The new installation should never be energised unless it has the insulation resistance at least of one mega ohm. The insulation resistance can be found out as
Insulation Resistance = 50 / No. of outlets (points + Switches)
There are the following different tests to be carried over before energising the new installation.
- Continuity test.
- Insulation resistance between the conductors and earth.
- Insulation resistance between conductors of opposite polarity.
- Polarity test.
- Earth continuity test.
Continuity test. It is also known as the open circuit test. The main object of this test is to test the continuity of the wires and of each point.
Put the main switch off and connect both L and E terminals of the megger to positive and negative or phase and neural respectively of the main switch. Put all the lamps in the lamp holder, short circuit the plug socket and for easy and effective testing put the switches ‘off’ before stalling the test. Rotate the handle and turn the switch ‘on’ turn by turn. The megger should show zero with every individual switch and if it does not, then there is a break in the circuit, check and remove the fault.
Insulation resistance between the conductors and earth. This test is performed to test the insulation resistance between the conductors (phase and neutral) and earth. If the insulation resistance is less there will be the leakage current from conductors to earth, take the following steps for this test:
(i) Insert all lamps in the holders and short circuit or put load in the sockets, outlets and ‘ON’ the switch.
(ii) Now short P and N, both the terminals Of the installation and connect to ‘L’ terminal Of the megger. Connect E terminal to earth wire as shown in Fig. 14.12.
(iii) On rotating the handle, it should show infinity or at least one megaohm.
The insulation resistance should be = 50 / No. of outlets (points + Switches)
Fig. 14.12. Insulation resistance test between conductors and earth.
Fig. 14.13. Insulation resistance between conductors of opposite polarity
Insulation resistance between the conductors of opposite polarity. The object of this test is to test the insulation resistance between the conductors of opposite polarity. Take the following steps:
(i) Take out all the lamps from the holders and short-circuiting wire from the socket outlets.
(ii) Put all the switches in on position.
(iii) Connect ‘L’ terminal of the megger with the line (positive) terminal or conductor and E with neutral (negative) terminal or conductor.
(iv) On rotating the handle the megger should show at least one mega
The insulation resistance can otherwise be seen as = 50 / No. of outlets (points + Switches)
If the megger shows zero it means there is a short circuit, check and remove the fault.
Polarity test. According to Electricity Rules the live wire should be connected to the switches. The object of this test is to test whether the live wire is connected to the switches or not. Take the following steps:
(i) Take out all the lamps from the holders and put the switches in off position.
(ii) Remove the cover of all switches.
(iii) Connect ‘L’ terminal Of the megger with positive or phase wire in the main switch and E terminal to the individual switch turn by turn. On rotating the handle the megger should indicate zero with every individual switch. If it shows infinity it means that connection are not according to Electricity Rules, check it and remove the fault.
Fig. 14.14. Polarity test.
Earth continuity test. The earth continuity throughout the installation should be maintained and all the metallic parts except the current-carrying conductor be properly earthed.
To perform this test take the following steps:
Connect ‘E’ terminal of the megger with earth wire at the main switch. The ‘L’ terminal of the megger should be connected to all the metallic parts of the installation except current carrying conductors; earth terminal of the three pin plug sockets. In every case the megger should show zero and if it indicates infinity at any point, there is a break in the earth wire, check and remove the fault.
