Interpretation of the Bonding Conductor Continuity Test

8.4.1 - Protective conductor continuity

All protective and bonding conductors must be tested to ensure that they are electrically safe and correctly connected. {8.7.1} gives test instrument requirements. Provided that the supply is not yet connected, it is permissible to disconnect the protective and equipotential conductors from the main earthing terminal to carry out testing. Where the mains supply is connected, as will be the case for periodic testing, the protective and equipotential conductors must not be disconnected because if a fault occurs these conductors may rise to a high potential above earth. In this case, an earth-fault loop tester can be used to verify the integrity of the protective system.

Where earth-fault loop impedance measurement of the installation is carried out, this will remove the need for protective conductor tests because that conductor forms part of the loop. However, the loop test cannot be carried out until the supply is connected, so testing of the protective system is necessary before supply connection, because connection of the supply to an installation with a faulty protective system could lead to danger.

There are three methods for measurement of the resistance of the protective conductor.

1. - Using the neutral conductor as a return lead
A temporary link is made at the distribution board between neutral and protective conductor systems. Don't forget to remove the link after testing. The low resistance tester is then connected to the earth and neutral of the point from which the measurement is taken (see {Fig 8.2}). This gives the combined resistance of the protective and neutral conductors back to the distribution board. Then

Rp = R x	An
	An + Ap

where

Rp - is the resistance of the protective conductor

R  - is the resistance reading taken

An - is the cross-sectional area of the neutral conductor

Ap - is the cross-sectional area of the protective conductor.

Note that the instrument reading taken in this case is the value of the resistance R1 + R2 calculated from {Table 5.5} (see {8.4. 4 }) . This method is only valid if both conductors have the same length and both are copper; in most cases where steel conduit or trunking is not used as the protective conductor, the test will give correct results.

Fig 8.2 - Protective conductor continuity test using the
neutral conductor as the return lead

2. - Using a long return lead
This time a long lead is used which will stretch from the main earthing terminal to every point of the installation.First, connect the two ends of this lead to the instrument to measure its resistance. Make a note of the value, and then connect one end of the lead to the main earthing terminal and the other end to one of the meter terminals.

Second, take the meter with its long lead still connected to the point from which continuity measurement is required, and connect the second meter terminal to the protective conductor at that point.The reading then taken will be the combined resistance of the long lead and the protective conductor, so the protective conductor value can be found by subtracting the lead resistance from the reading.

Rp = R - RL

where

Rp - is the resistance of the protective conductor

R - is the resistance reading taken

RL - is the resistance of the long lead

Some modem electronic resistance meters have a facility for storing the lead resistance at the touch of a button, and for subtracting it at a further touch.

3. - Where ferrous material forms all or part of the protective conductor
There are some cases where the protective conductor is made up wholly or in part by conduit, trunking, steel wire armour, and the like. The resistance of such materials will always be likely to rise with age due to loose joints and the effects of corrosion. Three tests may be carried out, those listed being of increasing severity as far as the current-carrying capacity of the protective conductor is concerned. They are:

1 - A standard ohmmeter test as indicated in 1 or 2 above. This is a low current test which may not show up poor contact effects in the conductor. Following this test, the conductor should be inspected along its length to note if there are any obvious points where problems could occur.

2 - If it is felt by the inspector that there may be reasons to question the soundness of the protective conductor, a phase-earth loop impedance test should be carried out with the conductor in question forming part of the loop. This type of test is explained more fully in {8.4.4}

3 - If it is still felt that the protective conductor resistance is suspect, the high current test using 1.5 times the circuit design current (with a maximum of 25 A) may be used (see {Fig 8.3}. The protective circuit resistance together with that of the wander lead can be calculated from:

voltmeter reading (V)

ammeter reading (A)

Fig 8.3 - High-Current ac test of a protective conductor

Subtracting wander lead resistance from the calculated value will give the resistance of the protective system.

The resistance between any extraneous conductive part and the main earthing terminal should he 0.05 Ohms or less; all supplementary bonds are also required to have the same resistance.

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Source: https://www.tlc-direct.co.uk/Book/8.4.1.htm

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