The aim of this article is to discuss the requirements and considerations for carrying out insulation resistance testing on low voltage circuits within electrical installations.
Such a test is used for verifying the condition of conductor insulation and connections between conductors and equipment.
Over time, terminations within accessories can become loose leading to high resistance joints, creating a risk of fire.
This typically occurs at accessories such as socket-outlets on radial and ring final circuits and may often go undetected through lack of periodic inspection and testing.
For example, where the main purpose is to identify any deficiencies present in an existing electrical installation and to verify it is safe for continued use.
Prior to carrying out periodic inspection and testing of the continuity of conductors, as required by Chapter 65 of BS 7671, the circuit(s) must be isolated from the supply.
An appropriate safe isolation procedure is outlined in the Electrical Safety First publication Best Practice Guide 2.
The presence of any protective bonding conductor(s) connected to metallic pipework at one or more points within an installation may also introduce parallel paths which could influence the test results.
Parallel paths may also conceal the fact that a protective conductor is not electrically continuous.
Where it is practical, such protective bonding should be temporarily disconnected before carrying out the test.
Regulation 643.2.1 states that resistance measurements should be undertaken to verify the continuity of conductors, including protective conductors, protective bonding conductors, and the live conductors of ring final circuits.
Circuits incorrectly connected or having open end loop conductors on ring final circuits may lead to overloading and hence damage of the circuit cables.
The expected measurements obtained during continuity testing should be of low ohmic value, although the measurements will vary with the circuit length and conductor cross-sectional area.
Typical instruments would include an ohmmeter having a low resistance range (typically 0-2 Ω or similar), or a multi-function instrument set to the continuity range.
Test instruments such as a multi-meter should not be used as these are unable to generate sufficient current for the test.
As part of the pre-test procedure, it will be necessary either to record the resistance of the test leads for subtraction from the overall measurement or use the auto-null function, a feature found on many instruments.
The end-to-end resistances obtained for each of the circuit conductors – the line (r1), neutral (rn) and the cpc (r2) – should be recorded in the three columns provided on the associated schedule of test results.
Ring final circuit
A ring final circuit by nature of its installation begins and ends at the same protective device to which the circuit is connected.
The line and neutral conductors forming the loop must remain unbroken without interconnections, as must the cpc, unless it is formed by a continuous metallic covering or enclosure, such as a steel conduit or trunking containment systems.
Typically, within a domestic property, the ring final circuit is more often than not wired using flat twin and earth cables having 2.5mm² line and neutral conductors and 1.5mm² cpc.
The resistance of the cpc will be proportionally higher than that of the line and neutral conductors due to the reduction in cross-sectional area (csa).
Where this is the case, the end-to-end resistance values measured for each conductor loop should result in the cpc having a resistance approximately 1.67 times that of the measured line or neutral conductor loop values.
This ratio relates to the difference in csa between conductors: 2.5/1.5 = 1.67. However, this may not always be the case.
It is important to recognise that conductors varying in length or having a csa other than 2.5mm² line and neutral and 1.5mm² for the cpc will result in a different resistance ratio other than 1.67.
Therefore, care must be applied when making a judgement in determining whether the test results are consistent for a particular size cable.
Any inaccurate decision may lead to the rejection of a healthy circuit and the recording of an inappropriate departure code on the report.
For some types of installations, it may be noticed that the ring final circuit has been wired using a flat twin and earth cable having an increased csa of conductors than is perhaps typical.
This may be, for example, to meet the design requirements for volt drop or where the cable is expected to pass through thermal insulation, or where high ambient temperatures are expected.
Where this is the case and the line and neutral conductors are larger than 2.5mm², the ratio of the resistance of the cpc to that of the line and neutral conductors will not be 1.67.
For example, where the flat twin and earth cable has 4.0mm² line and neutral conductors and a 1.5mm² cpc, the ratio would be approximately 2.67 times greater.
This method also applies for other sizes of flat twin and earth cables having a reduction of cpc.
Cables having same size conductors
Ring final circuits in many non-domestic properties may be wired using other types of cable such as a steel wire armoured (SWA), mineral insulated (MI) or single-core cables.
Such types of cable may result in the cpc having an equivalent csa as the line and neutral conductor; in which case, the end-to-end resistance of the cpc loop should be more or less equal to that of the line and neutral conductor loop, typically within ± 0.o5 Ω.
N.B. the value ± 0.o5 is based on a general rule of thumb relating to the characteristics of the testing instrument(s).
Exposed-conductive-part utilised as a protective conductor
Where cables have a metallic sheath or braid, or where circuits are installed within a metallic conduit or trunking system, and the circuit protective conductor forms part of the metallic path as permitted by Regulation 543.2.2, the ratio method previously mentioned would not be appropriate.
Where this is the case, the size of the cpc shall be calculated in accordance with Regulation 543.1.3, or alternatively selected in accordance with Regulation 543.1.4.
Care should be taken to ensure that the test results obtained for a particular circuit are consistent for cable size.
Any inaccurate decision may lead to the rejection of a healthy circuit.
Factors that can affect the measured value of the end-to-end loop resistance include the length of run, cross-sectional area and type conductor material.
In addition, the presence of parallel paths may also influence such test results.
For further guidance on maximum length of run for ring final circuits in domestic premises, refer to Appendix A of the Site Guide published by NICEIC and ELECSA.
For more news from the NICEIC, click here.