Code of Practice for the Electricity (Wiring) Regulations - 2009 Edition

(5) Insulation resistance
(a) A suitable d.c. insulation tester should be used to measure insulation
resistance. Care should be taken to ensure that the insulation of the
equipment under test can withstand the test voltage without
damage.
(b) The main switchboard and each distribution circuit should be tested
separately. To carry out this test, it is acceptable to divide large
installation into sections with groups of outlets. The term ‘outlet’ in
this case includes every point and every switch. A socket outlet or
appliance or luminaire incorporating a switch is regarded as one
outlet.
(c) When measured with all fuse links in place, all switches and circuit
breakers (including, if practicable, the main switch) closed and all
poles or phases of the wiring electrically connected together, the
insulation resistance to earth should not be less than the appropriate
values given in Table 21(1), as illustrated in Figure 21(4).
(d ) When measured between all conductors connected to any one
phase or pole of the supply and, in turn, all conductors connected to
each other phase or pole, as shown in Figure 21(5), the insulation
resistance should not be less than the appropriate values in Table
21(1).
(e) In carrying out the test:
(i) wherever practicable, all lamps should be removed and all
current using equipment should be disconnected and all local
switches controlling lamps or other equipment should be
closed;
(ii) where the removal of lamps and/or the disconnection of current
using equipment is impracticable, the local switches controlling
such lamps and/or equipment should be open;
(iii) electronic devices connected in the installation should be
isolated or short circuited where appropriate so that they are
not damaged by the test voltage.
(iv) where the circuits contain voltage sensitive devices, the test
should measure the insulation resistance to earth with all live
conductors (including the neutral) connected together.
(f ) Where equipment is disconnected for the test and the equipment
has exposed conductive parts required to be connected to protective
conductors, the insulation resistance between the exposed
conductive parts and all live parts of the equipment should be
measured separately and should have a minimum insulation
resistance not less than 1 megohm.
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