In the second part of our special Technical Update series, we take a closer look at the tests required as part of the verification process for a new electrical installation.
Before proceeding with any tests, it’s important that you give some consideration to the test sequence. This is because initial tests are carried out before the supply is connected – i.e. ‘power-off’ tests – but later tests are carried out with the power on.
You should be aware that all ‘power-on’ tests are regarded by the Health and Safety Executive (HSE) as live working, and therefore suitable safety precautions should be implemented to control this hazard. In some instances, they may have to be documented.
Guidance on suitable safety precautions is contained in the HSE document HSG85 Electricity at Work Safe Working Practices and should be referred to where necessary.
The adequacy of the test equipment, and associated test leads in particular, will need to be confirmed as satisfactory prior to testing, and a regime of self-check and regular calibration may help when assessing the condition of the equipment. It’s also important that you have a good working knowledge of the correct operation of the test equipment.
TEST 1: Continuity of protective conductors, including main and supplementary equipotential bonding
The first test in Chapter 61 is Continuity of Conductors, which requires in Regulation 612.2.1 that a continuity test be undertaken to confirm the presence and suitability of protective conductors including main and supplementary bonding.
This is particularly important where the protective measure automatic disconnection of supply (ADS) is used to provide protection against electric shock in an installation, which is generally the case.
ADS requires protective earthing and protective equipotential bonding for fault protection purposes and circuit protective conductors (CPCs) at each point in wiring and accessories to connect the exposed conductive parts of electrical equipment to the earthing system provided for the installation (TN-S, TN-C-S or TT).
The CPCs will need to be verified as being of suitable size and correctly terminated. They must also be of sufficiently low resistance to allow the required amount of fault current to flow to operate the circuit overcurrent protective device – fuse or circuit breaker – during a fault within the required 0.4 or 5 second disconnection time for compliance with Regulation 422.214.171.124.
Main protective bonding conductors are provided for slightly different reasons and connect any extraneous conductive parts, such as gas or water installation pipes, to the exposed conductive parts of the installation. They equalise the voltage that could appear between these parts during a fault before the disconnection occurs, limiting the touch voltage to a safe value which is generally regarded as being less than or equal to 50V.
The importance of ensuring that all these conductors are installed correctly and are unbroken and continuous to the end of the circuit is rather obvious. However, Regulation 612.2.1 does not provide information on how such a continuity test should be carried out, other than to require that the test equipment used for the test is capable of applying a test voltage between 4V and 24V DC or AC, and passes a short circuit current of not less than 200mA.
However, there is plenty of industry guidance available, and the test procedures that have evolved to test the continuity of protective conductors provide two specific options, known as Test Method 1 and Test Method 2. Be aware that these methods are only suitable for testing radial circuits. They should not be used to test the continuity of ring final circuit conductors, which requires a three-step test method to verify all conductors.
Benefits of Test Method 1
The decision to use one method over another is entirely down to your particular preference. However, Test Method 1 does make it somewhat easier to determine the earth fault loop impedances required later in the test.
Test Method 1 measures the sum of the resistance of the line conductor (R1) and the resistance of the CPC (R2) to give a value that is the combined (R1+R2) value for a circuit.
This can be used to determine the earth fault loop impedance of a circuit using the formula Zs = Ze + (R1+R2) later in the test process. Zs is the circuit earth fault loop impedance and Ze is the part of the earth fault loop impedance that is external to the installation (see Figure 1).
This also has the benefit of reducing the amount of live testing necessary because it only requires a power-on test to measure Ze, rather than multiple power-on tests to measure the earth fault loop impedances of all circuits. (This may have been specifically identified when assessing pre-testing safety precautions to reduce the risk to persons associated with working live!) Test Method 1 can also be used to verify polarity and does not require a long lead.
Disadvantages of Test Method 1
The downside of this method is that it’s limited to testing of all insulated wiring systems. Where there are connections from the wiring system to exposed conductive parts and/or extraneous conductive parts – for instance via a metal back box connected to steel conduit or trunking containment system or to the armour of SWA cables – this will result in parallel paths and the measured value of the protective conductors would be very low.
Any R1+R2 value obtained would therefore not be accurate for the purposes of verifying Zs and Test Method 2 may be a more suitable option to verify continuity. This is because it would be virtually impossible to disconnect all of these types of connection in an installation prior to testing.
Performing a continuity test using Test Method 1
This is done using a low resistance ohmmeter or a multi-function test instrument with a continuity setting capable of applying the required test voltage and current.
To carry out the test, you should identify the circuit to be tested, then disconnect both the line conductor and CPC at the distribution board or consumer unit. A temporary connection should be made to connect the conductors together, e.g. using a terminal strip connector block, and you can then proceed to test at the furthest part of the circuit (see Figure 1).
You should hopefully have identified the furthest part of the circuit during the inspection process carried out prior to testing. However, it may be difficult to clearly identify the furthest point of a particular circuit in some installations, in which case you may have to carry out a number of continuity tests at several different points in the circuit. For example, in a lighting circuit there may be a number of lighting points and connections to other equipment, such as an extractor fan.
Please note, the connection between the line conductor and CPC could alternatively be made at the furthest part of the circuit where known, and the measurement taken at the distribution board depending on your preference.
Before measuring the R1+R2 of the circuit, you should also remember to measure the resistance of the test leads, including any associated clip or probe, and note the value.
This will have to be deducted from the measured resistance value obtained when testing the circuit. Alternatively, if the test equipment has a test lead null feature, this can be used before carrying out the test.
The test leads can then be connected between the line conductor and the circuit’s protective conductor to obtain a measurement of resistance. Depending on the instrument being used, this may require the test instrument test button to be pressed. On other types, the test proceeds automatically when the test probes are connected to the terminals of the circuit.
If the circuit is complete and unbroken, the circuit resistance value will be displayed on the test instrument. The resistance of the conductors will obviously depend on the length and the cross-sectional area of the conductors of the circuit being measured, and also on the tightness of cable terminations, but typically will be less than 1Ω.
If there is a break or loose connection in the line or circuit protective conductor, the value shown will be a high value, usually displayed as the maximum value that the instrument can display >200Ω.
However, before assuming this, it may be useful to check if there are any switches in the circuit that haven’t been switched on to complete the circuit. Also remember to test the continuity of all line conductors in a circuit, including strappers or pass wires that may have been installed to facilitate two-way switching on lighting circuits.
On completion of the test, the result should then be recorded in the appropriate column of the schedule of test results, specifically the R1+R2 column (see Figure 2).
Performing a continuity test using Test Method 2
This measures only the R2 resistance of the CPC and, like Method 1, is performed using a suitable continuity test instrument or multi-function test instrument set to the continuity setting.
To carry out the test, a lead is required that is long enough to reach to the furthest point of the installation – commonly known as a ‘wandering lead’. One end is connected to the main earthing terminal, with the other connected to the continuity test instrument. The remaining test lead is then used to connect to the CPC of the circuit using a test probe, with the highest value measured being the furthest point of the circuit.
This method is therefore very useful if you haven’t been able to identify the furthest point prior to testing, as it’s relatively easy to carry out a test at various points of the circuit using a test probe (see Figure 3).
Before recording the highest value obtained, remember that this will need to be compensated for the resistance of both the test lead and the wandering lead. The compensated value can then be recorded in the relevant continuity column in the schedule of test results, specifically the R2 column.
The benefit of using Test Method 2 is that it’s relatively easy to verify the connection to earth at various points of the circuit, particularly accessible exposed conductive parts of the installation, such as metal accessories or equipment.
The same method will also be used to verify the connection to earth of the main protective conductors of the installation, including the earthing conductor, any main protective bonding conductors connected to extraneous conductive parts, and supplementary bonding conductors.
Although the connection and continuity of the installation earthing conductor and any main protective bonding conductors require to be verified – typically a low value of approximately 0.05Ω would be expected – the resistance values obtained will not require to be recorded on the schedule of test results. The connection and continuity of these important protective conductors does, however, have to be indicated as having been verified on Page 2 of the Electrical Installation Certificate.
Finally, after testing the continuity of protective conductors of all radial circuits supplied from the distribution board or consumer unit, remember to reinstate the circuits to their original completed condition before moving onto the next test in the test sequence.