8.6.1 - Testing earth electrodes

The earth electrode, where used, is the means of making contact with the general mass of earth. Thus it must be tested to ensure that good contact is made. A major consideration here is to ensure that the electrode resistance is not so high that the voltage from earthed metalwork to earth exceeds 50 V. Where an RCD is used, this means that the result of multiplying the RCD operating current (in amperes) by the electrode resistance (in ohms) does not exceed 50 (volts). for normal dry locations, or 25 (volts) for construction sites and agricultural premises.

If a 30 mA RCD is used, this allows a maximum electrode resistance of 1,666 Ohms, although it is recommended that earth electrode resistance should never be greater than 200 Ohms.  A maximum value of 100 ohm is proposed in a draft amendment of BS 7430, Code of Practice for Earthing.

There are several methods for measurement of the earth electrode resistance. In all cases, the electrode must be disconnected from the earthing system of the installation before the tests commence.

Fig 8.13 - Measurement of earth electrode resistance with a dedicated tester

1. -   Using a dedicated earth resistance tester
The instrument is connected as shown in {Fig 8.13} with terminals C1 and P1 being connected to the electrode under test (X). To ensure that the resistance of the test leads does not affect the result, separate leads should be used for these connections. If the test lead resistance is negligible, terminals Ci and P1 may be bridged at the instrument and connected to the earth electrode with a single lead.

Terminals C2 and P2 are connected to temporary spikes which are driven into the ground, making a straight line with the electrode under test. It is important that the test spikes are far enough from each other and from the electrode under test. If their resistance areas overlap, the readings will differ for the reason indicated in {Fig 8.14}. Usually the distance from X to Y will be about 25 m, but this depends on the resistivity of the ground. To ensure that resistance areas do not overlap, second and third tests are made with the electrode Z 10% of the X to Y distance nearer to, and then 10% further from, X. If the three readings are substantially in agreement, this is the resistance of the electrode under test. If not, test electrodes Y and Z must be moved further from X and the tests repeated.

The tester provides an alternating output to prevent electrolytic effects. If the resistance to earth of the temporary spikes Y and Z is too high, a reduction is likely if they are driven deeper or if they are watered.

Fig 8.14 - Effect of overlapping resistance areas
a) resistance areas not overlapping
b) resistance areas overlapping

 

2. - Using a transformer, ammeter and voltmeter
The system is connected as shown in {Fig 8.15}. Current, which can be adjusted by variation of the resistor R, is passed through the electrode under test (X) to the general mass of earth and hence to the test electrode Y. The voltmeter connected from X to Z measures the volt drop from X to the general mass of earth. The electrode resistance is calculated from:

voltmeter reading (V)

ammeter reading (A)

As in the case of the dedicated tester, the test electrode Z must again be moved and extra readings taken to ensure that resistance areas do not overlap. It is important that the voltmeter used has high resistance (at least 200 Ohms /V) or its low resistance in parallel with that of the electrode under test will give a false result.

Fig 8.15 - Measurement of earth electrode resistance
with a transformer, ammeter and voltmeter

3. - Using an earth fault loop impedance tester
The tester is connected between the phase at the origin of the installation and the earth electrode under test as shown in {Fig 8.16}. The test is then carried out, the result being taken as the electrode resistance although the resistance of the protective system from the origin of the installation to the furthest paint of the installation must be added to it before its use to verify that the 50 V level is not exceeded. If an RCD with a low operating current is used, the protective system resistance is likely to be negligible by comparison with the permissible electrode resistance.

Fig 8.16 - Measurement of earth electrode resistance using an earth-fault loop tester

It is most important to ensure that earthing leads and equipotential bonds are reconnected to the earth electrode when testing is complete.