Insulation Resistance Test


3. Insulation Resistance Test

An insulation resistance test is performed to ensure that the resistance between all live conductors and earth, or as the case maybe, all live parts and earth, is adequate to prevent electric shock hazards, fire hazards and equipment damage.
It is a requirement of Clause of the Wiring Rules that the insulation resistance between all live conductors (including the neutral) connected together, and earth, measured with all protective devices in circuit and all switches on, is not less than 1 Megohm (see Note below). This applies to the complete installation and all parts of the installation.
This does not preclude the use of appliances incorporating heating elements provided that, when tested with all elements in circuit, the insulation resistance between live parts and the earthing terminal or earthing contact of the appliance
is not less than 0.01 Megohm (10,000 ohms). Note: Whilst the minimum reading is 1 Megohm, the ideal reading is infinity. This indicates a sound installation.

Test Equipment
Where the supply voltage does not exceed 250 volts between conductors and earth, a 500 volt insulation tester should be used to perform this test.

Test Procedure

1. Ensure that all protective devices are in circuit and that all switches are on. If there are two way lighting circuits in the installation, tests must be carried out with the two way switches in all positions, so all circuit conductors are tested.

2. Set the insulation tester (eg. Megger) to Megohms.

3. Connect one lead of the insulation tester to the main earthing bar (MEN Link disconnected) and the other lead to the actives and neutrals connected together. Do not test between actives and neutrals.

4. Measure the insulation resistance of the total installation (including the consumers mains). If the insulation resistance is 1 Megohm or more – re-connect the MEN Link and return the installation to normal.

If the insulation resistance is less than 1 Megohm – test all circuits individually, starting with circuits supplying appliances incorporating heating elements. Where a circuit is less than 1 Megohm, disconnect the appliance or
appliances. Then test the circuit and appliance (separately) to ensure that the insulation resistance of both the circuit and the appliance complies with the following:
• not containing a heating element – is not less than 1 Megohm, or
• containing a heating element – is not less than 0.01 Megohm

Note: Where the insulation resistance of an appliance containing heating elements is found to be less than 0.01 Megohm and the defect cannot be rectified at the time of checking and testing, the electrical contractor must make arrangements for correction of the defect prior to submission of the Notice of Completion. Alternatively, the following procedure may be used:

a) Isolate the defective circuit, including the neutral conductor, by disconnecting at the appropriate switchboard.

b) Attach an “out of service” tag to the circuit protection device, advising that the circuit has been disconnected due to low insulation resistance.

c) Check that the earth conductor has been reconnected at the MEN point.

d) Then submit the Notice of Completion.

Ensure that all circuits and the MEN connection are re-connected when the testing is complete.

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