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5.7 Insulation Test 1


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5.7 Insulation Test 1

  1. 1. 8.3.6 Insulation resistance General 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. Exceptions: 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 verification test— (i) the FE connection shall be disconnected before carrying out the insulation resistance test on the equipment; or (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.
  2. 2. Results 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.