NAPIT’s Don Holmes details the two most common methods of measuring electrode resistance where there is a single earth electrode for the electrical installation.
With the growing number of electrical installations incoporating an earth electrode, electrical contractors are increasingly being called upon to measure the resistance to Earth of electrodes in order to comply with Regulation 643.7.2 of BS 7671. Contractors who are unaccustomed to this test, can find it challenging.
This article will offer some help by explaining the two most common methods of measuring electrode resistance where there is a single earth electrode for the electrical installation.
Measurement using a dedicated soil resistivity/earth electrode tester
This method of testing is usually used where an electricity supply isn’t available. Dedicated earth electrode resistance testers generally have test probes and coloured connecting test leads. Earth electrode resistance testers are available from different manufacturers and the test leads to the test probes and earth electrode under test must be in accordance with the manufacturer’s instructions.
The earth electrode test requires the use of two temporary test probes and is carried out as follows:
a) isolate the electrical installation if connected to the supply at the main switch, verify the electrical supply is off using an approved voltage indicating device, lock off and label.
b) disconnect the earthing conductor from the earth electrode.
c) prepare the test instrument for use, checking that the instrument, test leads and test probes are in good condition and suitable for use.
d) insert the temporary test probes into the ground with the first one being 15 to 25 metres from the electrode under test and the other 30 to 50 metres from the electrode under test, as illustrated in Fig 1.
Connection to the earth electrode is made using terminals C1 and P1 of a four terminal test meter. To be able to exclude the resistance of the test leads from the reading, individual leads should be taken from the test instrument terminals and connected separately to the electrode under test and the temporary test probes.
Where the resistance of the test leads is negligible, the test meter C1 and P1 terminals may be linked together and a single test lead used, as would be the case if using a three terminal test meter.
Take the following three resistance readings:
1. with the protection test probe T2 inserted in the soil midway between the electrode under test and the current test probe T1.
2. with the potential test probe T2 moved to position 3 metres back towards the electrode under test.
3. with the potential test probe T2 moved to a position 3 metres from its original position towards test probe T1.
The distance between the test spikes is very important. If they’re too close together their resistance areas will overlap, resulting in an inaccurate value of resistance for the earth electrode under test. In order to obtain reliable results, the distance between the electrode under test and the current test probe C2 should be at least ten times the maximum dimensions of the electrode system. As an example, when the earth electrode under test is 3 metres long, a distance of 30 metres is required.
On completion of the test and a satisfactory resistance reading, reconnect the earthing conductor to the earth electrode, remove the main switch locking device and label and restore the electrical supply.
Measurement using an earth fault loop impedance tester
The earth electrode test is carried out as follows:
• isolate the electrical installation at the main switch, verify the electrical supply is off using a voltage indicating device with proving unit, lock off and label.
• disconnect the earthing conductor to the electrode at the main earthing terminal – this method allows the test current to flow through the earth electrode alone and reduces the likelihood of any problems with parallel earth paths.
• prepare the test meter for use, checking that the test meter, leads, probes and clips are suitable for use.
• connect one lead of the earth loop impedance tester to the earthing conductor going to the electrode, then connect the other lead to the line conductor at the incoming side of the main switch for the installation (as illustrated in Fig 2) and carry out the test.
• note the reading which should be taken to be the earth electrode resistance.
• reconnect the earthing conductor, remove the main switch locking device and label and restore the electricity supply.
Reducing earth resistance
Table 41.5 note 2 of BS 7671 states that a value exceeding 200? may not be stable.
The resistance of an earth electrode to Earth can be reduced by installing it deeper using extendable rod- type electrodes. The resistance of an earth electrode to the general mass of Earth reduces rapidly with the first metre or so. This reduction in resistance is less marked at depths greater than 2 to 3 metres in soil of uniform resistivity.
Other factors
Moisture
The greater the moisture content of the soil, the more the resistivity is reduced. Moisture content is an important consideration in areas of large seasonal variation in rainfall. Ideally an earth electrode should be installed at a depth deep enough to reach the water table or present water level with salt content.
Temperature
When soil becomes frozen, its resistivity rises but this only becomes significant near or below freezing point. The permanent frost-free moisture level is often some metres below the surface. Regulation 542.2.4 should be consulted.
Chemical additives
The contact resistance of an earth electrode with the surrounding soil can be improved by adding chemical additives to the soil. However, rainwater usually disperses the chemicals over time and this method requires constant monitoring. Care must be taken to ensure the chemicals used don’t harm the environment or cause damage to the electrode.
Conclusion
An earth electrode is an important part of a protective measure of an electrical installation. The designer of the installation must be satisfied that it will provide a permanent and reliable connection to Earth with a suitably low value of resistance to Earth throughout its period of use.
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