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 REis the sum of the dissipation resistance RD, the resistance of the metal conductor that ser-ves as the earth electrode RMand the resistance of the earthing conductor RC, which runs be-tween the main earthing busbar and the earth electrode. The dissipation resistance RDis the resistance between the earth electrode and the surrounding soil. The reactance of the earthing system XEcan be expressed as:
(3) with
XM reactance of the metallic earth electrode XC reactance of the earthing conductor. For AC supply current the reactance of 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 preventiconnecti-on, 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 inin-creasing 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 I E M = RE=RD+RM+RC Z R X E= E+ E 2 2Measuring 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 XM XC XE
➊
Vector diagram of impedance in an earthing systemRddissipation resistance; REresistance of the earthing system (earth resistance);
RMresistance of the metal conductor that acts as the earth electrode; RCresistance of the earthing conductor (e.g. connection lug, cable); XEreactance of the earthing system;
XMreactance of the metal conductor that acts as the earth electrode; XCreactance of the earthing conductor; ZEearth 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
R
E2.1
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 the earth impedance by measuring the voltage across 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 are shown in figuresÀ
toÕ
. The unusually long names given here to the various methods ensu-re that the methods can be distinguished unam-biguously.Although there are clear differences between the individual measurement methods, no one particular method can be said to be ideal. Each method has its own particular disadvantages such as limited applicability, electric shock ha-zard, larger measurement errors or requiring greater time and effort to complete. The vario-us advantages and disadvantages of the indivi-dual measurement techniques are described in more detail in sections 3 and 4. All of the me-thods discussed must only be carried out by competent 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 used will be at least 5 Hz above or below the fre-quencies 16.7 Hz, 50 Hz and integer multi-ples thereof. This prevents interference from supply frequency currents (‘interference cur-rents’) that can falsify measurement results. In those methods that do draw current directly from the supply (columns 7 to 9 in table➊
), it is of course essential that the supply frequen-cy and measuring frequenfrequen-cy are identical. This means that the interference effects mentioned above cannot be ruled out when such methods are used. However, these methods are simpler to perform and offer advantages in terms of their applicability.2
Table
➊
Overview of earth resistance measuring methods1 2 3 4 5 6 7 8 9
Designation based on internal circuitry
Balanced-bridge methods Current-voltage methods
Distinction based on whether method draws current directly from supplyv)
yes no
Distinction 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’)
Figure
2 3b)z) 3c) 3a) 3b) 3c) 4a) 4b) 4c)
Detailed description in section
2.2 3.2 3.3 3.4 4.3 4.4 4.5
Detailed schematic of measurement method
5 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.
2.2
The balanced-bridge method
The balanced-bridge method as described by Behrend is one of the techniques for measuring earth resistance that does not involve drawing current directly from supply. Earth resistance meters based on this method are no longer ma-nufactured, as other more user-friendly instru-ments have now been developed for the same sorts of applications. These new meters use the so-called current-voltage method, which also does not involve current being drawn from the supply. Nevertheless the balanced-bridge me-thod is described here because it is of funda-mental importance to the development of earth resistance measurement techniques and be-cause meters based on this method are still in use.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 no influence on the measurement result; it simply lowers measurement sensitivity. Information on the alignment and separation of the auxiliaryEarth resistance meter
tr = I2 : I1 = 0.1; 1; 10; 100 CT R2 I2 I1 I1 RE U1 U2 I1 I3 = 0 I3 = 0 N REC ∼ REC PS AC E ES S H Earth electrode Auxiliary earth electrode Probe
À
The balanced-bridge method REC rectifier; I1test (or measuring) current;I2Reference current; I3current whose mag-nitude is zero when bridge is balanced; C capacitor; N null detector; REearth resis-tance being measured; R2reference resis-tance; CT current transformer; U1Voltage across earth electrode under test; U2 refe-rence voltage; tr transformation ratio of the CT; PSACAC power supply.
Earth resistance meter
REC ∼ REC∼ REC∼ Earth electrode Earth electrode Earth electrode Auxiliary earth electrode Functional earth electrode Functional earth electrode Probe Probe L1 L2 L3 PEN L1 L2 L3 PEN UM ––– I RE = UM ––– I Rloop = UM ––– I RE = RE < Rloop RE RF RE RF RE PSAC PSAC UM I U M I A V A V E ES S H E ES S H PSAC UM I A V E ES S H a) b) c)
Ã
Current-voltage methods that do not draw current directly from the power supply a) 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; Rlooploop resistance; UMtest voltage.
Õ
Current-voltage methods that draw current directly from the power supply a) with probe; b) using PEN conductor or neutral conductor instead of probe;c) no probe (measures resistance of conductor loop via earth return path);
U0conductor-to-earth voltage Functional earth electrode Functional earth electrode Earth resistance meter Earth electrode Earth electrode Earth electrode Probe UM ––– I RE = UM ––– I RE = UM ––– I RE = U0 – UM –––––––– I Rloop < RE < Rloop UE < U0 – UM RE RF RF RE RF RE a) b) c) L1 L2 L3 PEN or N L UM UM UE U0 I A V E S L UM I A V E S L UM I A V E S Functional earth electrode
earth electrode and the probe is provided in section 3.
The AC power source PSACis located between the connection point for the earth electrode un-der test (socket E) and that for the auxiliary earth electrode (socket H). The AC source is connected in series with the primary winding of a current transformer CT. Connected to the se-condary winding of the current transformer is a variable voltage divider. The setting chosen for the left part of the divider R2(‘reference resis-tance’) is displayed on the scale on the voltage divider’s control unit. A null detector N with a rectifier REC in series is located between the va-riable tap point of the voltage divider and the connection point for the probe (socket S). The rectifier is driven by the AC power source. A ca-pacitor C prevents any DC current from flowing across the probe. One end of the voltage divider is connected to the earth electrode being mea-sured via the instrument sockets ES and E. The transformation ratio tr of the current transfor-mer can be switched to achieve the required measurement range.
When balanced, the current I3in the probe is zero. The same current I1therefore flows in the auxiliary 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 U1corresponds to the earth electrode voltage that drives the test current I1 in the earth resistance REof the test object E, whereas U2is the voltage drop that maintains the current I2(‘reference current’) in the refe-rence resistor R2. The potential drops obey Ohm’s law as expressed by the equations U1= I1· REand U2= I2· R2. If the transforma-tion ratio of the current transformer tr = 1:1, then I2= I1and the value of the earth resis-tance REis equal to the selected reference re-sistance R2. The earth resistance can therefore be read off the voltage divider scale mentioned above. 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 methods
without supply current
Another group of methods for measuring earth resistance that do not draw current directly from the supply are the so-called current-volta-ge techniques illustrated in figureÃ
. The earth resistance RE is determined from the voltage UMthat appears across the earth electrode and across the sockets ES and S, and the measu-red current I.(4) Figure
Ã
simply illustrates the principle of the measurement and shows only a small part of the complex circuitry within the earth resis-tance meter. Usually, the voltage UMand cur-rent I are not shown separately and the meter only displays a digital reading of the earth re-sistance RE. If the AC supply source PSACis aconstant-current generator, the earth resistance can be displayed directly on the voltage meter. When the balanced-bridge method was first de-veloped, the only exterior circuit known was that shown in figures
À
andÃ
a). It was the-refore usual to consider the circuitry inside the meter and the exterior circuit as a single entity. However, as indicated in columns 2 and 3 in tableÀ
, the same meter can be used for mea-surements with the exterior circuits shown in fi-guresÃ
b) andÃ
c). Equally, the earth resis-tance meters used for the current-voltage me-thods that do not draw current directly from the supply can be used like the meters designed with the balanced-bridge circuit. The internal circuits can therefore be freely combined with the exterior circuits.2.4
Measurement methods with
current from the supply
These methods can only be used in networks with a direct connection to earth. As shown in figureÕ
, the measurement involves drawing the test current from the phase conductor of the supply system. The meters used in this type of measurement are primarily designed for testing electrical safety systems involving residual cur-rent devices. The meters are generally con-nected to the supply via a flexible power lead and an earthed safety plug.Current-voltage method that
draws no supply current
3.1
Earth resistance meters
The four connection sockets are labelled as shown in figureŒ
. 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 measuring the soil resistivity)
• S – Probe2)
Normally when measuring the resistance to earth, the sockets E and ES are connected to one another via a removable link or via a con-tact strip within the meter’s selector switch as this ensures that the earth electrode under test is connected to both the current and voltage measurement paths. If, in addition, a jumper is placed 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 frequencies 16.7 Hz and 50 Hz and any integer multiples thereof. Typically, the supply frequency is in the range 41–140 Hz, though in some meters a higher frequency is used. Some earth resistance meters also offer the option of selecting the quency. A number of meters with automatic fre-quency control (AFC) automatically switch to that frequency offering the lowest level of inter-ference.
To protect against electric shocks, the open-cir-cuit test voltage generated by the meter must not exceed 50 V (r.m.s.) and 70 V (peak). In the case of earth resistance meters used on agricultural sites, these values must be halved. Alternatively, the short-circuit current must not exceed 3.5 mA r.m.s. and a peak value of 5 mA (see [5], sec. 4.5). If neither of these conditions are met, the meter must switch off automatically.
The meter is powered either by a battery, a group of primary cells or a hand-driven genera-tor, though the latter method is now rare. The meter 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 drawn directly from the supply, the earth resistance RE is computed as the quotient of the measured voltage UMthat appears across the earth elect-rode (and across the meter sockets ES and S) and the measured current I (that flows through sockets E and H). Figure
Œ
only indicates the basic principle of the complex circuitry withinR U I E M =
3
Earth resistance meter
REC ∼ Earth electrode Auxiliary earth electrode Probe UM ––– I RE = RE RE' RE'1 RE' RE'2 PSAC UM I A V E ES S H 0.2 s 0.2 s s approximately flat section Distance
Œ
Current-voltage methods that do not draw current directly from the power sup-ply and that use a probe and an auxiliary earth electrodeI test current; REearth resistance being measured; RE´measured earth resistance;
the meter. Usually, the voltage UMand current I are not shown separately and the meter only displays a digital reading of the earth resistance RE. If the AC supply source is a constant-cur-rent generator, there is no need to measure the current and calculate the quotient. In this case a voltage meter can be calibrated to display the earth resistance directly.
Most meters are equipped with a switch for se-lecting the type of measuring circuit, the mea-surement frequency and/or the meamea-surement range, and for switching the power on and off. Most meters also have a button that is used to initiate measurement. The earth resistance me-ter must also indicate that the resistance of the auxiliary earth electrode and the probe are within the specified limits (see [5], sec. 4.4). However, it is not advisable to rely too heavily on a warning signal, because by the time a warning signal has been issued, the limit may have been exceeded by a significant amount. User-friendly devices offer additional functions such 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 a constant-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 an
auxiliary earth electrode
3.2.1 Principle
As shown in figure
Œ
, the earth electrode un-der test, an auxiliary earth electrode and the probe are connected to the earth resistance me-ter. The test current I flows through the earth electrode, the soil and the auxiliary earth elect-rode. The voltage UMthat appears across the earth resistance REalso appears across the me-ter sockets ES and S. The earth resistance is displayed as the value of UMdivided by I.3.2.2 Earth electrode (test object)
If socket E is connected to the beginning of the earthing conductor (at the main earthing termi-nal), the earthing conductor will be included in the measurement of the earth resistance. If, on the other hand, socket E is connected directly to the earth electrode, the resistance of the eart-hing conductor will not be included in the mea-surement. The difference, however, is usually slight.
The resistance of the measuring leads will be in-cluded in the measurement. This will result in an overestimation of the earth resistance and thus 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 meter close to the point of connection and to use a short measuring lead. The resistance of the
measuring lead can of course be measured and this value subtracted from the value displayed by the earth resistance meter. If the effect of the measuring lead’s resistance is to be avoided at all costs, the jumper linking sockets E and the ES 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 this would falsify the result of the measurement. In the TN earthing systems found in consumer in-stallations, the earthing conductor must be dis-connected from the main earthing busbar as the latter is connected to the PEN conductor of the supply network. This is not required in TT sys-tems as the main earthing busbar is not con-nected to the neutral conductor of the power supply network. If, nevertheless, the earthing conductor is disconnected, the entire system must be de-energized beforehand and locked out to prevent it being switched on again.
3.2.3 Auxiliary electrode
1)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 of overlap between the potential gradient areas (‘spheres of influence’) surrounding the two electrodes. The larger the electrodes, the farther apart they must be. As a rough guide, the mi-nimum distance apart can be taken to be three times the depth of a rod earth electrode or the average diameter of a ring earth electrode. The figure of 40 m that is found in the documenta-tion provided by some manufacturers can only be considered to be a rough average value. Whether the chosen distance is appropriate will be shown when the correct alignment and po-sitioning of the electrodes is carried out (see sec. 3.2.4).
The greater the resistivity of the soil, the longer the auxiliary electrode needs to be and the dee-per it needs to be driven into the ground. If the resistance of the auxiliary earth electrode is too large, measurement errors can arise, because, for example, the constant current normally ge-nerated by the AC supply cannot then flow. In such cases, it can prove useful to saturate the area of ground being used for the measurement with water.
3.2.4 Probe
2)As the internal resistance of the voltage measu-rement path is very large, the resistance of the probe and therefore the size of the probe is of minor importance. The preferred location of the probe is on the straight line between the earth electrode and the auxiliary earth electrode at a position where it has minimum interaction with the 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 the earth electrode and the probe the results would form a curve whose ends are relatively steep while the intermediate section of the curve is
flatter. If the distance between the earth elect-rode and the auxiliary electelect-rode is large enough, the curve will have an approximately horizontal central section in which the measured resis-tance to earth is essentially independent of electrode separation.
This central section must be determined by at least three measurements. The midpoint of the central section is not midway between the earth electrode and the auxiliary earth electrode, but lies closer to the auxiliary earth electrode as the spatial extent of the spheres of influence asso-ciated with the two earth electrodes differ. In general, the optimum separation between the earth electrode and the probe is about two thirds of the distance between the earth elec-trode and the auxiliary earth elecelec-trode3).
3.2.5 Limitations of method
If no portion of the resistance vs. distance cur-ve is approximately horizontal, then the dis-tance between the earth electrode under test and the auxiliary earth electrode is too small. If the curve exhibits an unusual profile, buried metal installations (e. g. water pipes) are very probably influencing the measurement. In such conditions it is not possible to achieve usable results from the measurement. Measu-rement may be possible if the electrodes can be laid out perpendicular to their original di-rection or perpendicular to the longitudinal axis of the buried metal installation or so that they run away from and not above the buried metal installation.
It is also not possible to achieve reliable results if the earth electrode under test is surrounded by other earth electrodes, for example in areas with a high density of buildings. Furthermore measurement is impossible whenever the au-xiliary earth electrode and the probe cannot be positioned in the right locations. In all such cases, another measurement technique must be selected.
3.3
Method using a probe but no
auxiliary earth electrode
3.3.1 Principle
As shown in figure
œ
, the functional earth of the supply network acts as a replacement for the auxiliary earth electrode. It is extremely 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 to as 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 electrode and the auxiliary earth electrode. That is incorrect. Other companies recommend placing the probe at a distance from the earth electrode that is always 62 % of the separation between the earth and the auxiliary earth electrode. This method is thus sometimes referred to as the 62 % method. The 62 % mark generally gives a good approximation of the correct location. But the optimum position must always be determined by moving the probe to neighbouring positions.
important to ensure that the connection is not accidentally made to one of the phase con-ductors.
In a TN system, the H socket of the meter has to be connected (for instance, via the earthing contact of a plug) to the protective earth (PE) conductor, which itself has been branched off the PEN conductor. The meter socket E is con-nected to the earthing conductor, which has to be disconnected from the main earthing busbar. Supply networks configured with the TT earth-ing system have a neutral conductor instead of the PEN conductor. This has to be treated as a live conductor even though it is connected to a functional earth. Applying this method of mea-suring earth resistance to a TT system would therefore involve connecting the earth resis-tance meter to the neutral conductor. The me-thod is therefore not approved for use with TT systems.
3.3.2 Problems in the TN system
The method does not function in a TN system if the electrode being measured is strongly cou-pled or if it is connected via a metal conductor to another earth electrode that itself is con-nected to the PEN conductor. This would result in the test current flowing in the wrong path so that the display on the earth resistance meter would 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 and
an auxiliary earth electrode
(‘stakeless method’)
3.4.1 Principle
This method (illustrated in figure
–
) is an earth-loop resistance measurement because it involves measuring the resistance of a con-ductor loop via an earth return path. The S and H sockets of the earth resistance meter are con-nected together. The advantage of this method is that neither an auxiliary earth electrode nor a probe need to be used.In a TN system the earthing conductor (EC) is disconnected from the main earthing busbar (MEB) and the earth resistance meter is inser-ted between them. This method is not suitable for measurements on a consumer installations with a TT earthing system.
The resistance measurement displayed on the meter includes the resistance to earth of the functional 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, they are difficult to determine, because the functio-nal earth in a TN system comprises not only the functional earth electrode shown in figure
–
, it is also connected to numerous earths in the consumer installations of neighbouring buil-dings. The error that is introduced by measu-ring these additional resistances results in an overestimation of the earth resistance, yielding a value that errs on the side of safety.3.4.2 Problems in TN systems
The problem mentioned earlier in secton 3.3.2 can also arise when measuring earth resistance without an auxiliary earth electrode and without a probe. Some examples of configurations whe-re problems can arise awhe-re shown in figuwhe-re
—
. Temporary remedial measures include: • Disconnecting the metal connection betweenthe earth electrodes as shown in figure
—
b). • Disconnecting the second earth electrode from the PEN conductor, if permitted by the owner. The residual influence of the second earth electrode on the earth resistance mea-surement is not a disadvantage, as it acts to improve the performance of the first earth electrode.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 method described in section 3.4. This technique differs from that shown in figure–
in that instead of inserting 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 contains both a current-to-voltage transformer (a voltageinducing clamp, VIC) and a current transformer (a current measuring clamp CMC). Models available 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 as the quotient of the voltage induced by the VIC in the earthing conductor and the resulting test current registered by the CMC. In this case the resistance is the loop resistance Rloop, or more precisely the loop impedance (see section 3.4.1).
Another solution (no separate diagram provi-ded) involves clamping two split-core current transformers around the earthing conductor, one of which functions like the voltage-inducing clamp VIC while the other corresponds to the current measuring clamp CMC that measures
Earth resistance meter
REC ∼ Earth electrode Functional earth electrode UM ––– I Rloop = Rloop < RE RF RE PSAC UM I A V E ES S H L1 L2 L3 PEN PEN N PE MEB EC
–
Current-voltage methods that do not draw current directly from the power supply and that use neither a probe nor an auxiliary earth electrode (resistance of conductor loop via earth return path)Rlooploop 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 resistance measurement techniques are also selective, e.g. those presented in sections 3.4, 3.5 and 4.6.
Earth resistance meter
REC ∼ Earth electrode Functional earth electrode Probe UM ––– I RE = RF RE PSAC UM I A V E ES S H L1 L2 L3 PEN PEN N PE MEB EC
œ
Current-voltage methods that do not draw current directly from the power sup-ply and that use a probe but no auxiliary earth electrodeEC earthing conductor; MEB main earthing busbar; RBresistance to earth of the functio-nal earth electrode
the test current. The clamps are connected to a special earth resistance meter (Fluke Earth Ground Tester 1623 or 1625). Depending on which of the Fluke meters is used, either EI-1623 or EI-1625 ‘selective/stakeless clamp set’ is required. The advantage in both cases is that the earthing conductor does not need to be disconnected, making measurement safer and quicker. The problem discussed in section 3.4.2 can also arise in these cases.
If this method is used to make measurements on consumer installations, they must be desig-ned with a TN earthing system. The method is suitable for measuring the resistance to earth of a pylon in an overhead power transmission line if the clamps can be fitted around the earthing conductor.
3.6
Selective earth resistance
measurements using a probe,
an auxiliary earth electrode
and a clamp-on ohmmeter
The earth resistance measurement described in this section6)is used if the earth electrode un-der test cannot or should not be disconnected from other earth electrodes to which it is wired in parallel. This method is based on the techni-que using a probe and auxiliary earth electrode that is discussed in section 3.2, but in this va-riant (see figure”
a)) a special earth resistance meter (Fluke 1623 or 1625) and an additional clamp-on current transformer (CMC) are requi-red. The current measuring clamp CMC is clam-ped around the earthing conductor ECcon-L1 L2 L3 PEN E ES S H IV Icpl Rcpl E2 RE1 IE1 I E2 RE2 I L1 L2 L3 PEN E ES S H IV Icpl E2 RE1 IE2 = 0 IE12 = 0 RE2 I I = IE1 + Icpl IE1 < Icpl RM < RE1 I = IE1 + Icpl IE1 < 0 Icpl < I RM ⪡ RE1
—
Cases involving a TN system in which the method shown in fig.–
is not suitable a) Small distance and therefore small coupling resistance Rcplbetween the earth electrode under test E1 and a second earth electrode E2 that is connected to the PEN conductor. b) Metallic connection to a second earth electrode that is itself connected to the PEN conductor.Icplcurrent causing measurement error; RE1earth resistance being measured; RMearth resis-tance displayed on meter.
L1 L2 L3 PEN PEN N PE MEB EC I UM RF RE EC CMC VIC COM for Rloop Earth electrode Functional earth electrode RE < Rloop
“
Method as in figure–
but with a clamp-on ohmmeter rather than an earth resistance meterEC earthing conductor; VIC voltage-induc-ing clamp; CMC current measurvoltage-induc-ing clamp; COM clamp-on ohmmeter.
Earth resistance meter Earth resistance meter
Other earth electrodes a) b) Other earth electrodes Test object Auxiliary earth electrode Auxiliary earth electrode
Probe Test object Probe
(pyton stubs) UM ––– I RE = UM ––– IE1 RE1 = RE RE1 RE2 RE3 RE4 IE IE1 IE2 IE3 IP IE IE4 IP I I I I E ES S H E ES S H CMC EC SCT Earthed conductor e. g. counterpoise Lattice-type pylon 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 pylon
IEpart of test current flowing through the earth electrode under test to the auxiliary earth electrode; IE1to IE4parts of IEflowing in the pylon legs and stubs; IPportion of the test current flowing to the auxiliary earth electrode via the other parallel earth electrodes (current path through soil not shown in part b) of figure).
nected to the earth electrode under test and connected to a multi-pole socket on the earth resistance meter. When the meter is connected in this way and the rotary selector switch has been set appropriately, IP, the portion of the test current I flowing via the other parallel earth electrodes, has no effect on measurement result so that the branch current IErecorded by cur-rent measuring clamp CMC is solely responsi-ble for determining the resistance to earth RE displayed by the meter.
Figure
”
b) shows the measurement circuit used 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 earthing conductor EC, it is clearly not possible to clamp a 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 four pylon legs that are connected to the four pylon stubs that act as earth electrodes. The earth re-sistance meter displays the rere-sistances RE1to RE4 consecutively. The resulting earth resis-tance REof the four mast feet, which are con-nected to one another through the steel lattice structure, can be calculated by equation (5):(5)
Measurement methods that
draw current from supply
4.1
Measuring equipment
The meters used for this type of measurement are designed primarily for testing electrical safety systems that make use of residual cur-rent devices. To ensure the simplest and safest connection to the power supply, the meters are typically equipped with a flexible power cable and an earthed safety plug. The meters also have a socket S for the probe (see figures and ). The socket E is used to connect the meter to the earth electrode under test un-less one of the cores (protective earth core) of the flexible power cable and the earth contacts on the plug are used for this purpose. As the test meters are classified as Class II equipment (see ref. [4], sec. 4.5), the core and the plug’s earth contact do not serve as protection against shock hazards.
The meters do not have their own power sour-ce unless this is needed for some other type of measurement. Some meters may have an addi-tional connector socket for a current measuring clamp. R R R R R E E1 E2 E3 E4 = + + + 1 1 1 1 1
4
A L1 L2 L3 PEN Earth resistance meter Earth resistance meter Earth electrode Functional earth electrode Probe Earth electrode Functional earth electrode Probe L1 L2 L3 N UM ––– I RE = UM ––– I RE = RF RE RF RE EC EC UM I A V E S N PE N MEB MEB PE L UM I V E S L RCD a) b) L1 L2 L3 PEN Earth resistance meter Earth electrode Functional earth electrode UM ––– I RE = RF RE EC UM I A V E S N PE MEB L a) L1 L2 L3 N Earth resistance meter Earth electrode Functional earth electrode UM ––– I RE = RF RE EC UM I A V E S N PE MEB L b) RCDCurrent-voltage methods that draw current directly from the power supply and that use a probe
a) Installation with TN system; b) Installation with TT system
EC earthing conductor; RCD residual current device; I test current; MEB Main earthing busbar;
RFresistance of functional earth; REearth resistance being measured; UMtest voltage
Current-voltage methods that draw current directly from the power supply and that use the PEN or neutral conductor instead of a probe
a) Installation with TN system; b) Installation with TT system 7) The jaws of the split-core transformer are
dimen-sioned for large rectangular-section conductors such as the legs of high-voltage pylons.
13
Figures to show the basic principles of the complicated circuitry inside these meters. In most of these meters, the actual measurement process (including any gradual increase in the test current that may be involved) is carried out automatically. 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, measurement circuit, parameter range and/or measurement sequence to be chosen. Most meters are fitted with 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, the meter must switch off automatically as soon as it causes a fault voltage greater than 50 V in the 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 measurements in which the current is only allowed to flow at maximum strength for a short period are both common.
The difficulty associated with drawing current directly from the power supply is that the mea-surement is made at the supply frequency and interference currents that originate in the power supply or that are carried via earth can easily in-troduce measurement errors. The larger the test current, the less effect these sources of interfe-rence will have. It is therefore expedient to work with a large test current. However, a large test current 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 can be avoided by using one of the following proce-dures:
• Ensuring that the magnitude of the test rent is only half that of the rated residual cur-rent IΔNof the RCD.
• Connecting the meter in front of the RCD or to a circuit that is not equipped with an RCD. • According to the manufacturer Chauvin Arnoux the patented ‘ALT system’ used in its C.A 6115 N and C.A 6456 Earth Clamps enables 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 conducted and the results compared with one another.
4.2
Connections to power supply
and earth electrode
The meter is typically connected to the power supply via its earthed safety plug. If the plug is
Functional earth electrode Earth resistance meter Earth resistance meter Earth electrode Functional earth electrode Earth electrode U0 – UM –––––––– I Rloop < RE < Rloop UE < U0 – UM U0 – UM –––––––– I Rloop < RE < Rloop UE < U0 – UM a) b) L1 L2 L3 PEN L1 L2 L3 N UM UE U0 UM UE U0 L UM RF RE RF RE EC EC I A V E S L UM I A V E S L N PE N PE MEB MEB RCD Earth resistance meter Earth resistance meter Functional earth electrode Probe Functional earth electrode Probe Earth electrode Other earth electrodes Earth electrode Other earth electrodes L1 L2 L3 N L1 L2 L3 N UM RF RE RF RE IE I IP I IP IE IE EC EC A V E S L UM IE A V E S L N N PE MEB 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 probe
a) Installation with TN system; b) Installation with TT system
Rlooploop resistance; U0conductor-to-earth voltage; UEvoltage 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 ammeter
a) Installation with TN system; b) Installation with TT system
IEportion of test current flowing to the earth electrode under test; IFportion of test current flowing to the other earth electrodes; CMC current measuring clamp
14
13
inserted incorrectly, no hazard arises but no measurement is possible. Although not shown in the figures, the internal circuitry of most of the 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 be taken into account when connecting the meter (see discussion in section 4.1 above). Depending on the type of meter used, the earth electrode 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 would yield erroneous results. It is for this reason that when measurements are made on consumer in-stallations with a TN earthing system, the earth-ing conductor EC has to be separated from the main earthing busbar MEB (see figures a) to a)) as the latter is connected via the PEN conductor of the service cable and the supply network to other earth electrodes. Disconnec-tion is not required in a TT system as the main earthing busbar is not linked to the neutral line of the supply network and the connection can be made as shown in figures b) to b).
4.3
Methods using a probe
This 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 of the measurement set-up is shown in figure . The probe has to be located so that it is outside the sphere of influence of the earth electrode. The voltage UMbetween the sockets E and S generates the test current I in the earth elec-trode.
4.4
Method using the PEN
con-ductor or neutral concon-ductor
instead of a probe
This measuring techniques can be used when-ever it is not possible to insert a probe into the ground at the right location. In this method (see figure ) the probe is replaced by connecting socket S of the meter to the PEN or PE ductor in a TN system or to the neutral con-ductor in a TT system. Caution! The neutral conductor must be treated as if it is live, even though it is earthed.
The value displayed by the meter includes the resistance to earth of the functional earth elec-trode. This will overestimate the resistance of the earth electrode and thus yield a value that errs on the side of safety.
The voltages generated by operating currents and by fault currents in the functional earth or in the PEN conductor or neutral conductor of the power supply system can result in erro-neous measurement results. The accuracy of this technique is therefore lower than that
achievable using the method described in section 4.3.
4.5
Method without a probe
This method (illustrated schematically in fig-ure ) involves measuring the resistance of a conductor loop via an earth return path. In this method, the voltage across the test object (UE) is not measured directly. It is determined as the difference between the potential drop between the phase conductor and earth when the test re-sistance is switched off (U0) and that when the test current I is flowing (UM). The resistance val-ue measured includes the resistances of the functional earth, the transformer and the phase conductor. This will result in an overestimation of the earth resistance and thus yield a value that errs on the side of safety.This method is particularly attractive as it can be performed with a minimum of effort. But it suffers from the weakness that supply load fluc-tuations that happen to occur simultaneously while the measurement is being made will cause significant additional measurement er-rors. To limit these errors, it is therefore expe-dient to work with a large test current. It is also advisable to perform numerous measurements, to reject any extreme values recorded and to compute the mean value from the remaining measurement data.
4.6
Selective earth resistance
measurements using a probe
and a clamp-on ammeter
The method selective earth resistance meas-urement8)is used if, for the purposes of the measurement, the earth electrode under test cannot 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 (see figure ) a special earth resistance meter (Chauvin Arnoux C.A. 6115N or C.A. 6456) and an additional current measuring clamp CMC are required. The current measuring clamp is connected to a multipole socket on the meter and the clamp jaws are placed around the earthing conductor EC connected to the earth electrode under test.If the meter is connected in this way and if the rotary selector switch set appropriately, IP, the portion of the measuring current I flowing via the other parallel earth electrodes, has no effect on measurement result so that the branch cur-rent IErecorded by the current measuring clamp CMC is solely responsible for determining the resistance to earth REdisplayed by the meter.
References
[1] IEC 60364-4-41:205 Erection of power installa-tions with nominal voltages up to 1000 V – Part 4-41: Protection for safety – Protection against electric 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 for testing, measuring or monitoring of protective measures.
[8] Hering, E.: Probleme mit einem der Erdungs-meßverfahren beim TN-System (engl.: Problems with an earth resistance measurement technique in 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.
8) On its own, the expression ‘selective earth mea-surement’ is ambiguous, as other earth resistance measurement techniques are also selective, e. g. those presented in sections 3.4, 3.5 and 3.6. 14
14
14
14
Reasons for and limits of
continuity testing
1.1
Earthing conductor and
main earthing busbar
The earthing system must be connected to the main earthing busbar (MEB) of the electrical in-stallation. This is necessary
a) 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 is not 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 and E.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 earthing conductor and/or an equipotential bonding con-ductor (protective bonding concon-ductor and/or lightning 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 electrodes
Continuity testing on ring earth electrodes is ad-visable for the following reasons:
a) Earth resistance measurements, which are required by the applicable standards but are not 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 the voltage protection, as the surge currents are forced to flow via another path.
b) Repeated continuity testing on older ring earth electrodes can, if carried out in the form of resistance measurements, identify reducti-ons in the conductor cross-section (as a re-sult of corrosion) by registering an increased resistance.
In order for continuity testing to be possible, a buried ring earth electrode must have at least
two soil entry points, while a foundation earth electrode must be equipped with at least two connection points. The more of these hook-up points 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 by the steel reinforcing bars within the foundation. In this case, continuity testing can only serve to verify that the connection between the con-nection points and the ring is intact.
1.3
Linear earth electrodes
Continuity testing is not possible with vertical earth 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 to where it enters the soil, this may be detectable as increased earth resistance.
Hazard avoidance
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. If the earth 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 only be broken if the electrical installation has been disconnected from the power source or power generator.
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 greater than 25 V should be avoided. Small voltages are anyway advisable for continuity testing (see [6], sec. 612.2). The testing and measuring equipment used must comply with the specifi-cations in the relevant standard (see ref. [7]). Resistance measurements are typically carried out using equipment that conforms with the specifications in reference [8]. However, earth resistance meters that meet the requirements in reference [9] can also be used.
Test methods
3.1
Principles
The person carrying out the test must be aware of all connections between the earthing system and other electrical and non-electrical installations. Examples of the latter include pipe systems or metallic structural components of a building. An unknown connection can fal-sify the test results. The continuity test can be carried out most simply if the conductor under test 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 be used when no other conductor is connected in parallel to the test object. The continuity tester shown in figure➊
a) consists of at least a power source and an indicator, e.g. a lamp. To in-crease the test current, a resistance can becon-3
2
1
Continuity testing
in earthing systems
E. Hering, Dresden (Germany)
Continuity tests are carried out to verify that conductors, in this case metal
conductors, are unbroken. This article describes the extent to which continuity
testing in earthing systems is required and possible and also discusses the test
equipment 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 are
treated separately in the relevant standards.
nected in parallel to the indicator. According to the recommendation in reference [6], sec-tion 612.2, the power source should generate an open circuit voltage of between 4 and 24 V and the test current should be at least 0.2 A. Some continuity testers produce an acoustic signal if the resistance measured exceeds a user-adjustable limit.
In figure
➊
b) the resistance is determined using a milliohmmeter. The advantage of the four-wire connection shown is that the result of the 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 meter only has two sockets and only two measuring leads are used, the resistances of the leads must be subtracted from the resistance value displayed by the meter. In order to measure the resistance of the leads their ends are connected together. Instead of using a milliohmmeter an earth resistance meter can be used as shown in figure➊
c) 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 figure➊
b) apply here anal-ogously.As indicated in figure
➊
d), the resistance can also be measured by means of a clamp-on ohm-meter such as the Earth Clamp1)C.A 6410 manufactured by Chauvin Arnoux. The resist-ance of the measuring lead needed to close the circuit must be subtracted from the resistance value displayed by the meter. If one or more conductors are connected in parallel to the test object, the same procedures discussed with re-gard to figureÀ
can be used. A power source is connected to the ends of the conductor under test. In figureÀ
a) continuity is verified whena current is detected by a clamp-on ammeter that is clamped around the conductor under test TC. If, as shown in figure
À
b), a voltmeter for measuring small voltages is connected across the conductor under test, the resistance of the conductor can be determined by dividing the measured voltage by the measured current. Continuity testing is carried out preferentially with alternating current, e.g. from a transformer or from an earth resistance meter. If a DC source is used, then a special type of clamp-on ammeter is required that contains a current sensor based on the Hall effect instead of a cur-rent transformer.3.2
Continuity testing in ring earth
electrodes that can be opened
This section describes examples of continuity tests performed on ring-shaped foundation earth electrodes. Many of these tests can also be carried out in the same way on buried ring earth electrodes.As already mentioned in section 1.2, the ease of performing a continuity test and the reliabil-ity of the results obtained depends on the num-ber and arrangement of connection points or soil entry points. Being able to open the ring earth electrode considerably simplifies continu-ity testing. These factors should be taken into account when planning the earthing system. The test circuits shown in figures
Ã
toŒ
can only be used if the ring can be opened at least one location. Foundation earth electrodes can be opened when there are two adjacent, flush-floor connection points that are linked conduc-tively to one another via a bridging strip that closes the ring. Buried ring earth electrodes may have an inspection joint located in an un-derfloor inspection box.Figure
Ã
shows the ring opened for testing. In the test circuits shown in figureÕ
continuity is verified by detecting current in the relevant parts 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-ureŒ
, the resistance of the ring is measured at the bridging strip or the underfloor inspection joint using a clamp-on ohmmeter (see ‘Earth Clamp’ in section 3.1).3.3
Continuity testing in ring
earth systems that cannot be
opened
If the ring earth electrode has at least four con-nection points, a break in ring continuity can be identified by sequentially measuring the resist-ance between two neighbouring connection points. The small resistances of the conductors that run between the point of connection of the meter leads and the ring have to be subtracted from 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, they have no significant effect on the measurement result because their resistivity is very much greater than that of steel.
If the ring is uninterrupted, the resistance mea-sured is that of a parallel circuit comprising the
TC PLC G CAM PS TC PLC G CAM PS CAM2 mV VM a) b) CT 1 2 3 CT 1 2 3 CT a) b) c) 1 2 3 a) b) G CA1 PS G CA2 PS 3 1 2 4 3 G CA1 CA2 CA3 CA4 I4 I3 I2 I1 I2 – I3 = I4 – I1 I c) 1 2 PS
À
Continuity testing circuits that mea-sure the current in a conductor that has another conductor connected to it in parallela) Without determining the resistance of the conductor
b) Resistance of conductor determined by making an additional voltage measurement PLC conductor connected in parallel (can also be a second test conductor) VM low-voltage voltmeter
CAM position of clamp-on ammeter CAM2 second position of clamp-on ammeter when PLC is a second test conductor
Ã
Examples of continuity tests when ring earth electrode is opena) Testing connections 1 and 3 and the left secti-on of the ring
b) Testing connections 2 and 3 and the right section of the ring
c) Testing the entire ring earth electrode
Õ
Examples of continuity tests when ring earth electrode has not been opened a) Analogous to fig.Ã
a);b) Analogous to fig.
Ã
b);c) Four tests on a foundation earth electrode with a cross connector and four connection points
I measured current; I1to I4partial currents; CA1 to CA4 positions of clamp-on ammeter
1) The semicircular jaws of the clamp contain two transformers. The purpose of one is to induce a voltage in the conductor, the other measures the resulting current. The clamp is closed when the jaws have been placed around the conductor. The display on the clamp meter displays a resistance value computed by dividing the value of the indu-ced voltage by the current flowing in the con-ductor.
