Test of insulation resistance is carried out by applying a test voltage approximately twice the normal r.m.s value of working voltage. The value of testing voltage is 500v. Testing instruments can be either the battery-operated type or the hand driven type Remember that the large installations may be divided into groups of outlets. For this the term ‘outlet’ means every point and every switch. For example, a lighting point controlled by a switch constitutes two outlets, but a switched socket-outlet only constitutes one.
The result of this test is in mega ohms. The testing set (megger) is simple to use. The ‘L’ terminal is connected to the conductor and, the earth terminal marked ‘E’ to a good earth and the handle of the megger is turned at a steady constant speed of about 120 r.p.m.
According to LEE regulations, the formula for wiring insulation is that
Insulation resistance of P.V.C cable = 12.5/no of outlets
Insulation resistance of V.I.R cable (mega-ohms) 50/no of outlets
This test should he made with
(I) All switches closed.
(ii) All fuses are inserted in distribution fuse hoard.
(iii) All the lamps are inserted in lamp holders.
(iv) Main switch is OFF.
(v) The phase and neutral leads to the main switch from the supply side of the installation should, be connected together.
After that the live terminal of megger is connected between the linked phase and neutral leads and the other terminal of the merger is connected to the consumer’s earthing terminal. It is recommended that the insulation resistance to earth should not be less than one mega ohms (1MΩ). If the value obtained is less than 1MΩ, then the insulation must c sectionalized until faulty circuit is located.
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