Resistance Reading From Line 1 to Neutral

Generally, it is more than probable for conductors to suffer harm 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 curtained 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 advisable times during structure and on completion of an installation, earlier the supply is continued, to verify the condition of the installed cables (Regulation 641.i).

In doing and so, any harm that might have occurred can be identified and rectified at an early stage of the projection, rather than at the completion phase: the structure may, for example, be tiled, busy and/ or fitted with cabinets or units making the remedial work much more than intrusive and costly.

Every bit required past Regulation 643.iii.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 exist connected together, where appropriate.)

Test precautions

When insulation resistance testing is carried out, especially on completion of electric work, precautions may exist necessary to avert 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 impairment from exposure to the exam voltage being applied should exist 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 depression voltage circuits may adopt ane of the following approaches:

Perform the test with the live (line and neutral) conductors connected together and examination between the live conductors and Earth only. (The term 'Globe' means that the protective conductor(southward) are connected to the earthing arrangement.)

Perform the insulation resistance test betwixt conductors at a reduced test voltage of 250 V DC. Nonetheless, 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 Excursion Details and Exam Results', and a characterization should exist fixed to the appropriate distribution board or consumer unit of measurement (Regulation 514.9.1 refers).

Performing the insulation resistance test

Wherever practicable, insulation resistance testing of low voltage circuits should be performed at 500 5 DC, and where appropriate the whole consumer unit or distribution board should exist subjected to the exam. However, before testing care must be exercised to confirm that it is safe for circuits to be tested collectively.

More often than not, to minimise the risk of damaging RCDs it may be advisable to examination circuits in groups but where necessary circuits may demand 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 chief switch, RCD and the protective device in the 'OFF' position.

For initial verification insulation resistance test readings should exist loftier 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 200 MΩ would exist expected for individual boards/circuits.

Parallel resistances

It should be recognised that, due to the parallel human relationship, the total insulation resistance of a group of circuits or a whole consumer unit of measurement will be less than the everyman individual circuit value. Therefore, given the private insulation resistance values shown in Fig 2, the guess insulation resistance value for all the circuits tested together will exist less than the lowest individual value, i MΩ.

Consequently, where circuits are tested collectively and a depression reading is obtained, private circuit testing should be undertaken to identify whether it is an private circuit that is responsible for the low reading.

Note: Having identified the circuit, inspection should be fabricated to confirm that all loads were asunder prior to testing.

Consumer unit replacement

Information technology should be noted that where an existing consumer unit of measurement is intended to exist 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 exam readings of similar circuits evidence marked differences in insulation resistance the circuits should be investigated to identify the cause (Regulation 643.3.2 refers). Low insulation resistance may exist 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 exist 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 exist possible to disconnect continued equipment, and so the examination and the test voltage may demand to be adjusted appropriately. 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 cavalcade for recording the voltage at which the insulation resistance test was performed.

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Source: https://professional-electrician.com/technical/insulation-resistance-low-voltage-circuits/

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