4 Types of Tests Conducted on Internal Electrical Installations

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Internal electrical installation of a building consists of cabling and associated accessories such as switches, sockets, distribution boards, and light fittings. Their primary function is to provide illumination and power supply to internal lighting and electrical appliances.

This article explains the tests to be conducted in completed internal electrical installations before commissioning. 

Tests on Internal Electrical Installations

On completion of internal electrical installation, the following tests shall be carried out:

1. Insulation resistance test
2. Polarity test of the switch
3. Earth continuity test
4. Earth electrode resistance test

The testing shall be carried out for the completed installations in the presence of and to the satisfaction of the engineer-in-charge, and the test results shall be recorded.

1. Insulation Resistance Test

• The completed installation shall be tested by applying resistance between the earth and the whole system of conductors, with all fuses in place and all switches closed.
• Except in earthed concentric wiring, all lamps in position or poles of the installation shall be tested with direct current pressure of not less than twice the working pressure. The test results must not exceed 500 volts for medium voltage circuits. 
• The working pressure shall be assumed to be that which is maintained between the phase conductor and the neutral, where the supply is derived from a three-wire D.C. or a polyphase A.C system.
• The insulation resistance shall also be tested between all the conductors connected to one pole and all the conductors connected to the neutral or phase conductors of the supply with all the lamps in position and switches in the "off" position.
• The insulation resistance measured in megaohms for the conductors in the above step shall not be less than 12.5 megaohms for the wiring with PVC insulated cables, subject to a minimum of 1 megaohm variation. 
• In the case where an entire installation is tested, a lower value than that given by the formula, subject to a minimum of 1 megaohm variation, is acceptable. 
• A similar test may be carried out before the lamps, bulbs, etc., are installed, and in this event, the insulation resistance to earth should not be less than 25 megaohms for the wiring with PVC insulated cables subject to a minimum of 2 megaohms. 

2. Polarity Test of Switch 

• A test shall be carried out in a two-wire installation to verify that all the switches in the circuit are fitted in the same conductor throughout the supply.
• In a three-wire or a four-wire installation, a test shall be carried out to verify every non-linked single pole switch is fitted in a conductor labeled or marked for connection to one of the phase conductors of the supply.
• The installation shall be connected to the supply for testing. A test lamp shall test the terminals of all switches, one lead of that is connected to the earth. 
• Glowing the test lamp to its full intensity when the switch is in the "on" position irrespective of the appliance is in position or not shall indicate that the switch is connected to the right polarity.

3. Earth Continuity Test

• The earth continuity conductor, including metallic envelopes of cables and metal conduits, shall be tested for electric continuity.
• The electrical resistance of the circuit and the earthing lead is measured from the connection with the earth electrode to any point in the earth continuity conductor. This shall not exceed 1 ohm. 

4. Earth Electrode Resistance Test

• Besides the test electrode, two auxiliary earth electrodes are placed at a suitable distance from the test electrode (see figure). 
• A measured current is passed between the electrode 'A' to be tested and an auxiliary current electrode 'C.' The potential difference between the electrode 'A' and the auxiliary potential 'B' is measured. 
• The resistance of the test electrode 'A' is then given by the formula: 

R = V/I         

Where,
R = Resistance of the test electrode in ohms,
V = Reading of the voltmeter in volts,
I = Reading of the ammeter in amps.

• A hand-driven generator is used to eliminate stray currents flowing in the soil, which produce serious errors in the measurement of earth resistance. 
• If the hand-driven generator's supply frequency coincides with the frequency of the stray current, the instrument pointer fluctuates.
• An increase or decrease in generator speed will cause this to disappear.  
• During the test, the test electrode shall be disconnected from the earthing system.
• For testing, an auxiliary electrode of 13 mm diameter, the mild steel rod is driven up to 1 m into the ground. 
• All three electrodes must be positioned so that their resistance areas are independent of one another.
• If the test electrode is in the form of a pipe, plate, or rod, the auxiliary current electrode 'C' shall be placed 30 m away, and the auxiliary potential electrode 'B' shall be placed between them. 
• Suppose three consecutive test electrode resistance readings do not concur. In that case, the test must be repeated by extending the distance between electrodes A and C to 50 m and inserting electrode B in between them.

Test Certificate 

Upon completing an electrical installation (or an extension to an installation), a certificate shall be furnished, countersigned by the certified supervisor under whose direct supervision the installation was carried out. This certificate shall be in the prescribed form in addition to the test certificate required by the local electrical supply authorities.

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