Measuring earth resistance · tween the main earthing busbar and the earth electrode. The dissipation resistance R ... ring resistance to earth [5] • IEC 61557-6:2007 Equipment

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 canbe seen from figure ➊, the earth impedancecan be expressed as in equation (1):

(1)

(2)

As shown in equation (2), the earth resistanceRE is the sum of the dissipation resistance RD,the resistance of the metal conductor that ser-ves as the earth electrode RM and the resistanceof the earthing conductor RC, which runs be-tween the main earthing busbar and the earthelectrode. The dissipation resistance RD is theresistance between the earth electrode and thesurrounding soil. The reactance of the earthingsystem XE can be expressed as:

(3)

withXM reactance of the metallic earth electrodeXC reactance of the earthing conductor.

ForACsupply current the reactanceof the earth-ing conductor is only significant in the case ofextended horizontal earthing strips or long earthrods. In all other cases, the difference betweenearth impedance and earth resistance is sosmall that frequently no distinction is made be-tween these two quantities. The relevant indus-trial standards also treat earth impedance andearth resistance as identical.As earthing measurements are carried out usingan AC supply, it is actually the earth impedan-ce that is measured. If the measurement fre-quency is greater than 50 Hz, a slightly largerearth 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 intransformer 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 evenwhen there is no requirement to comply with aspecific value. The results of repeat tests mustbe compared with those of earlier measure-ments.

1.3 Standards for measuring instruments

The standards contain the requirements thathave to be met by the manufacturers of mea-suring equipment. For users, these standardsserve only informational purposes.In low-voltage systems earthing measurementsmust be made using equipment that complieswith the VDE 0413 standards (VDE: Verbandder Elektrotechnik Elektronik Informationstech-nik e.V./ engl.: Association for Electrical, Elect-ronic & Information Technologies) (see [3],sec. 61.1). All equipment must comply withthe specifications in IEC 61557-1:2007 [4]. Inaddition, equipment must also comply with thefollowing standards depending on the type ofdevice 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 measuresinvolving residual current devices [6]

• IEC 61557-10:2007 Combined measuringequipment [7].

Equipment manufactured in accordance withearlier 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 bythe manufacturer, they need to be aware of andunderstand the measuring method they want to apply. Measuring instruments that do notmake it clear which measuring method is beingapplied 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, ifpossible, 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 standardconnections 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 alightning protection system) must never be car-ried out during a storm or whenever a stormcould be expected. Failure to comply could behazardous, particularly for the person perfor-ming the operation. After the measurement hasbeen completed, any connections that werebroken must be properly restored.If the test current is split so that part of it runsparallel to the earth electrode being measured,the earth resistance displayed by the meter willbe too small. The person conducting the mea-surement must therefore be aware of everythingthat is connected to the earth electrode undertest [8]. Measurements must only be carriedout 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 minorconsequence for foundation earth electrodes inbuildings with a basement or for long (vertical)rod electrodes, they have to be taken into ac-count in the case of horizontal surface earthelectrodes. Measurements made during cold, dry weatherremain unaffected, but measurement data re-corded in warm weather or after a rain showerhave to be adjusted upward.

1

RU

IEM=

R R R RE D M C

= + +

Z R XE E E

= +2 2

1

Measuring earth resistanceE. Hering, Dresden (Germany)

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

ϕ

RD

RE

ZE

RM RC

XM

XC

XE

➊ Vector diagram of impedance in anearthing systemRd dissipation resistance; RE resistance ofthe earthing system (earth resistance); RM resistance of the metal conductor thatacts as the earth electrode; RC resistance ofthe earthing conductor (e.g. connection lug,cable); XE reactance of the earthing system;XM reactance of the metal conductor thatacts as the earth electrode; XC reactance ofthe 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 compliedwith and even in the absence of interference ef-fects. The magnitude of an instrument’s opera-ting error is listed on its technical specificationsheet or in its instruction manual. In those me-thods of measuring earth resistance that drawcurrent directly from the power source (seesections 2.4 and 4), additional measurementuncertainty can be caused by random currentand voltage fluctuations in the supply duringthe 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 selectorswitch 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 shouldbe compared with the project specifications, re-sults of repeat tests should be compared withthose of earlier measurements. If significant dif-ferences are apparent, the possible causes ofthe discrepancy should be determined. The in-fluence of weather on the measurement resultsand how this can be taken into account is dis-cussed in section 1.6.

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

systems [9]. In general, the results from all thetests are contained in a single test report. Themeasurements performed and any accompany-ing action that is taken must be described pre-cisely so that they can be reproduced at a laterdate. 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 statedclearly and unambiguously. This also applies toany weather-related adjustments of the resultsthat 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 PrinciplesThere is a wide degree of variation in the inter-nal circuitry of the measuring instruments usedand the layout and arrangement of the externalmeasuring circuits. However, a common featu-re of all the methods is that they determine theearth impedance by measuring the voltageacross the earthing system for a known test cur-rent. Leads that carry the test current outside of

the instrument are shown in red in the dia-grams.Known measurement methods are listed in table ➊. The underlying circuit principles areshown in figures À to Õ. The unusually longnames given here to the various methods ensu-re that the methods can be distinguished unam-biguously.Although there are clear differences betweenthe individual measurement methods, no oneparticular method can be said to be ideal. Eachmethod has its own particular disadvantagessuch as limited applicability, electric shock ha-zard, larger measurement errors or requiringgreater time and effort to complete. The vario-us advantages and disadvantages of the indivi-dual measurement techniques are described inmore detail in sections 3 and 4. All of the me-thods discussed must only be carried out bycompetent persons exercising due care and at-tention.In those methods that do not draw current directly from the supply (columns 1 to 6 in table ➊), the measurement frequency usedwill be at least 5 Hz above or below the fre-quencies 16.7 Hz, 50 Hz and integer multi-ples thereof. This prevents interference fromsupply frequency currents (‘interference cur-rents’) that can falsify measurement results.In those methods that do draw current directlyfrom the supply (columns 7 to 9 in table ➊),it is of course essential that the supply frequen-cy and measuring frequency are identical. Thismeans that the interference effects mentionedabove cannot be ruled out when such methodsare used. However, these methods are simplerto perform and offer advantages in terms of theirapplicability.

2

2

Table ➊ Overview of earth resistance measuring methods

1 2 3 4 5 6 7 8 9Designation based on internal circuitry

Balanced-bridge methods Current-voltage methodsDistinction based on whether method draws current directly from supplyv)

yes noDistinction based on use of probe and/or auxiliary electrodew)

probe and probe, no no probe, no probe and probe, no no probe, no probe PEN or neutral no probex)

auxiliary auxiliary auxiliary auxiliary auxiliary auxiliary conductor in-electrode electrode electrodex) electrode electrode electrodex), y) stead of probe

(‘stakeless method’)

Figure2 3b)z) 3c) 3a) 3b) 3c) 4a) 4b) 4c)

Detailed description in section2.2 3.2 3.3 3.4 4.3 4.4 4.5

Detailed schematic of measurement method5 6 7 11 12 13

v) For current-voltage techniques, this distiction is included in the method name.w) All methods include this distinction as part of the method name.x) Measures resistance of conductor loop via earth return path.y) An earth resistance meter does not need to be inserted into the earthing conductor if a clamp-on resistance meter is placed around the earthing

conductor.z) In the case of balance-bridge methods, figure only applies to the exterior circuit.

2.2 The balanced-bridge methodThe balanced-bridge method as described byBehrend is one of the techniques for measuringearth resistance that does not involve drawingcurrent directly from supply. Earth resistancemeters based on this method are no longer ma-nufactured, as other more user-friendly instru-ments have now been developed for the samesorts of applications. These new meters use theso-called current-voltage method, which alsodoes not involve current being drawn from thesupply. Nevertheless the balanced-bridge me-thod is described here because it is of funda-mental importance to the development of earthresistance measurement techniques and be-cause meters based on this method are still inuse.The measurement circuit for the balanced-bridge method is shown in figure À. The me-thod involves driving an auxiliary earth elec-trode1) and a probe2) temporarily into the soil.When the earth meter is in its balanced state,there is no current flowing in the probe. The re-sistance to earth of the probe has therefore noinfluence on the measurement result; it simplylowers measurement sensitivity. Information onthe alignment and separation of the auxiliary

3

Earth resistance meter

tr = I2 : I1 = 0.1; 1; 10; 100

CT

R2I2

I1

I1

RE

U1

U2

I1I3 = 0 I3 = 0

N

REC∼

REC PSAC

EES S H

Earthelectrode

Auxiliaryearth electrode

Probe

À The balanced-bridge methodREC rectifier; I1 test (or measuring) current;I2 Reference current; I3 current whose mag-nitude is zero when bridge is balanced; C capacitor; N null detector; RE earth resis-tance being measured; R2 reference resis-tance; CT current transformer; U1 Voltageacross earth electrode under test; U2 refe-rence voltage; tr transformation ratio of theCT; PSAC AC power supply.


REC∼

REC∼

REC∼

Earthelectrode

Earthelectrode

EarthelectrodeAuxiliary

earth electrode

Functionalearth electrode


ProbeProbe

L1L2L3PEN

L1L2L3PENUM–––

IRE =

UM–––I

Rloop =

UM–––I

RE = RE < RloopRE RE RERF RF

PSAC PSAC

UMI UM

I

A V A V

E ES S H E ES S H

PSAC

UMI

A V

E ES S H

a) b) c)

Ã Current-voltage methods that do not draw current directly from the power supplya) with probe and auxiliary electrode; b) with probe, but without an auxiliary electrode; c) no probe, no auxiliary electrode (measures resistance of conductor loop via earth return path).I test current; Rloop loop resistance; UM test voltage.

Õ Current-voltage methods that draw current directly from the power supplya) with probe; b) using PEN conductor or neutral conductor instead of probe;c) no probe (measures resistance of conductor loop via earth return path);U0 conductor-to-earth voltage




Earthelectrode

Earthelectrode

Earthelectrode

Probe

UM–––I

RE =UM–––I

RE =UM–––I

RE =

U0 – UM––––––––I

Rloop <

RE < Rloop

UE < U0 – UM

RERF RERF RERF

a) b) c)

L1L2L3PENor N

L

UM

UM

UE

U0

IAV

ES

L

UM IAV

ES

L

UM IAV

ES


earth electrode and the probe is provided insection 3.The AC power source PSAC is located betweenthe connection point for the earth electrode un-der test (socket E) and that for the auxiliaryearth electrode (socket H). The AC source isconnected in series with the primary winding ofa current transformer CT. Connected to the se-condary winding of the current transformer is avariable voltage divider. The setting chosen forthe left part of the divider R2 (‘reference resis-tance’) is displayed on the scale on the voltagedivider’s control unit. A null detector N with arectifier REC in series is located between the va-riable tap point of the voltage divider and theconnection point for the probe (socket S). Therectifier is driven by the AC power source. A ca-pacitor C prevents any DC current from flowingacross the probe. One end of the voltage divideris connected to the earth electrode being mea-sured via the instrument sockets ES and E. Thetransformation ratio tr of the current transfor-mer can be switched to achieve the requiredmeasurement range.When balanced, the current I3 in the probe iszero. The same current I1 therefore flows in theauxiliary earth electrode and in the earth elec-trode under test. Additionally, the voltages U2(‘reference voltage’) and U1 are of the same size. The voltage U1 corresponds to the earthelectrode voltage that drives the test current I1in the earth resistance RE of the test object E,whereas U2 is the voltage drop that maintainsthe current I2 (‘reference current’) in the refe-rence resistor R2. The potential drops obeyOhm’s law as expressed by the equationsU1 = I1 · RE and U2 = I2 · R2. If the transforma-tion ratio of the current transformer tr = 1:1,then I2 = I1 and the value of the earth resis-tance RE is equal to the selected reference re-sistance R2. The earth resistance can thereforebe read off the voltage divider scale mentionedabove. If another transformation ratio is used,this must be multiplied by the value of the re-ference resistance R2, i. e. RE = tr · R2.

2.3 Other measurement methodswithout supply current

Another group of methods for measuring earthresistance that do not draw current directlyfrom the supply are the so-called current-volta-ge techniques illustrated in figure Ã. The earthresistance RE is determined from the voltageUM that appears across the earth electrode andacross the sockets ES and S, and the measu-red current I.

(4)

Figure Ã simply illustrates the principle of themeasurement and shows only a small part ofthe complex circuitry within the earth resis-tance meter. Usually, the voltage UM and cur-rent I are not shown separately and the meteronly displays a digital reading of the earth re-sistance RE. If the AC supply source PSAC is a

constant-current generator, the earth resistancecan be displayed directly on the voltage meter.When the balanced-bridge method was first de-veloped, the only exterior circuit known wasthat shown in figures À and Ãa). It was the-refore usual to consider the circuitry inside themeter and the exterior circuit as a single entity.However, as indicated in columns 2 and 3 intable À, the same meter can be used for mea-surements with the exterior circuits shown in fi-gures Ãb) and Ãc). Equally, the earth resis-tance meters used for the current-voltage me-thods that do not draw current directly from thesupply can be used like the meters designedwith the balanced-bridge circuit. The internalcircuits can therefore be freely combined withthe exterior circuits.

2.4 Measurement methods withcurrent from the supply

These methods can only be used in networkswith a direct connection to earth. As shown infigure Õ, the measurement involves drawingthe test current from the phase conductor of thesupply system. The meters used in this type ofmeasurement are primarily designed for testingelectrical safety systems involving residual cur-rent devices. The meters are generally con-nected to the supply via a flexible power leadand an earthed safety plug.

Current-voltage method thatdraws no supply current

3.1 Earth resistance metersThe four connection sockets are labelled asshown in figure Œ. Sockets for the supply current path and for current measurement:• E – Earth electrode (test object)• H – Auxiliary earth electrode1)

Sockets for the voltage measurement path:• ES – Earth electrode (or the probe located

close to the earth electrode when measuringthe soil resistivity)

• S – Probe2)

Normally when measuring the resistance toearth, the sockets E and ES are connected toone another via a removable link or via a con-tact strip within the meter’s selector switch asthis ensures that the earth electrode under testis connected to both the current and voltagemeasurement paths. If, in addition, a jumper isplaced between sockets H and S, the earth re-sistance meter can be used as a simple ohm-meter.The frequency of the AC supply PSAC is at least 5 Hz above or below the frequencies16.7 Hz and 50 Hz and any integer multiplesthereof. Typically, the supply frequency is in therange 41–140 Hz, though in some meters ahigher frequency is used. Some earth resistancemeters also offer the option of selecting the fre-quency. A number of meters with automatic fre-quency control (AFC) automatically switch tothat frequency offering the lowest level of inter-ference.

To protect against electric shocks, the open-cir-cuit test voltage generated by the meter mustnot exceed 50 V (r.m.s.) and 70 V (peak). Inthe case of earth resistance meters used onagricultural sites, these values must be halved.Alternatively, the short-circuit current must notexceed 3.5 mA r.m.s. and a peak value of5 mA (see [5], sec. 4.5). If neither of theseconditions are met, the meter must switch offautomatically.The meter is powered either by a battery, agroup of primary cells or a hand-driven genera-tor, though the latter method is now rare. Themeter must indicate whether the end-point vol-tage of the power supply is sufficient to main-tain proper instrument function (see [4],sec. 4.3).When earth resistance is measured by a me-thod that does not involve current being drawndirectly from the supply, the earth resistance REis computed as the quotient of the measuredvoltage UM that appears across the earth elect-rode (and across the meter sockets ES and S)and the measured current I (that flows throughsockets E and H). Figure Œ only indicates thebasic principle of the complex circuitry within

RU

IEM=

3

4


REC∼

Earthelectrode

Auxiliaryearth electrodeProbe

UM–––I

RE =

RE

RE'

RE

'1

RE

'2

RE

'

PSAC

UMIA V

E ES S H

0.2 s 0.2 ss

approximatelyflat section

Distance

Œ Current-voltage methods that do notdraw current directly from the power sup-ply and that use a probe and an auxiliaryearth electrodeI test current; RE earth resistance being measured; RE´ measured earth resistance;UM test voltage

the meter. Usually, the voltage UM and currentI are not shown separately and the meter onlydisplays a digital reading of the earth resistanceRE. If the AC supply source is a constant-cur-rent generator, there is no need to measure thecurrent and calculate the quotient. In this casea voltage meter can be calibrated to display theearth resistance directly.Most meters are equipped with a switch for se-lecting the type of measuring circuit, the mea-surement frequency and/or the measurementrange, and for switching the power on and off.Most meters also have a button that is used toinitiate measurement. The earth resistance me-ter must also indicate that the resistance of the auxiliary earth electrode and the probe arewithin the specified limits (see [5], sec. 4.4).However, it is not advisable to rely too heavilyon a warning signal, because by the time a warning signal has been issued, the limit mayhave been exceeded by a significant amount.User-friendly devices offer additional functionssuch as:• warning signal or automatic cut-out if too

great an interference voltage is detected• warning signal or disabling of measurement

function if test current is too small• display of test current (for monitoring purpo-

ses only when measurements made with aconstant-current generator)

• automatic measurement range selection• display hold function• data storage for transmitting or printing mea-

surement results.

3.2 Methods using a probe and anauxiliary earth electrode

3.2.1 PrincipleAs shown in figure Œ, the earth electrode un-der test, an auxiliary earth electrode and theprobe are connected to the earth resistance me-ter. The test current I flows through the earthelectrode, the soil and the auxiliary earth elect-rode. The voltage UM that appears across theearth resistance RE also appears across the me-ter sockets ES and S. The earth resistance isdisplayed as the value of UM divided by I.

3.2.2 Earth electrode (test object)If socket E is connected to the beginning of theearthing conductor (at the main earthing termi-nal), the earthing conductor will be included inthe measurement of the earth resistance. If, onthe other hand, socket E is connected directlyto the earth electrode, the resistance of the eart-hing conductor will not be included in the mea-surement. The difference, however, is usuallyslight.The resistance of the measuring leads will be in-cluded in the measurement. This will result inan overestimation of the earth resistance andthus yield a value that errs on the side of safety.To reduce the magnitude of the error, it is ex-pedient to position the earth resistance meterclose to the point of connection and to use ashort measuring lead. The resistance of the

measuring lead can of course be measured andthis value subtracted from the value displayedby the earth resistance meter. If the effect of themeasuring lead’s resistance is to be avoided atall costs, the jumper linking sockets E and theES must be removed and each socket con-nected to the earthing system by its own mea-suring lead.The earth electrode under test must not be con-nected to any other earth electrodes as thiswould falsify the result of the measurement. Inthe TN earthing systems found in consumer in-stallations, the earthing conductor must be dis-connected from the main earthing busbar as thelatter is connected to the PEN conductor of thesupply network. This is not required in TT sys-tems as the main earthing busbar is not con-nected to the neutral conductor of the powersupply network. If, nevertheless, the earthingconductor is disconnected, the entire systemmust be de-energized beforehand and lockedout to prevent it being switched on again.

3.2.3 Auxiliary electrode1)

The auxiliary earth electrode should be positio-ned as far away as possible from the earth elect-rode under test, so as to minimize the degree ofoverlap between the potential gradient areas(‘spheres of influence’) surrounding the twoelectrodes. The larger the electrodes, the fartherapart they must be. As a rough guide, the mi-nimum distance apart can be taken to be threetimes the depth of a rod earth electrode or theaverage diameter of a ring earth electrode. Thefigure of 40 m that is found in the documenta-tion provided by some manufacturers can onlybe considered to be a rough average value.Whether the chosen distance is appropriate willbe shown when the correct alignment and po-sitioning of the electrodes is carried out (seesec. 3.2.4).The greater the resistivity of the soil, the longerthe auxiliary electrode needs to be and the dee-per it needs to be driven into the ground. If theresistance of the auxiliary earth electrode is toolarge, measurement errors can arise, because,for example, the constant current normally ge-nerated by the AC supply cannot then flow. Insuch cases, it can prove useful to saturate thearea of ground being used for the measurementwith water.

3.2.4 Probe2)

As the internal resistance of the voltage measu-rement path is very large, the resistance of theprobe and therefore the size of the probe is ofminor importance. The preferred location of theprobe is on the straight line between the earthelectrode and the auxiliary earth electrode at aposition where it has minimum interaction withthe spheres of influence of the two electrodes(see diagram in figure Œ).If one were to carry out a series of measure-ments with different distances between theearth electrode and the probe the results wouldform a curve whose ends are relatively steepwhile the intermediate section of the curve is

flatter. If the distance between the earth elect-rode and the auxiliary electrode is large enough,the curve will have an approximately horizontalcentral section in which the measured resis-tance to earth is essentially independent ofelectrode separation. This central section must be determined by at least three measurements. The midpoint of thecentral section is not midway between the earthelectrode and the auxiliary earth electrode, butlies closer to the auxiliary earth electrode as thespatial extent of the spheres of influence asso-ciated with the two earth electrodes differ. Ingeneral, the optimum separation between theearth electrode and the probe is about twothirds of the distance between the earth elec-trode and the auxiliary earth electrode3).

3.2.5 Limitations of methodIf no portion of the resistance vs. distance cur-ve is approximately horizontal, then the dis-tance between the earth electrode under testand the auxiliary earth electrode is too small.If the curve exhibits an unusual profile, buriedmetal installations (e. g. water pipes) are veryprobably influencing the measurement. Insuch conditions it is not possible to achieveusable results from the measurement. Measu-rement may be possible if the electrodes canbe laid out perpendicular to their original di-rection or perpendicular to the longitudinalaxis of the buried metal installation or so thatthey run away from and not above the buriedmetal installation.It is also not possible to achieve reliable resultsif the earth electrode under test is surroundedby other earth electrodes, for example in areaswith a high density of buildings. Furthermoremeasurement is impossible whenever the au-xiliary earth electrode and the probe cannot be positioned in the right locations. In all suchcases, another measurement technique mustbe selected.

3.3 Method using a probe but noauxiliary earth electrode

3.3.1 PrincipleAs shown in figure œ, the functional earth ofthe supply network acts as a replacement forthe auxiliary earth electrode. It is extremely

5

1) In some publications the auxiliary electrode is also referred to as the outer test electrode, or cur-rent test stake.

2) In some publications the probe is also referred toas the inner test electrode, or voltage test stake.

3) Some manufacturers state that the distance be-tween the earth electrode and the probe should be half the distance between the earth electrodeand the auxiliary earth electrode. That is incorrect.Other companies recommend placing the probe ata distance from the earth electrode that is always62 % of the separation between the earth and the auxiliary earth electrode. This method is thus sometimes referred to as the 62 % method. The62 % mark generally gives a good approximationof the correct location. But the optimum positionmust always be determined by moving the probeto neighbouring positions.

important to ensure that the connection is notaccidentally made to one of the phase con-ductors.In a TN system, the H socket of the meter hasto be connected (for instance, via the earthingcontact of a plug) to the protective earth (PE)conductor, which itself has been branched offthe PEN conductor. The meter socket E is con-nected to the earthing conductor, which has tobe disconnected from the main earthing busbar. Supply networks configured with the TT earth-ing system have a neutral conductor instead ofthe PEN conductor. This has to be treated as alive conductor even though it is connected to afunctional earth. Applying this method of mea-suring earth resistance to a TT system wouldtherefore involve connecting the earth resis-tance meter to the neutral conductor. The me-thod is therefore not approved for use with TTsystems.

3.3.2 Problems in the TN systemThe method does not function in a TN systemif the electrode being measured is strongly cou-pled or if it is connected via a metal conductorto another earth electrode that itself is con-nected to the PEN conductor. This would resultin the test current flowing in the wrong path sothat the display on the earth resistance meterwould be smaller than the true value of the re-sistance to earth. This is discussed in more de-tail in section 3.4.2.

3.4 Method without a probe andan auxiliary earth electrode(‘stakeless method’)

3.4.1 PrincipleThis method (illustrated in figure –) is anearth-loop resistance measurement because itinvolves measuring the resistance of a con-ductor loop via an earth return path. The S andH sockets of the earth resistance meter are con-nected together. The advantage of this methodis that neither an auxiliary earth electrode nor aprobe need to be used.In a TN system the earthing conductor (EC) isdisconnected from the main earthing busbar(MEB) and the earth resistance meter is inser-ted between them. This method is not suitablefor measurements on a consumer installationswith a TT earthing system.The resistance measurement displayed on themeter includes the resistance to earth of thefunctional earth and the resistance of the PEN conductor. If they were accurately known,these values could be subtracted from the re-sistance displayed on the meter. However, theyare difficult to determine, because the functio-nal earth in a TN system comprises not only thefunctional earth electrode shown in figure –,it is also connected to numerous earths in theconsumer installations of neighbouring buil-dings. The error that is introduced by measu-ring these additional resistances results in anoverestimation of the earth resistance, yieldinga value that errs on the side of safety.

3.4.2 Problems in TN systemsThe problem mentioned earlier in secton 3.3.2can also arise when measuring earth resistancewithout an auxiliary earth electrode and withouta probe. Some examples of configurations whe-re problems can arise are shown in figure —.Temporary remedial measures include:• Disconnecting the metal connection between

the earth electrodes as shown in figure—b).• Disconnecting the second earth electrode

from the PEN conductor, if permitted by theowner. The residual influence of the secondearth electrode on the earth resistance mea-surement is not a disadvantage, as it acts toimprove the performance of the first earthelectrode.

More details can be found in reference [8].

3.5 Stakeless methods (no probe,no auxiliary earth electrode)using a clamp-on ohmmeter

This is a variation on the measurement methoddescribed in section 3.4. This technique differsfrom that shown in figure – in that instead ofinserting an earth resistance meter into the eart-hing conductor, a clamp-on ohmmeter (COM)is placed around the earthing conductor (see fi-gure “). The clamp-on ohmmeter containsboth a current-to-voltage transformer (a voltage

inducing clamp, VIC) and a current transformer(a current measuring clamp CMC). Modelsavailable include: • Chauvin Arnoux Earth Clamps C.A 6410,

C.A 6412 or C.A 64154); • Fluke Earth Ground Clamp Meter 16305).The meter displays the resistance calculated asthe quotient of the voltage induced by the VICin the earthing conductor and the resulting testcurrent registered by the CMC. In this case theresistance is the loop resistance Rloop, or moreprecisely the loop impedance (see section 3.4.1).Another solution (no separate diagram provi-ded) involves clamping two split-core currenttransformers around the earthing conductor,one of which functions like the voltage-inducingclamp VIC while the other corresponds to thecurrent measuring clamp CMC that measures

6


REC∼

Earthelectrode


UM–––I

Rloop =

Rloop < RE

RF RE

PSAC

UMI

A V

E ES S H

L1L2L3PEN

PEN

NPEMEB

EC

– Current-voltage methods that do notdraw current directly from the powersupply and that use neither a probe noran auxiliary earth electrode (resistanceof conductor loop via earth return path)Rloop loop resistance.

4) Induced voltage: approx. 60 mV; frequency:2403 Hz; inner diameter of clamp jaw: 32 mm.Data provided without warranty.

5) Induced voltage: approx. 30 mV; frequency:1667 Hz; inner diameter of clamp jaw: 23 mm.Data provided without warranty.

6) On its own, the expression ‘selective earth mea-surement’ is ambiguous, as other earth resistancemeasurement techniques are also selective, e.g.those presented in sections 3.4, 3.5 and 4.6.


REC∼

Earthelectrode


Probe

UM–––I

RE =RF RE

PSAC

UMI

A V

E ES S H

L1L2L3PEN

PEN

NPEMEB

EC

œ Current-voltage methods that do notdraw current directly from the power sup-ply and that use a probe but no auxiliaryearth electrodeEC earthing conductor; MEB main earthingbusbar; RB resistance to earth of the functio-nal earth electrode

the test current. The clamps are connected to aspecial earth resistance meter (Fluke EarthGround Tester 1623 or 1625). Depending onwhich of the Fluke meters is used, either EI-1623 or EI-1625 ‘selective/stakeless clampset’ is required. The advantage in both cases isthat the earthing conductor does not need to bedisconnected, making measurement safer andquicker. The problem discussed in section3.4.2 can also arise in these cases. If this method is used to make measurementson consumer installations, they must be desig-ned with a TN earthing system. The method issuitable for measuring the resistance to earth ofa pylon in an overhead power transmission lineif the clamps can be fitted around the earthingconductor.

3.6 Selective earth resistancemeasurements using a probe,an auxiliary earth electrodeand a clamp-on ohmmeter

The earth resistance measurement described inthis section6) is used if the earth electrode un-der test cannot or should not be disconnectedfrom other earth electrodes to which it is wiredin parallel. This method is based on the techni-que using a probe and auxiliary earth electrodethat is discussed in section 3.2, but in this va-riant (see figure ”a)) a special earth resistancemeter (Fluke 1623 or 1625) and an additionalclamp-on current transformer (CMC) are requi-red. The current measuring clamp CMC is clam-ped around the earthing conductor EC con-

7

L1L2L3PEN

E ES S H

IV

Icpl

Rcpl

E2

RE1

IE1 IE2

RE2

I

L1L2L3PEN

E ES S H

IV

Icpl

E2

RE1

IE2 = 0IE12 = 0

RE2

I

I = IE1 + IcplIE1 < IcplRM < RE1

I = IE1 + IcplIE1 < 0Icpl < IRM ⪡ RE1

— Cases involving a TN system in which the method shown in fig. – is not suitablea) Small distance and therefore small coupling resistance Rcpl between the earth electrode undertest E1 and a second earth electrode E2 that is connected to the PEN conductor. b) Metallicconnection to a second earth electrode that is itself connected to the PEN conductor.Icpl current causing measurement error; RE1 earth resistance being measured; RM earth resis-tance displayed on meter.

L1L2L3PEN

PEN

NPEMEB

EC

I

UM

RF RE

EC

CMC

VIC

COMfor Rloop

Earthelectrode


RE < Rloop

“ Method as in figure – but with aclamp-on ohmmeter rather than anearth resistance meterEC earthing conductor; VIC voltage-induc-ing clamp; CMC current measuring clamp;COM clamp-on ohmmeter.

Earth resistance meter Earth resistance meter

Otherearthelectrodesa) b)

Other earthelectrodes

Testobject



Probe ProbeTest object(pyton stubs)

UM–––I

RE =

UM–––IE1

RE1 =

RE RE1 RE2 RE3 RE4

IE

IE1 IE2 IE3

IPIE

IE4

IP

II

I I

E ES S H E ES S H

CMC

EC SCT

Earthed conductore. g. counterpoise

Lattice-typepylon

EC

” Selective earth resistance measurements using a probe, an auxiliary earth electrode and split-core current transformersa) Test object whose earthing conductor can be clamped by a split-core current transformer; b) Pylon whose legs can be clamped by a split-core current transformer near the foundation of the pylonIE part of test current flowing through the earth electrode under test to the auxiliary earth electrode; IE1 to IE4 parts of IE flowing in the pylon legs and stubs; IP portion of the test current flowing to the auxiliary earth electrode via the other parallel earth electrodes (currentpath through soil not shown in part b) of figure).

nected to the earth electrode under test andconnected to a multi-pole socket on the earthresistance meter. When the meter is connectedin this way and the rotary selector switch hasbeen set appropriately, IP, the portion of the testcurrent I flowing via the other parallel earthelectrodes, has no effect on measurement resultso that the branch current IE recorded by cur-rent measuring clamp CMC is solely responsi-ble for determining the resistance to earth REdisplayed by the meter.Figure ”b) shows the measurement circuitused when dealing with a steel-lattice electrici-ty pylon that cannot be electrically discon-nected from the earthed conductor (e. g. coun-terpoise, PEN conductor or neutral conductor).As the pylon structure serves as the earthingconductor EC, it is clearly not possible to clampa CMC around the earthing conductor as in fi-gure ”a). In this case, measurements are ma-de by consecutively clamping a splitcore trans-former SCT (Fluke EI-162BN7)) around the fourpylon legs that are connected to the four pylonstubs that act as earth electrodes. The earth re-sistance meter displays the resistances RE1 toRE4 consecutively. The resulting earth resis-tance RE of the four mast feet, which are con-nected to one another through the steel latticestructure, can be calculated by equation (5):

(5)

Measurement methods thatdraw current from supply

4.1 Measuring equipmentThe meters used for this type of measurementare designed primarily for testing electricalsafety systems that make use of residual cur-rent devices. To ensure the simplest and safestconnection to the power supply, the meters aretypically equipped with a flexible power cableand an earthed safety plug. The meters also have a socket S for the probe (see figures

and ). The socket E is used to connectthe meter to the earth electrode under test un-less one of the cores (protective earth core) ofthe flexible power cable and the earth contactson the plug are used for this purpose. As the testmeters are classified as Class II equipment (seeref. [4], sec. 4.5), the core and the plug’searth contact do not serve as protection againstshock hazards.The meters do not have their own power sour-ce unless this is needed for some other type ofmeasurement. Some meters may have an addi-tional connector socket for a current measuringclamp.

R

R R R R

E

E1 E2 E3 E4

=+ + +

11 1 1 1

4

8

A

L1L2L3PEN



Earthelectrode


Probe Earthelectrode


Probe

L1L2L3N

UM–––I

RE =UM–––I

RE =

RF RFRE RE

EC

EC

UM I

AV

ES

N NPEMEB MEB

PE

L

UM I

V

ES

L

RCD

a) b)

L1L2L3PEN


Earthelectrode


UM–––I

RE =

RF RE

EC

UM IAV

ES

N PE

MEB

L

a)

L1L2L3N


Earthelectrode


UM–––I

RE =

RF RE

EC

UM I

AV

ES

N PE

MEB

L

b)

RCD

Current-voltage methods that draw current directly from the power supply and that use a probea) Installation with TN system; b) Installation with TT systemEC earthing conductor; RCD residual current device; I test current; MEB Main earthing busbar;RF resistance of functional earth; RE earth resistance being measured; UM test voltage

Current-voltage methods that draw current directly from the power supply and that use the PEN or neutral conductor instead of a probea) Installation with TN system; b) Installation with TT system

7) The jaws of the split-core transformer are dimen-sioned for large rectangular-section conductorssuch as the legs of high-voltage pylons.

13

12

Figures to show the basic principles of thecomplicated circuitry inside these meters. Inmost of these meters, the actual measurementprocess (including any gradual increase in thetest current that may be involved) is carried outautomatically. Rather than displaying the mea-sured voltage and current separately, the resis-tance to earth is computed and displayed digi-tally on the meter.A selector switch enables the type of measure-ment, measurement technique, measurementcircuit, parameter range and/or measurementsequence to be chosen. Most meters are fittedwith a ‘START’ button to initiate the measure-ment process. User-friendly devices offer addi-tional functions such as:• Multiple measurements with display of ave-

rage result• Smoothing function• Display hold function• Data storage for transmitting or printing mea-

surement results.To provide protection against electric shock, themeter must switch off automatically as soon asit causes a fault voltage greater than 50 V inthe earthing system being measured. If a varia-ble resistor is used to increase the test current,the current must not exceed 3.5 mA at the be-ginning of the measurement (see ref. [6],sec. 4.7). Measurements in which the test cur-rent is increased gradually and measurementsin which the current is only allowed to flow atmaximum strength for a short period are bothcommon.The difficulty associated with drawing currentdirectly from the power supply is that the mea-surement is made at the supply frequency andinterference currents that originate in the powersupply or that are carried via earth can easily in-troduce measurement errors. The larger the testcurrent, the less effect these sources of interfe-rence will have. It is therefore expedient to workwith a large test current. However, a large testcurrent can itself be problematic when the me-ter is connected behind a residual current devi-ce, as it can cause the RCD to trigger. This canbe avoided by using one of the following proce-dures:• Ensuring that the magnitude of the test cur-

rent is only half that of the rated residual cur-rent IΔN of the RCD.

• Connecting the meter in front of the RCD orto a circuit that is not equipped with an RCD.

• According to the manufacturer ChauvinArnoux the patented ‘ALT system’ used in itsC.A 6115 N and C.A 6456 Earth Clampsenables these devices to make earth resis-tance measurements using a larger test cur-rent even if connected behind a 30 mA RCD.

Whenever interference effects may play a role,several measurements should be conductedand the results compared with one another.

4.2 Connections to power supplyand earth electrode

The meter is typically connected to the powersupply via its earthed safety plug. If the plug is

9




Earthelectrode


Earthelectrode

U0 – UM––––––––I

Rloop <

RE < Rloop

UE < U0 – UM

U0 – UM––––––––I

Rloop <

RE < Rloop

UE < U0 – UM

a) b)

L1L2L3PEN

L1L2L3N

UM

UE

U0

UM

UE

U0

L

UM

RF RE RF RE

EC

EC

I

AV

ES

L

UM I

AV

ES

L

N NPE PEMEB

MEB

RCD




Probe Functionalearth electrode

ProbeEarthelectrode

Otherearthelectrodes

Earthelectrode

Otherearthelectrodes

L1L2L3N

L1L2L3N

UM

RF RE RF RE

IE

I IIP IP

IE IE

EC EC

A

V

ES

L

UM

IE

A

V

ES

L

N N PEMEB

MEB

UM–––I

RE =

UM–––I

RE =

UE = UM

CMC CMC

RCD

Current-voltage methods that draw current directly from the power supply and that do not use a probea) Installation with TN system; b) Installation with TT systemRloop loop resistance; U0 conductor-to-earth voltage; UE voltage across tested earth electrode

Selective earth resistance measurement methods that draw current directly from the power supply and that use a probe and a clamp-on ammetera) Installation with TN system; b) Installation with TT systemIE portion of test current flowing to the earth electrode under test; IF portion of test current flowing to the other earth electrodes; CMC current measuring clamp

14

13

14

inserted incorrectly, no hazard arises but nomeasurement is possible. Although not shownin the figures, the internal circuitry of most ofthe meters only functions if the meter is con-nected to the phase conductor and to the neu-tral conductor.The test current can induce accidental trigger-ing of an upstream RCD. This may need to betaken into account when connecting the meter(see discussion in section 4.1 above).Depending on the type of meter used, the earthelectrode to be measured is• either connected directly to socket E of the

meter (see fig. Õ in section 2)• or (in most cases) is connected to the meter

via the plug’s earth contact as shown in fig-ures to .

Connections between the earth electrode under test and other earth electrodes wouldyield erroneous results. It is for this reason thatwhen measurements are made on consumer in-stallations with a TN earthing system, the earth-ing conductor EC has to be separated from themain earthing busbar MEB (see figures a) to

a)) as the latter is connected via the PENconductor of the service cable and the supplynetwork to other earth electrodes. Disconnec-tion is not required in a TT system as the mainearthing busbar is not linked to the neutral lineof the supply network and the connection canbe made as shown in figures b) to b).

4.3 Methods using a probeThis method is the most accurate of the tech-niques that draw current directly from the sup-ply provided that the probe can be inserted in-to the soil at a suitable location. A schematic ofthe measurement set-up is shown in figure .The probe has to be located so that it is outsidethe sphere of influence of the earth electrode.The voltage UM between the sockets E and Sgenerates the test current I in the earth elec-trode.

4.4 Method using the PEN con-ductor or neutral conductorinstead of a probe

This measuring techniques can be used when-ever it is not possible to insert a probe into theground at the right location. In this method (seefigure ) the probe is replaced by connectingsocket S of the meter to the PEN or PE con-ductor in a TN system or to the neutral con-ductor in a TT system. Caution! The neutralconductor must be treated as if it is live, eventhough it is earthed.The value displayed by the meter includes theresistance to earth of the functional earth elec-trode. This will overestimate the resistance ofthe earth electrode and thus yield a value thaterrs on the side of safety.The voltages generated by operating currentsand by fault currents in the functional earth orin the PEN conductor or neutral conductor ofthe power supply system can result in erro-neous measurement results. The accuracy ofthis technique is therefore lower than that

achievable using the method described in section 4.3.

4.5 Method without a probeThis method (illustrated schematically in fig-ure ) involves measuring the resistance of aconductor loop via an earth return path. In thismethod, the voltage across the test object (UE)is not measured directly. It is determined as thedifference between the potential drop betweenthe phase conductor and earth when the test re-sistance is switched off (U0) and that when thetest current I is flowing (UM). The resistance val-ue measured includes the resistances of thefunctional earth, the transformer and the phaseconductor. This will result in an overestimationof the earth resistance and thus yield a valuethat errs on the side of safety.This method is particularly attractive as it canbe performed with a minimum of effort. But itsuffers from the weakness that supply load fluc-tuations that happen to occur simultaneouslywhile the measurement is being made willcause significant additional measurement er-rors. To limit these errors, it is therefore expe-dient to work with a large test current. It is alsoadvisable to perform numerous measurements,to reject any extreme values recorded and tocompute the mean value from the remainingmeasurement data.

4.6 Selective earth resistancemeasurements using a probeand a clamp-on ammeter

The method selective earth resistance meas-urement8) is used if, for the purposes of themeasurement, the earth electrode under testcannot or should not be disconnected from oth-er earth electrodes to which it is wired in paral-lel. It is based on the method using a probe dis-cussed in section 4.3, but in this variant (seefigure ) a special earth resistance meter(Chauvin Arnoux C.A. 6115N or C.A. 6456)and an additional current measuring clampCMC are required. The current measuringclamp is connected to a multipole socket on themeter and the clamp jaws are placed aroundthe earthing conductor EC connected to theearth electrode under test.If the meter is connected in this way and if therotary selector switch set appropriately, IP, theportion of the measuring current I flowing viathe other parallel earth electrodes, has no effecton measurement result so that the branch cur-rent IE recorded by the current measuring clampCMC is solely responsible for determining theresistance to earth RE displayed by the meter.

References[1] IEC 60364-4-41:205 Erection of power installa-

tions with nominal voltages up to 1000 V – Part4-41: Protection for safety – Protection againstelectric shock.

[2] Hering, E.: Schutzerder des TT-Systems (engl.:Protective earthing in the TT system). Elektro-praktiker, Berlin 59 (2005) 5, p. 370-373.

[3] IEC 60364-6:2006-02 Low-voltage electrical in-stallations – Part 6: Verification.

[4] IEC 61557-1:2007 Equipment for testing, mea-suring or monitoring of protective measures –Part 1: General requirements.

[5] IEC 61557-5:2007 Equipment for testing, mea-suring or monitoring of protective measures –Part 5: Resistance to earth.

[6] IEC 61557-6:2007 Equipment for testing, mea-suring or monitoring of protective measures –Part 6: Effectiveness of residual current devices(RCD) in TT, TN and IT systems.

[7] IEC 61557-10:2000 Equipment for testing,measuring or monitoring of protective measures– Part 10: Combined measuring equipment fortesting, measuring or monitoring of protectivemeasures.

[8] Hering, E.: Probleme mit einem der Erdungs-meßverfahren beim TN-System (engl.: Problemswith an earth resistance measurement techniquein a TN system). Elektropraktiker, Berlin 53(1999) 9, p. 820-822.

[9] Hering, E.: Durchgangsprüfungen an Erdungsan-lagen [Continuity testing in earthing systems].Elektropraktiker, Berlin 59 (2005) 11, p. 888-891 und in diesem Sonderdruck.

[10]DIN EN 62305-3 (VDE 0185-305-3):2006-10:Protection against lightning – Part 3: Physical da-mage to structures and life hazard.

10

8) On its own, the expression ‘selective earth mea-surement’ is ambiguous, as other earth resistancemeasurement techniques are also selective, e. g.those presented in sections 3.4, 3.5 and 3.6.

14

14

14

14

12

Reasons for and limits ofcontinuity testing

1.1 Earthing conductor and main earthing busbar

The earthing system must be connected to themain earthing busbar (MEB) of the electrical in-stallation. This is necessarya) to provide protection against electric shock

([1], sec. 441.3.1; [2], sec. 542.1.2)b) to provide a connection to earth for overvol-

tage protection devices even if the building isnot equipped with a lightning protection sys-tem ([3], figures A.1 to A.5)

c) in buildings with a lightning protection sys-tem ([4], sections 5.4.1, 6.2.1, 6.2.5 andE.6.2)

d) in buildings with an antenna requiring light-ning protection

e) for foundation earth electrodes ([5], secti-ons 4 and 5.4).

The conductor between the main earthing busbar and the earthing system is an earthingconductor and/or an equipotential bonding con-ductor (protective bonding conductor and/orlightning protection bonding conductor).The continuity of this conductor must be tested([6], section 612.2; [4], section E.7.2.4; [5],section 7).

1.2 Ring earth electrodesContinuity testing on ring earth electrodes is ad-visable for the following reasons:a) Earth resistance measurements, which are

required by the applicable standards but arenot dealt with in this article, are unable to de-tect any break in the ring. While any discon-tinuity will not lead to an increase in the re-sistance to earth, it can have a significant de-trimental effect on the efficiency of thevoltage protection, as the surge currents areforced to flow via another path.

b)Repeated continuity testing on older ringearth electrodes can, if carried out in the formof resistance measurements, identify reducti-ons in the conductor cross-section (as a re-sult of corrosion) by registering an increasedresistance.

In order for continuity testing to be possible, aburied ring earth electrode must have at least

two soil entry points, while a foundation earthelectrode must be equipped with at least twoconnection points. The more of these hook-uppoints there are available, the easier it is to car-ry out continuity testing.In reinforced concrete foundations, it is not pos-sible to detect a break in the ring of the foun-dation earth electrode. This is not dangerous,however, as the break is effectively bridged bythe steel reinforcing bars within the foundation.In this case, continuity testing can only serve toverify that the connection between the con-nection points and the ring is intact.

1.3 Linear earth electrodesContinuity testing is not possible with verticalearth rods and elongated horizontal earth elect-rodes (star or crow’s foot configurations) as the-se electrodes only have a single soil entry point.If there is a break in the earth electrode near towhere it enters the soil, this may be detectableas increased earth resistance.

Hazard avoidance

The process of measurement and any accom-panying procedures (e.g. breaking standardconnections and making non-standard con-nections) must not pose a safety hazard. If theearth electrode also functions as the protective

earthing of a TT or IT earthing system as detai-led in [1], its connection to the MEB may onlybe broken if the electrical installation has beendisconnected from the power source or powergenerator.Tests that involve breaking connections (e.g.opening the inspection joint of a lightning pro-tection system) must never be carried out du-ring a storm or whenever a storm could ex-pected. Failure to comply could be hazardous,particularly for the person doing the testing.For safety reasons, the use of voltages greaterthan 25 V should be avoided. Small voltagesare anyway advisable for continuity testing (see[6], sec. 612.2). The testing and measuringequipment used must comply with the specifi-cations in the relevant standard (see ref. [7]).Resistance measurements are typically carriedout using equipment that conforms with thespecifications in reference [8]. However, earthresistance meters that meet the requirements inreference [9] can also be used.

Test methods

3.1 PrinciplesThe person carrying out the test must be aware of all connections between the earthingsystem and other electrical and non-electrical installations. Examples of the latter include pipe systems or metallic structural componentsof a building. An unknown connection can fal-sify the test results. The continuity test can becarried out most simply if the conductor undertest can be disconnected at least on one side.This assumes, however, that the broken con-nection can be reliably re-stored after testing.To facilitate continuity testing on lightning pro-tection systems, it is normal to open the in-spection joints at the soil entry points or con-nection terminals.Figure ➊ illustrates test circuits that can beused when no other conductor is connected inparallel to the test object. The continuity testershown in figure ➊a) consists of at least a powersource and an indicator, e.g. a lamp. To in-crease the test current, a resistance can be con-

3

2

1

11

Continuity testing in earthing systemsE. Hering, Dresden (Germany)

Continuity tests are carried out to verify that conductors, in this case metalconductors, are unbroken. This article describes the extent to which continuitytesting in earthing systems is required and possible and also discusses the testequipment that can be used. In terms of continuity there is no significant diffe-rence between the initial test and repeat tests, despite the fact that they aretreated separately in the relevant standards.

nected in parallel to the indicator. According tothe recommendation in reference [6], sec-tion 612.2, the power source should generatean open circuit voltage of between 4 and 24 Vand the test current should be at least 0.2 A.Some continuity testers produce an acousticsignal if the resistance measured exceeds auser-adjustable limit.In figure ➊b) the resistance is determinedusing a milliohmmeter. The advantage of thefour-wire connection shown is that the result ofthe measurement is not affected by the resis-tances of the measuring leads. If a bridge is in-stalled between the connection sockets 1 and 2 or between sockets 3 and 4 or if the meteronly has two sockets and only two measuringleads are used, the resistances of the leadsmust be subtracted from the resistance valuedisplayed by the meter. In order to measure theresistance of the leads their ends are connectedtogether. Instead of using a milliohmmeter anearth resistance meter can be used as shown infigure ➊c) provided that the meter has a resis-tance measuring range down to 0.1 Ω, or low-er if possible. The remarks made about the me-ter connections in figure ➊b) apply here anal-ogously.As indicated in figure ➊d), the resistance canalso be measured by means of a clamp-on ohm-meter such as the Earth Clamp1) C.A 6410manufactured by Chauvin Arnoux. The resist-ance of the measuring lead needed to close thecircuit must be subtracted from the resistancevalue displayed by the meter. If one or moreconductors are connected in parallel to the testobject, the same procedures discussed with re-gard to figure À can be used. A power sourceis connected to the ends of the conductor undertest. In figure Àa) continuity is verified when

a current is detected by a clamp-on ammeterthat is clamped around the conductor undertest TC. If, as shown in figure Àb), a voltmeterfor measuring small voltages is connectedacross the conductor under test, the resistanceof the conductor can be determined by dividingthe measured voltage by the measured current.Continuity testing is carried out preferentiallywith alternating current, e.g. from a transformeror from an earth resistance meter. If a DCsource is used, then a special type of clamp-onammeter is required that contains a currentsensor based on the Hall effect instead of a cur-rent transformer.

3.2 Continuity testing in ring earthelectrodes that can be opened

This section describes examples of continuitytests performed on ring-shaped foundationearth electrodes. Many of these tests can alsobe carried out in the same way on buried ringearth electrodes.As already mentioned in section 1.2, the easeof performing a continuity test and the reliabil-ity of the results obtained depends on the num-ber and arrangement of connection points orsoil entry points. Being able to open the ringearth electrode considerably simplifies continu-ity testing. These factors should be taken intoaccount when planning the earthing system.The test circuits shown in figures Ã to Œ canonly be used if the ring can be opened at leastone location. Foundation earth electrodes canbe opened when there are two adjacent, flush-floor connection points that are linked conduc-tively to one another via a bridging strip thatcloses the ring. Buried ring earth electrodesmay have an inspection joint located in an un-derfloor inspection box.

Figure Ã shows the ring opened for testing. Inthe test circuits shown in figure Õ continuityis verified by detecting current in the relevantparts of the ring by means of a clamp-on am-meter at the bridging strip or the underfloor in-spection joint. In the test scenario shown in fig-ure Œ, the resistance of the ring is measuredat the bridging strip or the underfloor inspectionjoint using a clamp-on ohmmeter (see ‘EarthClamp’ in section 3.1).

3.3 Continuity testing in ring earth systems that cannot beopened

If the ring earth electrode has at least four con-nection points, a break in ring continuity can beidentified by sequentially measuring the resist-ance between two neighbouring connectionpoints. The small resistances of the conductorsthat run between the point of connection of themeter leads and the ring have to be subtractedfrom the resistance displayed on the ohmmeter.Although the soil and, in the case of a founda-tion earth electrode, also the concrete are con-nected in parallel to the metal conductor, theyhave no significant effect on the measurementresult because their resistivity is very muchgreater than that of steel.If the ring is uninterrupted, the resistance mea-sured is that of a parallel circuit comprising the

12

TC

PLC

G

CAM

PS

TC

PLC

G

CAM

PS

CAM2

mV VMa) b)

CT

1 2

3 CT

1 2

3

CT

a) b)

c)

1 2

3

a) b)

G

CA1

PS

G

CA2

PS3

1 2

4 3

GCA1

CA2

CA3CA4

I4

I3

I2

I1

I2 – I3 = I4 – I1

I

c)

1 2

PSÀ Continuity testing circuits that mea-sure the current in a conductor that hasanother conductor connected to it inparallela) Without determining the resistance of the

conductorb) Resistance of conductor determined by

making an additional voltage measurementPLC conductor connected in parallel (can also be a second test conductor)VM low-voltage voltmeterCAM position of clamp-on ammeterCAM2 second position of clamp-on ammeterwhen PLC is a second test conductor

Ã Examples of continuity tests when ringearth electrode is opena) Testing connections 1 and 3 and the left secti-

on of the ringb) Testing connections 2 and 3 and the right

section of the ringc) Testing the entire ring earth electrode

Õ Examples of continuity tests when ringearth electrode has not been opened a) Analogous to fig. Ãa);b) Analogous to fig. Ãb);c) Four tests on a foundation earth electrode

with a cross connector and four connectionpoints

I measured current; I1 to I4 partial currents;CA1 to CA4 positions of clamp-on ammeter

1) The semicircular jaws of the clamp contain twotransformers. The purpose of one is to induce avoltage in the conductor, the other measures theresulting current. The clamp is closed when thejaws have been placed around the conductor. Thedisplay on the clamp meter displays a resistancevalue computed by dividing the value of the indu-ced voltage by the current flowing in the con-ductor.

section of the earth ring enclosed by the mea-suring leads and the series circuit that makesup the remaining part of the ring. If the earthring electrode is broken at some point betweenthe measuring leads (see figure œ), the resis-tance measured is that of the remaining part ofthe ring. If the measurement is made on onepart of the ring, the resistance of that part aloneis measured.

Example calculation 1The ring earth electrode has four connectionpoints. Those parts of the ring located betweenthe connection points have the resistancesR1…R4. If the ring is unbroken, the resistanceRO (‘O’ is used here to symbolize the intactearth ring electrode) measured across the twoconnection points that enclose the part of thering with the resistance R1 is given by:

(1)

However, if the part of the ring with the resist-ance R1 is broken, the resistance RC measuredacross those same connection points (‘C’ isused here to symbolize a ring with a continuitybreak) will now be given by equation (2).

(2)

Example calculation 2Situation as in example calculation 1 but eachof the four sections of the ring has the same re-sistance RP.

(3)

If the ring is unbroken, the resistance measuredacross any pair of neighbouring connectionpoints is expressed by equation (4):

(4)

If the ring is broken (i.e. open), the resistanceRC measured across the two connection pointsenclosing the break will be:

(5)

(6)

If the measurement is therefore made acrossthose connection points enclosing the disconti-nuity, the resistance measured will be threetimes the resistance of the partial resistance RPand four times the resistance that would bemeasured if the ring were unbroken.If the number of connection points is greaterthan four, the same computational methodolo-gy can be used with the exception that equa-tions (1) and (2) would then contain addition-al terms for the additional sections of the ring,and the factors in equations (4) and (5) wouldbe larger.If the earth ring electrode has only two connec-tion points, it may be possible to carry out test-ing in accordance with figure Àb) (see sec-tion 3.1). However, this would require beingable to clamp a clamp-on ammeter around thetwo parts of the ring that are connected in par-allel to the power source. If this is to be at-tempted on a buried ring earth electrode, theclamp can, for instance, be located on eachside of a soil entry point. If the measurement isan initial measurement, then the earthing chan-nel in the relevant areas must not be backfilleduntil the measurement has been made. If themeasurement is a repeat measurement, it canbe carried out, for example, when material hasto be removed from the earthing channel in or-der to check on the state of corrosion of theearth electrode.

Test report

Continuity testing is only one of several teststhat have to be performed on earthing systems.In general, the results from all the tests are con-tained in a single test report.The tests performed and any accompanying ac-tion that is taken must be described preciselyso that they can be reproduced at a later date.All test and measurement results must be stat-ed clearly and precisely. If the earth electrodeserves as a lightning protection earth electrode,the test report must comply with the specifica-tions in reference [4], sec. E.7.2.5.

References[1] IEC 60364-4-41:2005 Erection of power instal-

lations with nominal voltages up to 1000 V – Part4-41: Protection for safety – Protection againstelectric shock.

[2] DIN VDE 0100-540 (VDE 0100-540):2007-06– Low-voltage electrical installations – Part 5-54:Selection and erection of electrical equipment –Earthing arrangements, protective conductors andprotective bonding conductors.

[3] Prestandard DIN V VDE V 0100-534 (VDE V0100-534):1999-04 Electrical installations ofbuildings – Part 5-34: Selection and erection ofequipment – Devices for protection against over-voltages.

[4] EN 62305-3:2006 Protection against lightning –Part 3: Physical damage to structures and life ha-zard (IEC 62305-3:2006).

[5] DIN 18014:2007-09 Foundation earth electro-des.

[6] IEC 60364-6:2006-02 Low-voltage electrical in-stallations – Part 6: Verification.

[7] EN 61557-1:2007 Equipment for testing, mea-suring or monitoring of protective measures – Part1: General requirements (equivalent to IEC61557-1).

[8] EN 61557-4:2007 Equipment for testing, mea-suring or monitoring of protective measures – Part4: Resistance of earth connection and equipoten-tial bonding (equivalent to IEC 61557-4).

[9] EN 61557-5:2007 Equipment for testing, mea-suring or monitoring of protective measures – Part5: Resistance to earth (equivalent to IEC 61557-5).

R RC P

3=

R RR

RR

o P

P

P

P

3

4=

( )( )

=34

R R R R RP 1 2 3 4

= = = =

R R R RC 2 3 4

= + +

R

R R R R

E

E1 E2 E3 E4

=+ + +

11 1 1 1

4

13

COM

Discontinuity

mΩ OM

Œ Continuity testing using a clamp-onohmmeter positioned at the point COM

œ Verifying a discontinuity as an in-creased resistance resulting from thelonger current path

Rc = 3(4)Ro = 4Ro3

Documents

Measuring earth resistance · tween the main earthing busbar and the earth electrode. The dissipation resistance R ... ring resistance to earth [5] • IEC 61557-6:2007 Equipment