Measuring earth resistance

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Earth resistance is a key parameter in determining the efficiency of earthingsystems. In this article we look at the measurement of earth resistance.

Text of Measuring earth resistance

Measuring earth resistanceE. Hering, Dresden (Germany) Earth resistance is a key parameter in determining the efficiency of earthing systems. In this article we look at the measurement of earth resistance.

1.5

Avoiding hazards and measuring errors

11.1

A few fundamentals Earth resistance and earth impedance

The efficiency of an earthing system is principally determined by its impedance ZE. As can be seen from figure , the earth impedance can be expressed as in equation (1):2 2 Z E = RE + X E

(1) (2)

tems in low-voltage installations ([1], sections 411.5 and 411.6; [2]); Joint earthing system for high-voltage protective earthing and functional earthing in transformer substations; Earthing system for the neutral earthing reactor of a medium-voltage distribution system. In the case of lightning protection systems, earthing measurements must be made even when there is no requirement to comply with a specific value. The results of repeat tests must be compared with those of earlier measurements.

RE = RD + RM + RC

1.3

As shown in equation(2), the earth resistance RE is the sum of the dissipation resistance RD, the resistance of the metal conductor that serves as the earth electrode RM and the resistance of the earthing conductor RC, which runs between the main earthing busbar and the earth electrode. The dissipation resistance RD is the resistance between the earth electrode and the surrounding soil. The reactance of the earthing system XE can be expressed as: RE = UM I (3)

Standards for measuring instruments

with XM reactance of the metallic earth electrode XC reactance of the earthing conductor. For AC supply current the reactance of the earthing conductor is only significant in the case of extended horizontal earthing strips or long earth rods. In all other cases, the difference between earth impedance and earth resistance is so small that frequently no distinction is made between these two quantities. The relevant industrial standards also treat earth impedance and earth resistance as identical. As earthing measurements are carried out using an AC supply, it is actually the earth impedance that is measured. If the measurement frequency is greater than 50Hz, a slightly larger earth impedance is displayed. However, overestimating the earth impedance is not a problem, as it errs on the side of safety.

The standards contain the requirements that have to be met by the manufacturers of measuring equipment. For users, these standards serve only informational purposes. In low-voltage systems earthing measurements must be made using equipment that complies with the VDE0413 standards (VDE: Verband der Elektrotechnik Elektronik Informationstechnik e.V./ engl.: Association for Electrical, Electronic & Information Technologies) (see [3], sec.61.1). All equipment must comply with the specifications in IEC 61557-1:2007 [4]. In addition, equipment must also comply with the following standards depending on the type of device or measuring method for which it is used: IEC 61557-5:2007 Equipment for measuring resistance to earth [5] IEC 61557-6:2007 Equipment for testing, measuring or monitoring protective measures involving residual current devices [6] IEC 61557-10:2007 Combined measuring equipment [7]. Equipment manufactured in accordance with earlier editions of the VDE0413 series of standards can of course also be used.

The process of measurement and any accompanying procedures (e.g. breaking standard connections and making non-standard connections) must not pose a safety hazard ([3], sec.61.1.3). The magnitude of the test voltage or the test current must be limited (see sections 3.1 and 4.1). Before breaking a connection that is required for electric shock prevention, the entire power installation must be disconnected from the supply and locked out to prevent it being switched on again. Any measurement that involves breaking connections (e.g. opening the inspection joint of a lightning protection system) must never be carried out during a storm or whenever a storm could be expected. Failure to comply could be hazardous, particularly for the person performing the operation. After the measurement has been completed, any connections that were broken must be properly restored. If the test current is split so that part of it runs parallel to the earth electrode being measured, the earth resistance displayed by the meter will be too small. The person conducting the measurement must therefore be aware of everything that is connected to the earth electrode under test [8]. Measurements must only be carried out by competent persons.

1.6

Taking the effects of weather into account

The specific resistance of soil decreases with increasing temperature and increasing soil moisture levels. Whereas these effects are of minor consequence for foundation earth electrodes in buildings with a basement or for long (vertical) rod electrodes, they have to be taken into account in the case of horizontal surface earth electrodes. Measurements made during cold, dry weather remain unaffected, but measurement data recorded in warm weather or after a rain shower have to be adjusted upward.

XM XC RM RC

ZE

RD

1.4

Selecting the right measuring equipment

RE

1.2

Requirements for earthing measurements

Earthing measurements are necessary whenever compliance with a specified earth resistance or a particular earth impedance is required, as is the case in the following earthing systems: Protective earth for TT and IT earthing sys-

It is not enough for users to simply follow the (frequently unclear) instructions provided by the manufacturer, they need to be aware of and understand the measuring method they want to apply. Measuring instruments that do not make it clear which measuring method is being applied should not be used. Before purchasing equipment, users should request technical descriptions of the devices of interest as well as their performance data and, if possible, instruction manuals, and should assess the equipment on the basis of this documentation.

Vector diagram of impedance in anearthing systemRd dissipation resistance; RE resistance of the earthing system (earth resistance); RM resistance of the metal conductor that acts as the earth electrode; RC resistance of the earthing conductor (e.g. connection lug, cable); XE reactance of the earthing system; XM reactance of the metal conductor that acts as the earth electrode; XC reactance of the earthing conductor; ZE earth impedance; impedance angle.

XE

1

1.7

Assessing measurement results

Earth resistance meters are not error free. Measurement errors can occur even if the conditions specified in the relevant standards and instrument instruction manuals are complied with and even in the absence of interference effects. The magnitude of an instruments operating error is listed on its technical specification sheet or in its instruction manual. In those methods of measuring earth resistance that draw current directly from the power source (see sections2.4 and 4), additional measurement uncertainty can be caused by random current and voltage fluctuations in the supply during the measurement. Examples of possible operator errors include: failure to take account of connections detrimental to the measurement process connecting the instrument leads incorrectly or selecting the wrong setting on the selector switch of the instrument inserting the auxiliary earth electrode or probe in the wrong location meter reading errors failure to implement measures to reduce systematic measurement errors. Results from first-time measurements should be compared with the project specifications, results of repeat tests should be compared with those of earlier measurements. If significant differences are apparent, the possible causes of the discrepancy should be determined. The influence of weather on the measurement results and how this can be taken into account is discussed in section1.6.

systems [9]. In general, the results from all the tests are contained in a single test report. The measurements performed and any accompanying action that is taken must be described precisely so that they can be reproduced at a later date. Information that must be provided includes: the measurement method used the type of measuring instrument used the positions of any selector switches, if relevant details of any connections that were broken or made for the purposes of the measurement. The results of the measurement must be stated clearly and unambiguously. This also applies to any weather-related adjustments of the results that may have been made. The test report is required by [3], section61.1.6 concerning earthing systems in low-voltage networks [10], annexE, sectionE.7.2.5 concerning lightning protection systems both standards apply if the earthing system serves both purposes.

22.1

Overview of measurement methods for RE Principles

1.8

Test report

Measuring earth resistance is only one of several tests that have to be performed on earthing

There is a wide degree of variation in the internal circuitry of the measuring instruments used and the layout and arrangement of the external measuring circuits. However, a common feature of all the methods is that they determine the earth impedance by measuring the voltage across the earthing system for a known test current. Leads that carry the test current outside of

the instrument are shown in red in the diagrams. Known measurement methods are listed in table . The underlying circuit principles are shown in figures to . The unusually long names given here to the various methods ensure that the methods can be distinguished unambiguously. Although there are