Insulation resistance test

A d.c. insulation resistance test or polarization index reveals only the surface condition of the insulation and does not allow a realistic assessment of internal condition. Loss tangent values are true reflections of the insulation condition to detect moisture content, voids, cracks or general deterioration. The tan 5 versus test voltage curve may be drawn and compared with the original curve provided by the manufacturer, and inferences drawn regarding the condition of the insulation. The different starting tan lvalues will reveal the condition of the insulation in terms of amount of contamination, as noted in Table 10.4 (See lEE, Vol. 127, May 1980). 

Insulation resistance tests apply a substantial test voltage, typically 500 V, between the live and neutral bonded together and the earth. The PAT Tester then calculates the insulation resistance and either gives an actual reading... 

The use of an insulation resistance test as described by the lET Regulations must be avoided with any electronic equipment. The working voltage of this instrument can cause total devastation to modern electronic equipment. When carrying out an insulation resistance test as part of the prescribed series of tests for an electrical installation, all electronic equipment must first be disconnected or damage will result. 

There is an increased use of electronic devices in electrical installation work for example, in dimmer switches and ignitor circuits of discharge lamps. These devices should be temporarily disconnected so that they are not damaged by the test voltage of, for example, the insulation resistance test (IET Regulation 612.3). 

To measure accurately the resistance of the conductors in an electrical installation, we must use an instrument which is capable of producing an open-circuit voltage of between 4 and 24V a.c. or d.c., and delivering a short-circuit current of not less than 200mA (lET Regulation 612.2.1). The functions of continuity testing and insulation resistance testing are usually combined in one test instrument. 

Where the circuit inciudes electronic equipment which might be damaged by the insulation resistance test, a measurement between all live conductors (i.e. live and neutral conductors connected together) and the earthing arrangements may be made. The insulation resistance of these tests should be not less than 1. OMH (lET Regulation 612.3.3). 

The value of a satisfactory insulation resistance test on each final circuit of a 230V installation must be ... 

These designators were determined by a series of tests, including the copper mirror test, a qualitative silver chromate paper test for chlorides and bromides, a qualitative spot test for fluorides, a quantitative test for halides (chloride, bromide, and fluoride), a corrosion test for flux residue activity, and a surface insulation resistance test at accelerated temperatoe and humidity conditions. 

Fully separated systems, where a safety isolating transformer is inserted but the outgoing supply is not earthed, is another option frequently employed in production testing areas where an operator may have to handle or be exposed to contact with uninsulated live conductors. Measures must be taken to prevent one pole of the secondary side of the system becoming inadvertently connected to earth the separated circuit should be kept as short as possible and the conductors should be well insulated and visible so far as possible. Periodic inspections and insulation resistance tests should be carried out to confirm the continuing earth-free integrity of the system. 

BS 7671 specifies that d.c. test voltages should be used for insulation resistance tests, and that the minimum acceptable insulation resistance values should be in accordance with Table 17.1. The instruments used should be capable of providing an output of not less than 1 mA. For installations designed to operate at voltages up to 500 V, e.g. those connected to the 400/ 230 V public supply and the 110 V systems supplying portable apparatus, a 500 V d.c. insulation tester is adequate and suitable for the prescribed test. A 1000 V tester should be used to test systems over 500 V up to 1000 V. Large installations should be tested in sections, each of not more than 50 outlets. An outlet, for example, consists of a lighting point, a switch or a socket outlet. 

One test lead should be connected to the probe and the other to the nearest protective conductor. The test voltage should be at least 2 kV applied for one minute and the leakage current must not exceed 1 mA (see Fig. 17.9). This is followed by a 500 V d.c. insulation resistance test applied in the same way and at the same positions. The reading should be not less than 0.5 megohms for voltages up to 500 V or 1 megohms for voltages over 500 V. 

Limiting to a safe level the amount of current that can be supplied into a load representative of the human body. In general this means that the current should be limited to a maximum of 5 mA. As an example, this limitation is commonly applied to the probes of insulation resistance test units which may be energised in excess of 1000 V but which can be... 

Perfomi insulation resistance test (when appropriate). 

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