Insulation Resistance.

Before proceeding with the insulation test, the engineer should make sure that all pilot lamps and neon indicators have been left out of circuit, plus that all switches are in the off position if lamps are left installed. Any electronic packages or items likely to be damaged by the application of the test voltage should also be isolated. The incoming neutral must be disconnected.
Another important condition the engineer must comply with is to verify the type of system he (she) is about to test. Is the system suitable for testing with the instrument to hand, its no good testing an extra low voltage circuit with a 500 v insulation tester for example. Intrinsically safe instruments may be required on some installations.
The instrument to be used must be capable of providing the test voltage indicated in Table 71A of BS 7671 when loaded with 1mA.

Table 71A , Minimum values of insulation resistance:

The insulation resistance is deemed satisfactory if the circuits as tested result in values not less than those appropriate to Table 71A above.
As can be seen, values for a 400/230 volt system are acceptable down to 0.5M/ohms, should any circuits record values around 2 M/ohms then further investigation would be recommended. Obviously, should any values be recorded that are near to the values indicated in the above table then further investigation would certainly be required. However, the values shown are the minimum requirements for the circuit to be passed as satisfactory.
Should an insulation test on a distribution board result in values that are low enough to cause concern, then the circuits should be disconnected and individually examined and tested until (in all probability) the 1 or 2 circuits that have given the low readings have been identified.
The insulation resistance is normally measured between live conductors (phase and neutral), and between live conductors and earth. When testing circuits for a Periodic Inspection, if it is impractical to disconnect electronic devices from a circuit (as is usually the case), a measurement to earth only should be made, with the phase and neutral conductors connected together.

Insulation Test Procedure:

Why do we need to test insulation resistance?

An insulation test will give the engineer an indication of the condition of the conductor insulation throughout an installation. Effective insulation is a requirement to provide adequate protection against direct contact, and to prevent the likelihood of short circuits and earth faults. Faulty insulation can give rise to undue leakage currents and can present a risk to persons and especially livestock. Obviously faulty insulation and the possibility of a spark give rise to the risk of fire.

Circuit Nominal Voltage
Test Voltage d.c.
Minimum Insulation Resistance in M/ohms
Up to and including 500v
with the exception of the
above systems
Above 500v
Insulation Resistance.
Page 5 of 7
Tony Spry's Electrical Engineering Pages.
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