How to determine the resistance of earth electrode?

Source: City and Guilds

Reason for earth electrode resistance testing

The purpose of this test is to establish that the resistance of the soil surrounding an earth electrode is suitable and that the electrode makes contact with the soil

Recognised types of earth electrode

The following types of earth electrode are recognised:
1.    earth rods or pipes
2.    earth tapes or wires
3.    earth plates
4.    underground structural metalwork, embedded in foundations
5.    welded metal reinforcement of concrete, embedded in the earth
6.    metal sheaths and coverings of cables (subject to Regulation 542.02 to 05 in BS 7671)
7.    other suitable underground metalwork .

Example (Refer to the diagram above)

There are two temporary test electrodes/spikes (T1 and T2) which must be inserted into the ground. These are normally supplied with the test instrument.
  • C2 -  terminal on the meter is connected to T1 via a long lead, ideally 30-50 m away from the electrode under test.
  • P2 -  terminal on the meter is connected to T2 via a long lead, and is centrally positioned between T1 and the electrode under test Ideally, the distance between the earth electrode and test spike T1 should be ten times the length of the electrode under test, but this dimension is likely to be affected by the location of the electrode and any surrounding buildings, paths or driveways, for example.
  • C1 - earth electrode being tested (Ra)

Three readings are taken during this test, with test spike T2 moved for each reading. The distance T2 is moved for the second and third readings depends on the distance between the electrode and spike T1 . 

If the distance between them is 30 m then, typically, T2 will be moved 10% of that distance, which is 3 meter. So, the first test is taken with spike T2 in the central position, the second test with the spike moved 10% closer to the earth electrode and the third test with the spike moved 10% from the centre, away from the earth electrode.

Here we will consider example readings for an earth electrode in good soil or clay 

(the earth electrode is 3 meter long, so the distance between the electrode and test spike T2 is 30 m):

·         with T2 central = 72 ohms
·         with T2 3 m closer to the electrode under test = 70.5 ohms
·         with T2 3 m closer to T1 = 73.5 ohms

Evaluation of test results
Once the three results have been obtained, the average of the three is found. So, using the example values given above, the average reading is: 72 ohms

The three readings obtained should fall within a tolerance of 5% of the average, so 5% of 72 is 3.6 ohms so a tolerance of ± 5% gives 75.6 ohms and 68.4 ohms.

As the three readings all fall within this 5% tolerance, they are acceptable and the average value (72 ohms) would be recorded as the earth electrode resistance (Ra = 72 ohms).

Note: If the deviation exceeds 5%, further tests must be carried out with a larger separation between the earth electrode under test and spike T1.

 Acceptable test values for an earth electrode

Earth electrode resistance values can differ greatly, dependent on the type of ground and environmental conditions, the material of the electrode used and area of contact with the general mass of earth. 

It is recommended that the earth electrode resistance test is carried out when the ground conditions are least favorable, such as during dry weather. 

Note: If the reading of the three values are above 200 ohms, the soil condition may not be stable, as soil conditions change due to factors such as soil drying and freezing.


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