3 – failure of electrical and electronic equipment

The value of the damage caused will depend on:

Number of people and time when they are in building Type of service provided to the public The value of goods and services affected.

There are four types of losses to be considered:

L1 – loss of human life L2 – loss of service to the public L3 – loss of cultural heritage L4 – loss of economic value (structure, content, activity)

The total risk would be the sum of all risk factors, both direct and indirect: x d iR R R= ∑ = ∑ +∑R

Each risk factor is given by: x x x xR N Pδ= where: frequency of dangerous events xN = probability of damage or injury xP = xδ = relative amount of damage or injury If protection measure is adopted, the probability Px may be reduced by a reduction factor kx. The total risk should not exceed the acceptable risk:

. aR R≤


Possible risk components [Table 2.3 AS1768:2003]



Further parts in Section 2 of AS/NZS1768:2003 discuss the following in detail:

Risks due to lightning, risk components and calculations Procedure for risk assessment and management Risk management calculation tool

Flow diagram for risk management procedure

[Fig 2.2 AS1768:2003]

4.2 Protection of structures Lightning protection measures include:

An LPS for the structure and its occupants: this comprises an air terminal network to intercept the lightning strike, a down-conductor system to conduct the discharge current safely to earth, and an earth termination network to dissipate the current into the earth.

Protection of sensitive electronic equipment against lightning electro-magnetic pulses (LEMP): use of a mesh of down-conductors to minimise internal magnetic field, selection of lightning protection zones, equipotential bonding and earthing, installation of SPDs.

Transient protection (TP) of incoming services: use of isolation devices, shielding of cables, installation and coordination of SPDs.

Designers of LPS may select between 4 levels of protection:

Protection level, PL LPS efficiency, η I II III IV

0.98 0.95 0.90 0.80

LPS main design rules:

Firstly, provide air terminals to protect the most vulnerable parts of building (points and corners). Then use rolling sphere method (RSM) to check if less vulnerable parts (edges) are protected. If not, add more


air terminals. Then check least vulnerable parts (flat surfaces).

At least 2 down-conductors are required on any structure exceeding 10m in height. The spacing between down-conductors shall not exceed 20m.

The whole earth termination network must achieve a resistance <10Ω. If SPDs are required, they must be installed and bonded to the termination network.

The air terminals establish zones of protection and if properly positioned, these zones would shield the whole structure. A sphere of some specified radius (a) is imagined to be rolled along the ground towards the building, up the side and over the top of the building, and down the other side to ground. This process is repeated in various orientations in relation to the building. Any point on the building touched by the sphere is a possible lightning attachment point. All sections on the building touched by the sphere are considered vunerable to direct lightning strokes and need to be protected by air terminals. These need to be positioned so that the sphere only touches their interception surfaces.

Protection level PL

Sphere radius a, m (ai)

Interception current Imin, kA

I II III IV

20 (60) 30 (60) 45 (90)

60 (120)

2.9 5.4 10.1 15.7




The RSM is unduly conservative for large flat surfaces (e.g. building roof) and on the side of tall structures. The method is modified as follows:

Design air terminal network using standard radius a to provide protection for points, corners, and edge surfaces.

With this network, check if protection is provided to all flat surfaces using a larger radius ai (see Table). If not then more air terminals are to be added.

The striking distance, ds, is the distance between the tip of the downward leader and the eventual strike attachment point at the moment of initiation of an upward intercepting streamer when it has become inevitable that the gap ds will be bridged by the lightning discharge channel. The following relationship has been proposed: (m) 0.65

max10sd = × i

where is the peak current of the return stroke in kA. It can be seen that the radius value used in the RSM is closely related to the striking distance.

maxi

Typical LPS using metal in or on a building [Fig 4.4 AS1768:2003]


Using horizontal and vertical air terminals [Fig 4.5 AS1768:2003]


4.3 Calculation of lightning discharge voltages Appendix D of AS1768:2003 describes methods to calculate the expected lightning discharge voltages throughout an installation. For design purposes, lightning stroke currents are idealised as shown in Fig D1 and the approximate breakdown strength of air is given in Fig D2.

[Fig D1, AS/NZS1768:2003]

[Fig D2, AS/NZS1768:2003]


(a) Travelling wave analysis The injected charge propagates along the conductor as a travelling wave with velocity:

1 1

1vL C

= (m/s)

where: inductance per unit length (H/m) 1L = capacitance per unit length (F/m) 1C = For a single bare conductor (non-magnetic material) of radius r at a distance h above a perfect ground plane in air:

1 12 2ln and 2 ln

2o

oh hL C

rrμ πεπ

⎛ ⎞= = ⎜ ⎟⎝ ⎠

and thus: 81 3 10 m/so o

v cμ ε

= = = ×

For an insulated cable with earth sheath and dielectric with relative permittivity k (typically between 3-6):

cvk

= m/s

Voltage e generated by a travelling wave with current i is:

e i Z= ×where:


1

1

= conductor surge impedanceLZC

=

260ln hr

Ω (case of bare conductor)

When the travelling wave arrives at an electrical discontinuity, it results in:

reflected components: ' ; 'v bv i bi= = − transmitted components: ( ) ( )" 1 ; " 1v av b v i b i= = + = −

where:

2

1 2

2 transmission coefficientZaZ Z

= =+

2 1

1 2

reflection coefficientZ ZbZ Z

−= =

+

2Z = impedance seen by surge at the disconinuity. The total response must take into account the effect of multiple reflections and transmissions. The travelling wave analysis permits calculation of the voltage response at any point on the LPS because of the distributed constant representation of the system. The Bewley lattice diagram is often used to facilitate the calculation. Example: A 40m high building protected by 4 vertical air terminals. Four down-conductors, surge impedance 480Ω. Combined



resistance of earth termination network is 2.5Ω. Want to know voltage responses to the two idealised lightning stroke currents.

Surge voltage by Bewley lattice diagram

Surge voltage at the tip of the air terminal (node 1) is:

( ) ( ) for 0 2v t Zi t t T= ≤ ≤ ( ) ( ) ( )+2 2 for 2 4v t Zi t bZi t T T t T= − ≤ ≤ etc. The first voltage peaks of the response to idealised stroke currents are 1043V and 4800V respectively. ( ) ( ) and a bi t i t Figure below shows responses for a similar structure of 40m high, earth resistance of 2.5Ω, 4 down-conductor of 40Ω surge impedance, and 1500Ω surge impedance for the lightning discharge channel.

(b) Lumped circuit approximation This method is not able to determine the transient voltage oscillations associated with travelling waves. Nevertheless, it is still useful in that it gives ‘base lines’ about which transient oscillations occur.

R

1L Earth resistance

Down-conductori(t)

Air terminal R

1L Earth resistance

Down-conductori(t)

Air terminal


The voltage at node 1 with respect to remote earth is given by:

( ) ( ) ( )1

di tv t i t R L

dt= +

Example: Figure below shows responses for a structure of 40m high, earth resistance of 2.5Ω, 4 down-conductor of 1.5μH/m inductance.


4.4 Earthing and bonding Appendix E of AS1768:2003 describes acceptable methods of equipotential bonding. A bonding bar is used to facilitate the bonding of various services. It can be the main earthing bar (MEB) in the main switchboard (MSB) or a separate bar (see Fig E1) which is bonded to the MEB with a short bonding conductor (<1m). Note the MEB and LPS earth termination network are bonded directly to electricity supply service earth electrode. For new buildings, the preferred method (Fig E2) is to use a combined utilities enclosure. When bonding conductors can not be made short, there are two options:

provide a common bonding network (Fig E3), formed by interconnection of existing structural steel, e.g. steel mesh in reinforced concrete.

install a ring earth (Fig E4), formed by a bare conductor below ground. Require additional earthing electrodes.


Bonding of services [Fig E1, AS/NZS1768:2003]

Combined utilities enclosure [Fig E2, AS/NZS1768:2003]


Common bonding network (CBN) [Fig E3, AS/NZS1768:2003]

Ring earth [Fig E4, AS/NZS1768:2003]


4.5 Transient waveshapes Appendix F of AS/NZS1768:2003 deals with waveshapes for assessing susceptibility of equipment to transient overvoltages due to lightning. The waveshapes vary widely because of the random nature of lightning discharges and the variable characteristics of the transmission media (electrical supply lines, telecommunication lines). However, the majority of transients encountered in practice can be classified in terms of three standard waveshapes:

The 1.2/50μs unidirectional pulse. The 8/20μs unidirectional pulse. The 0.8μs/100kHz ring wave.

These are shown in Figs F1 and F2, with recommended applications for various parts of an installation given in Table F1 and Fig F4. The exposure peak amplitudes can be used to specify the test input levels to protection devices. The residual voltage level is that seen at the output (i.e. the equipment side of protection device) when the transient is applied at the crest of the AC voltage. The tolerable input voltage variations to electronic equipment are dependent on time and magnitude, as shown in Fig F3. The residual voltage level should be within the tolerable envelope.


Standard uni-directional waveshape

[Figure F1, AS/NZS 1768:2003]

0.5μs/100kHz ring wave (open-circuit voltage)



Recommended application for waveshapes of Figs F1 and F2

[Table F1, AS/NZS 1768:2003]

Typical voltage/time tolerance of computing equipment.



Location categories for application of waveshapes in Table F1



5. Overvoltage protection for LV systems

1) Limit voltage across sensitive insulation 2) Divert surge current away from load equipment. 3) Block surge current from entering the load. 4) Bond ground references together at the equipment. 5) Reduce or prevent surge current from flowing between

grounds. 6) Create a low-pass filter using limiting and blocking

principles.

Overvoltage protection devices are also known as surge protection devices (SPD), surge arresters, surge diverters or transient voltage surge suppressors (TVSS). However, arrester/diverter is most frequently used to describe the device in the service entrance (and thus may have more energy-handling capability) while TVSS is generally used to describe a similar device used at the load equipment.


There are a number of overvoltage protective devices available for LV/MV installations and equipment. They cover a very wide range of response times, current handling capacity and voltage clamping levels. 5.1 Crowbar devices Crowbar device is one which abruptly changes from a high impedance state to a low impedance conduction state when subjected to a voltage of sufficient level, called the breakdown voltage. During the conduction state, the crowbar maintains a very low voltage across it. This enables the crowbar to handle very large surge currents for a certain energy rating or size. (1) Air spark gaps or gas discharge tubes (GDT)

can safely conduct large currents (5kA for 50μs) low voltage in arcing mode very small parasitic capacitance (<2pF) require large voltage (>100V) to conduct can be slow to conduct possible “follow current” (sustained short circuit)

(2) SCRs and triacs

small voltage (0.7 to 2V) across conducting switch can tolerate sustained large currents slow to turn on or turn off (2μs) possible sustained conduction



5.2 Clamping devices Clamping device is one which limits the voltage transient to a certain level, by varying its internal resistance to the applied voltage. Since the transient energy is absorbed at the clamping voltage, these devices cannot withstand very high currents (unlike crowbar devices). These devices are governed by the power law, I=kVα, where k is a constant and α is the degree of non-linearity of conduction. The higher the α value, the better the clamping.

(1) Metal oxide varistors (MOV): fast response time (<0.5ns) large energy absorption can safely conduct large currents (1kA for 20μs) inexpensive


large parasitic capacitance (1 to 10nF)

(2) Avalanche diodes (Zener diodes)

determined clamping voltages

100A for 100μs)

) Switching and rectifier silicon diodes

)

c capacitance

urrently, MOV is the most commonly used. It has two

fast response (<0.1ns) selection of precisely (between about 6.8 and 200V)

small max. allowable current (< large parasitic capacitance (1 to 3nF)

(3 small clamping voltage (0.7 to 2V inexpensive small parasiti

Cimportant ratings. The first is maximum continuous operating voltage (MCOV), which must be higher than the line voltage and is often at least 125% of system nominal voltage. The second rating is the energy dissipation rating (in Joules).

.3 Isolators5


(1) Optoisolators tion voltages (5kV)

ata e) are expensive

) Isolation transformers

tages (5kV)

ode overvoltages

) Common-mode filters attenuating short-duration

ne as SPD

large isola good common-mode rejection easy to use to receive data difficult to use to transmit d fast devices (<1μs switching tim

(2 large isolation vol good common-mode rejection no attenuation of differential-m

(3 can be effective incommon-mode overvoltages

many problems when used alo

Surge diverter protection for electricity supply circuits


Low-pass filter to reduce rate of rise of voltage that reaches

the equipment.

Multi-stage protection for telephone and signalling circuits

Combination units.

Floating computer common (separate earth).


APPENDICES

A1. Other design methods for lightning protection The RSM is an implementation of simple electrogeometrical model (EGM) for striking distance. It does not account for the upward leader inception process, the effect of the structure height, or geometry of objects on the structure. Apart from RSM, other methods are:

Cone of Protection method: also called protection angle method. The volume protected by a vertical rod air terminal is illustrated below together with plots of relationship between the rod height and protection angle.


Volume protected by a catenary wire air termination.

Volume protected with vertical rod near building’s edge.

Faraday Cage method: also called mesh method, typically comprised of a series of horizontal air terminals such as copper tape which are bonded to vertically descending downconductors.


The method is detailed in IEC61024-1 Standard. Minimum values for the mesh size are given in the following Table:

Protection level Mesh size, m I II III IV

5 x 5 10 x 10 15 x 15 20 x 20

Collection Volume method: this is an improved Electrogeometric model developed by Eriksson which allows for computation of parabolic-like lightning collection volumes for all potential strike points on a building.



A2. Thunderstorm and lightning occurence The frequency of occurrence of lightnings and thurderstorms varies significantly depending on location. Furthermore, the severity of lightning storms (not frequency of occurrence) also varies with location. In addition, local topographical features may cause variations in the occurrence of ground flashes. Then on a smaller scale, tall objects (building rooftop, tree top, overhead lines) tend to attract lightning flashes to themselves and thus shielding a certain surrounding area from direct strikes.

Distribution of worldwide lightning strikes (flashes/km2/yr)

[Source: NSSTC]



A3. Example of a commercial LPS Erico has its Eritech System 3000 installed on the Centerpoint Tower in Sydney. A key feature is the design of its air terminal (Eritech Dynasphere) for controlled emission of a streamer. The streamer is produced only when the ambient field can sustain the upward leader initiation and propagation. The Eritech Interceptor is for smaller structures (<20m tall).

EritechDynasphere

EritechInterceptor

Air terminalsDownconductor(cutaway view)

Uppertermination

EritechDynasphere

EritechInterceptor

Air terminalsDownconductor(cutaway view)

Uppertermination


The Eritech Ericore downconductor is a low inductance, low impedance cable designed to minimise voltage build-up due to lightning impulses.

[Source: www.erico.com]


Documents

LIGHTNING PROTECTION IN BUILDINGS 1. … Lec08...... ground flashes are less common than cloud ... any pair of conducting objects in the building. If direct . ELEC9713: Lightning