Generally, it is more likely for conductors to suffer damage during the construction phase of an installation than when the installation is in service.
For example, cables pulled through conduit without due care may suffer abrasion whereas those concealed in walls and partitions might suffer damage as a result of misplaced fixings.
In view of the risks, BS 7671 requires insulation resistance testing to be carried out at appropriate times during construction and on completion of an installation, before the supply is connected, to verify the condition of the installed cables (Regulation 641.1).
In doing so, any damage that might have occurred can be identified and rectified at an early stage of the project, rather than at the completion stage: the structure may, for example, be tiled, decorated and/ or fitted with cabinets or units making the remedial work much more intrusive and costly.
As required by Regulation 643.3.1 of BS 7671: 2018, insulation resistance should be verified by measuring between:
live conductors (line and neutral); and
between live conductors and the protective conductor connected to the earthing arrangement.
(For the purpose of this test, the line and neutral conductors may be connected together, where appropriate.)
Test precautions
When insulation resistance testing is carried out, especially on completion of electrical work, precautions may be necessary to avoid damage to electronic devices, such as heating timers, controllers, lighting dimmer controls and the like. Equipment which might affect the test results and/or which might suffer damage from exposure to the test voltage being applied should be disconnected for the purpose of the test.
Where it is not reasonably practicable to disconnect electronic devices, such as where a fixed low voltage socket-outlet incorporates a surge protective device (SPD), the person carrying out insulation resistance testing on low voltage circuits may adopt one of the following approaches:
Perform the test with the live (line and neutral) conductors connected together and test between the live conductors and Earth only. (The term ‘Earth’ means that the protective conductor(s) are connected to the earthing arrangement.)
Perform the insulation resistance test between conductors at a reduced test voltage of 250 V DC. However, where this option is used, the minimum acceptable insulation resistance remains 1 MΩ.
For the benefit of those who undertake future testing on the installation, whether as part of planned maintenance work or periodic testing, any circuit that has equipment connected to it which is vulnerable to damage from insulation resistance testing should be identified on the ‘Schedule of Circuit Details and Test Results’, and a label should be fixed to the appropriate distribution board or consumer unit (Regulation 514.9.1 refers).
Performing the insulation resistance test
Wherever practicable, insulation resistance testing of low voltage circuits should be performed at 500 V DC, and where appropriate the whole consumer unit or distribution board should be subjected to the test. However, before testing care must be exercised to confirm that it is safe for circuits to be tested collectively.
Generally, to minimise the risk of damaging RCDs it may be appropriate to test circuits in groups but where necessary circuits may need to be tested individually. As shown in Fig 1, where circuits are tested individually, the test should be performed on the load side of each protective device, with the main switch, RCD and the protective device in the ‘OFF’ position.
For initial verification insulation resistance test readings should be high and values much higher than the minimum value of 1 MΩ given in Table 64 of BS 7671 should be obtained. Generally readings of more than 200 MΩ would be expected for individual boards/circuits.
Parallel resistances
It should be recognised that, due to the parallel relationship, the total insulation resistance of a group of circuits or a whole consumer unit will be less than the lowest individual circuit value. Therefore, given the individual insulation resistance values shown in Fig 2, the approximate insulation resistance value for all the circuits tested together will be less than the lowest individual value, 1 MΩ.
Consequently, where circuits are tested collectively and a low reading is obtained, individual circuit testing should be undertaken to identify whether it is an individual circuit that is responsible for the low reading.
Note: Having identified the circuit, inspection should be made to confirm that all loads were disconnected prior to testing.
Consumer unit replacement
It should be noted that where an existing consumer unit is intended to be replaced the condition of the existing cables should be determined before proceeding with the replacement. Otherwise, insulation defects that previously were undetected may cause the operation of RCDs.
Where the test readings of similar circuits show marked differences in insulation resistance the circuits should be investigated to identify the cause (Regulation 643.3.2 refers). Low insulation resistance may be due to the age of the installation, or alternatively it may be caused by an individual circuit, which would merit further investigation.
In summary, insulation resistance testing should be undertaken at relevant times during and on completion of an installation. When insulation resistance testing is performed at the completion stage of an installation, it may not be possible to disconnect connected equipment, so the test and the test voltage may need to be adjusted accordingly. In view of this the Schedule of Circuit Details and Test Results which forms part of an NICEIC Electrical Installation Certificate (EIC) and a Domestic Electrical Installation Certificate (DEIC) includes a column for recording the voltage at which the insulation resistance test was performed.
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