8.5.1 - Testing insulation resistance

A low resistance between phase and neutral conductors, or from live conductors to earth, will result in a leakage current. This current could cause deterioration of the insulation, as well as involving a waste of energy which would increase the running costs of the installation. Thus, the resistance between poles or to earth must never be less than half of one meg ohm (0.5 M Ohms) for the usual supply voltages. In addition to the leakage current due to insulation resistance, there is a further current leakage in the reactance of the insulation, because it acts as the dielectric of a capacitor. This current dissipates no energy and is not harmful, but we wish to measure the resistance of the insulation, so a direct voltage is used to prevent reactance from being included in the measurement. Insulation will sometimes have high resistance when low potential differences apply across it, but will break down and offer low resistance when a higher voltage is applied. For this reason, the high levels of test voltage shown in {Table 8.8} are necessary. {8.7.1} gives test instrument requirements.

Before commencing the test it is important that:

1. - electronic equipment which could be damaged by the application of the high test voltage should be disconnected. Included in this category are electronic fluorescent starter switches, touch switches, dimmer switches, power controllers, delay timers, switches associated with passive infrared detectors (PIRs), RCDs with electronic operation etc. An alternative to disconnection is to ensure that phase and neutral are connected together before an insulation test is made between them and earth.

2. - capacitors and indicator or pilot lamps must be disconnected or an inaccurate test reading will result.

Table 8.8 - Required test voltages and minimum resistance
Nominal circuit voltage	Test voltage (V)	Minimum insulation resistance (M Ohms)
Extra-low voltage circuits supplied from a safety transformer	250	0.25
Up to 500 V except for above	500	0.5
Above 500 V up to 1000 V	1000	1.0
The insulation resistance tester must be capable of maintaining the required voltage when providing a steady state of current of 1mA.

Where any equipment is disconnected for testing purposes, it must be subjected to its own insulation test, using a voltage which is not likely to result in damage. The result must conform with that specified in the British Standard concerned, or be at least 0.5 M Ohms if there is no Standard.

The test to earth {Fig 8.10} must be carried out on the complete installation with the main switch off, with phase and neutral connected together, with lamps and other equipment disconnected, but with fuses in, circuit breakers closed and all circuit switches closed. Where two-way switching is wired, only one of the two strapper wires will be tested. To test the other, both two-way switches should be

Fig 8.10 - Insulation test to earth

Fig 8.11 - Insulation tests between poles

operated and the system retested. If desired, the installation can be tested as a whole, when a value of at least 0.5 M Ohms should be achieved, see {Fig 8.10}. In the case of a very large installation where there are many earth paths in parallel, the reading would be expected to be lower. If this happens, the installation should be subdivided and retested, when each part must meet the minimum requirement.

The tests to earth {Fig 8.10} and between poles {Fig 8.11} must be carried Out as indicated, with a minimum acceptable value for each test of 0.5 M Ohms. However, where a reading of less than 2 M Ohms is recorded for an individual circuit, (the minimum value required by the Health and Safety Executive), there is the possibility of defective insulation, and remedial work may be necessary. A test result of 2 M Ohms may sometimes be unsatisfactory. If such a reading is the result of a re-test, it is necessary to consult the data from previous tests to identify deterioration. A visual inspection of cables to determine their condition is necessary during periodic tests; perished insulation may not always give low insulation readings

As indicated above, tests on SELV and PELV circuits are carried out at 250 V. However tests between these circuits and the live conductors of other circuits must be made at 500 V. Tests to earth for PELV circuits are at 250 V, whilst FELV circuits are tested as LV circuits at 500 V.  Readings of less than 5 M will require further investigation.