All new completed electrical installation should be tested before connection to the supply, to ensure that the installation is technically sound and free from any possible short circuits, etc. the main reasons, to test a new electrical installation or house wiring before it is switched on to the mains are as follows:
- To know the cause of failure of a particular circuit or circuits or equipment and to locate the exact position of break down.
- To ensure that it is free from faults and is as per electricity rules.
- These tests will receive the attention of the owner before any possible undue damage occurs.
The tests should be made on a new electrical installation before it is switched on to the mains are as under:
- Insulation resistance test between installation and earth.
- Insulation resistance test between conductors.
- Testing of polarity.
- Testing of earth continuity paths.
- Earth resistance test.
Insulation Resistance Test between Installation and Earth
This test is performed to know the standard of insulation of wires and cables used in the installation.
It also ensures that the insulation is sufficient enough to avoid any possible leakage of current to earth.
The leakage of the current to earth should not exceed 0.02% of the full load current.
Before performing insulation resistance test between installation and earth the conditions to be fulfilled for the position of the main switch, fuses, switches, and other points should be as under:-
- main switch in OFF position,
- fuses beyond the main switch should be in position,
- all switches in ON position.
- all lamps and other equipment should be in their position.
For testing the whole installation, the test is conducted on the main switch. A testing set known as megger is used for the test. It is a special form of the ohmmeter.
To perform this test, the phase and the neutral is short-circuited temporarily at any suitable point as shown in Figure.
The ‘L’ (line terminal) of the megger is connected to the short circuit point in the main switch and the earth terminal marked (E) is connected to earth continuity conductor or some good earth point near-by.
The handle of the tester is turned at a high speed so that sufficient testing voltage is produced. The reading on the dial of the megger is noted.
The insulation resistance thus measured should not be less than 0.5 MΩ on a firm, sound and fixed wiring.
If the insulation resistance is below this value, the wiring section giving that value should be rewired or checked thoroughly until the required value is obtained.
Insulation Resistance Test between Wiring Conductors
To ensure that the insulation of the cables or wires is not damaged and there is no leakage between them, this test is performed.
Before performing this test, the position of the main switch, fuses, switches, etc. should be as under:
- main switch in OFF position,
- all switches in ON position,
- all lamps and other appliances should be removed,
- fuses beyond the main switch should be in position.
The line terminal of the megger is connected to phase terminal of the installation and the earth terminal of megger is connected to neutral wire.
The insulation resistance so measured should not be less than 0.5 MΩ and not more than 1 MΩ.
Polarity Test in House Wiring
In a low voltage installation, this test is performed to verify that all single pole switches have been connected to phase wire throughout the installation.
It is very necessary to place all switches on phase so that when a switch is made OFF, the connected appliance is quite dead.
If the switch is connected to the neutral wire then the connected appliance will get phase even if the switch is in OFF position and remain alive.
There is absolutely no difference in the functioning of the switch in either case, but from the safety point of view to avoid shock, etc. the phase should always be given through the switch and neutral direct to the point.
The simple method of conducting the polarity test is by using a test lamp.
Before performing this test the position of the main switch, fuses, switches, etc. should be as under:
- main switch in ON position,
- all switches in OFF position,
- all lamps and other appliances should be removed.
One end of the test lamp is connected to earth wire and the other end to the incoming terminal of the switch.
If the lamp lights, it indicates that the switch is connected to phase wire, otherwise to neutral wire.
Earth Continuity Test of Electrical Installation
To perform this test with the help of megger, the main switch is opened, the main fuses are withdrawn, all the switches are made ON and all the lamps are put in position.
The ‘L’ (line terminal) of the megger is connected to the phase conductor in the main switch and ‘E’ (earth terminal) of the megger is connected to an earth point.
In this test, megger should indicate a resistance value between 0.5 and 1 mega ohm.
In this case, if earthing of all the metallic parts and the earth wire will be in good condition, a sufficient amount of current will flow through test circuit and megger will show a reading up to 1 MΩ.
If it will be in bad condition then it will offer high resistance to the current. As a result, a very low quantity of current will flow and megger will show a reading more than 1 MΩ .
Therefore, if the megger shows a high reading (more than 1 MΩ), it means that the main switch or conduit is not properly earthed or the earth wire is broken somewhere requiring correction.
Earth Resistance Test
I have explained this test earlier in an article. You can refer to that article for earth resistance test.
Thanks for reading about “electrical installation testing procedures”.