The publication of Amendment 2 of BS 7671 (AMD2) introduced several changes to the requirements for insulation resistance testing within Part 6 of that standard. This article from the experts at NICEIC explains these changes and outlines the considerations for those carrying out such testing.
There is a fundamental requirement within Section 134 of BS 7671 that every electrical installation is subjected to appropriate inspection and testing:
• during construction; and
• on completion
before it is put into service to confirm compliance with the relevant requirements of that standard (134.2.1).
This is supported by regulation 641.1 which calls for inspection and testing to be carried out both during the construction of the installation and on completion of the work. This requirement is particularly relevant to the insulation resistance testing of cables during construction.
The requirements for insulation resistance testing have been amended to clarify that all installed conductors must be subjected to testing at a voltage in excess of the voltage that they will carry in normal service to confirm that they remain serviceable after being installed.
It is not sufficient to try and verify this by applying a DC test voltage which is more-or-less the same as the normal operating voltage. The use of a higher-than-normal operating voltage test is more likely to detect conductor or insulation damage.
Why do we carry out insulation resistance tests on cables?
Any wiring can suffer damage to the insulation or conductors during the installation process or once it is installed, but prior to being energised. Unfortunately, it is often overlooked that regulation 641.1 calls for inspection and testing to be carried out, not only on completion, but also during construction.
Testing at relevant times during the construction phase helps to ensure that any damage that may have occurred is identified and rectified at an early stage of the project, rather than after completion, which would make any necessary remedial work much more intrusive and costly.
Effective insulation of conductors is necessary to provide basic protection and to prevent short-circuits and earth faults. Unintended leakage currents due to inadequate insulation can present a risk of electric shock to persons and livestock. This can also lead to further deterioration to the insulation and conductors if allowed to persist, which may present a fire risk.
It is important therefore that a test conducted at a voltage in excess of the voltage at which the circuit will subsequently operate, is carried out using a suitable test instrument according to BS EN 61557-2, to highlight any such damage.
What are the testing requirements in BS 7671?
Regulation 643.3.1 requires that the insulation resistance of installed circuits is measured:
• between live conductors; and
• between live conductors and the protective conductor connected to the earthing arrangement, with the line and neutral conductors connected together, where appropriate.
This measurement shall take place at the appropriate DC test voltage given in Table 64 of BS 7671, reproduced as Table 1, according to the circuit’s nominal voltage at least once after the installation of the cables.
Regulation 643.3.3 has been modified and now states that:
• Where connected equipment is likely to influence the measurement or the result obtained from testing, or be damaged, the test at the appropriate DC test voltage given in Table 64, shall be applied prior to the connection of such equipment; and
• After connection of the equipment, a test at 250 V DC shall be applied between live conductors and the protective conductor connected to the earthing arrangement. The measured insulation resistance obtained from this test shall have a value of at least 1 MΩ.
Such testing carried out after the connection of the equipment is primarily to confirm the effectiveness of those connections.
A new note to this requirement states that some manufacturer’s instructions may still advise that certain equipment must be disconnected prior to carrying out the 250 V DC test to avoid its influencing any test results obtained.
Summary
Anyone carrying out inspection and testing of electrical installations designed and installed in accordance with the requirements of BS 7671:2018+A2:2022 (AMD2) must familiarise themselves with the changes made to the requirements for insulation resistance testing.
In the case of initial verification, all conductors shall be tested at least once after installation at the test voltage indicated in Table 64 of BS 7671, appropriate to the circuit’s nominal voltage. This should preferably be done prior to the connection of any equipment that is likely to influence the measurement or result of the test, or to be damaged by the test voltage.
This test is to verify that the conductors remain in a serviceable condition post-installation. Subsequently, once equipment is connected, a test at 250 V DC shall be performed between the live conductors and the protective conductor connected to the earthing arrangement. This test verifies the connections made since the 500 V DC test was performed.
Although not stated in BS 7671, when carrying out testing during a periodic inspection, it would be logical to perform insulation resistance testing at 250 V DC initially due to the presence of items of connected equipment that may either be damaged by a higher applied test voltage or might influence the results obtained during testing.
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