Measuring earth resistance - Electrical Engineering Measuring earth resistance E. Hering, Dresden (Germany)

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  • A few fundamentals

    1.1 Earth resistance and earth impedance

    The efficiency of an earthing system is princi- pally determined by its impedance ZE. As can be seen from figure ➊, the earth impedance can be expressed as in equation (1):

    (1)

    (2)

    As shown in equation (2), the earth resistance RE is the sum of the dissipation resistance RD, the resistance of the metal conductor that ser- ves as the earth electrode RM and the resistance of the earthing conductor RC, which runs be- tween 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:

    (3)

    with XM reactance of the metallic earth electrode XC reactance of the earthing conductor.

    ForACsupply current the reactanceof the earth- ing 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 be- tween these two quantities. The relevant indus- trial 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 impedan- ce that is measured. If the measurement fre- quency is greater than 50 Hz, a slightly larger earth impedance is displayed. However, over- estimating the earth impedance is not a pro- blem, as it errs on the side of safety.

    1.2 Requirements for earthing measurements

    Earthing measurements are necessary when- ever compliance with a specified earth resis- tance or a particular earth impedance is requi- red, as is the case in the following earthing sys- tems: • Protective earth for TT and IT earthing sys-

    tems in low-voltage installations ([1], sec- tions 411.5 and 411.6; [2]);

    • Joint earthing system for high-voltage pro- tective earthing and functional earthing in transformer substations;

    • Earthing system for the neutral earthing reac- tor 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 measure- ments.

    1.3 Standards for measuring instruments

    The standards contain the requirements that have to be met by the manufacturers of mea- suring equipment. For users, these standards serve only informational purposes. In low-voltage systems earthing measurements must be made using equipment that complies with the VDE 0413 standards (VDE: Verband der Elektrotechnik Elektronik Informationstech- nik e.V./ engl.: Association for Electrical, Elect- ronic & 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 measu-

    ring 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 VDE 0413 series of stan- dards can of course also be used.

    1.4 Selecting the right measuring equipment

    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 re- quest technical descriptions of the devices of in- terest as well as their performance data and, if possible, instruction manuals, and should as- sess the equipment on the basis of this docu- mentation.

    1.5 Avoiding hazards and measuring errors

    The process of measurement and any accom- panying procedures (e.g. breaking standard connections and making non-standard con- nections) must not pose a safety hazard ([3], sec. 61.1.3). The magnitude of the test volta- ge or the test current must be limited (see secti- ons 3.1 and 4.1). Before breaking a connecti- on that is required for electric shock prevention, the entire power installation must be discon- nected from the supply and locked out to pre- vent it being switched on again. Any measurement that involves breaking con- nections (e.g. opening the inspection joint of a lightning protection system) must never be car- ried out during a storm or whenever a storm could be expected. Failure to comply could be hazardous, particularly for the person perfor- ming 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 mea- surement 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 in- creasing temperature and increasing soil mois- ture 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 ac- count in the case of horizontal surface earth electrodes. Measurements made during cold, dry weather remain unaffected, but measurement data re- corded in warm weather or after a rain shower have to be adjusted upward.

    1

    R U

    IE M=

    R R R R E D M C

    = + +

    Z R X E E E

    = +2 2

    1

    Measuring earth resistance E. 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.

    ϕ

    RD

    RE

    ZE

    RM RC

    X M

    X C

    X E

    ➊ Vector diagram of impedance in an earthing system Rd 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; ϕ impedance angle.

  • 1.7 Assessing measurement results

    Earth resistance meters are not error free. Mea- surement errors can occur even if the conditi- ons specified in the relevant standards and in- strument instruction manuals are complied with and even in the absence of interference ef- fects. The magnitude of an instrument’s opera- ting error is listed on its technical specification sheet or in its instruction manual. In those me- thods of measuring earth resistance that draw current directly from the power source (see sections 2.4 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 detri-

    mental 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 sys-

    tematic measurement errors. Results from first-time measurements should be compared with the project specifications, re- sults of repeat tests should be compared with those of earlier measurements. If significant dif- ferences are apparent, the possible causes of the discrepancy should be determined. The in- fluence of weather on the measurement results and how this can be taken into account is dis- cussed in section 1.6.

    1.8 Test report Measuring earth resistance is only one of seve- ral tests that have to be performed on earthing

    systems [9]. In general, the results from all the tests are contained in a single test report. The measurements performed and any accompany- ing action that is taken must be described pre- cisely so that they can be reproduced at a later date. Information that must be provided inclu- des: • 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 measure- ment.

    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], section 61.1.6 concerning earthing sys-

    tems in low-voltage networks • [10], annex E, section E.7.2.5 concerning

    lightning protection systems • both standards apply if the earthing system

    serves both purposes.

    Overview of measurement methods for RE

    2.1 Principles There is a wide degree of variation in the inter- nal circuitry of the measuring instruments used and the layout and arrangement of the external measuring circuits. However, a common featu- re of all the methods is that they determine