8.3.6 Insulation resistance
An insulation resistance test is necessary to ensure that the insulation resistance between all live conductors
and earth or, as the case may be, all live parts and earth is adequate to ensure the integrity of the insulation.
This is to prevent—
(a) electric shock hazards from inadvertent contact; and
(b) ﬁre hazards from short-circuits; and
(c) equipment damage.
In addition, an insulation resistance test between conductors is necessary for consumers mains and submains
to minimize potential for insulation breakdown, injury or property damage because of failure of such conductors.
The integrity of the insulation is stressed by applying a direct current at 500 V.
1 Where equipment, such as electromagnetic compatibility (EMC) ﬁlters, equipment containing surge protective
devices connected to earth, or electronic equipment, is likely to be damaged— (i) such equipment shall be
disconnected or switched off before carrying out the insulation resistance test on the equipment; or (ii) the test
voltage for the particular circuit may be reduced to 250 V d.c., in which case, the insulation resistance should be
not less than 1 MΩ. 2 Where equipment, such as an RCD with an FE connection, is likely to inﬂuence the
(i) the FE connection shall be disconnected before carrying out the insulation resistance test on the equipment;
(ii) the insulation resistance of the FE connection shall be not less than 0.05 MΩ, or as prescribed by the
manufacturer. The insulation resistance tester used shall be able to maintain its terminal voltage within +20%
and -10% of the nominal open circuit terminal voltage, when measuring a resistance of 1 MΩ on the 500 V
range or 10 MΩ on the 1000 V range.
The insulation resistance between—
(a) the conductors of consumers mains and submains; and
(b) live and earthed parts of an electrical installation, or parts thereof, including
consumers mains and submains shall be not less than 1 MΩ. Sheathed heating
elements of appliances, RCDs with a functional earth connection and some other
electrical equipment may cause test results lower than 1 MΩ. These should be
disconnected from the circuit prior to testing.
Acceptable insulation resistance values for such items are—
(i) 0.01 MΩ for sheathed heating elements of appliances; or
(ii) value permitted in the Standard applicable to electrical equipment.
NOTE: Where consumers mains or submains are not of signiﬁcant length, the insulation
resistance of the circuit should be signiﬁcantly greater than 1 MΩ, e.g. with short lengths
(say 50 m) of polymeric cables, a value in excess of 50 MΩ would be expected.