A building’s internal electrical installation comprises cabling and various accessories like switches, sockets, distribution boards, and light fittings. Its main purpose is to provide power to internal electrical appliances and lighting. However, before commissioning the installation, several tests must be performed to ensure its safety and functionality.
These tests are essential as they ensure that the installation meets the required safety standards and complies with the regulations. Additionally, conducting these tests reduces the risk of electrical shocks and potential fire hazards, which can occur due to faulty wiring or defective equipment.
The tests involve checking the electrical circuits, insulation resistance, earthing, and polarity. During circuit testing, the electrician ensures that the wires are correctly connected and have the correct ratings. Insulation resistance testing involves checking the insulation between the wires and the ground to ensure that it is adequate. Earthing testing checks if the electrical system is adequately grounded, while polarity testing verifies that the electrical system’s polarity is correct.
Once all these tests are completed, the electrician issues a certificate confirming that the electrical installation meets the safety standards and regulations. This certification is necessary as it proves that the electrical installation has been tested and is safe to use.
In summary, the internal electrical installation of a building plays a crucial role in providing power to electrical appliances and lighting. However, before commissioning the installation, tests must be conducted to ensure its safety and functionality. These tests involve checking the circuits, insulation resistance, earthing, and polarity. Upon completion of these tests, a certificate is issued to confirm that the installation meets the safety standards and regulations.
Tests on Internal Electrical Installations
Upon the completion of the internal electrical installation, a series of tests must be conducted. These tests include the insulation resistance test, polarity test of the switch, earth continuity test, and earth electrode resistance test. The tests must be carried out in the presence of the engineer-in-charge, who must also be satisfied with the results. It is crucial that the test results are recorded accurately for future reference.
1. Insulation Resistance Test
The testing process for a completed electrical installation involves several steps. First, resistance between the earth and the entire system of conductors must be applied, with all fuses in place and all switches closed. This test is not applicable for earthed concentric wiring.
Next, all lamps in position or poles of the installation must be tested with a direct current pressure of at least twice the working pressure, except for medium voltage circuits where the test results must not exceed 500 volts. The working pressure is assumed to be the maintained pressure between the phase conductor and the neutral for three-wire DC or polyphase AC systems.
The insulation resistance between all the conductors connected to one pole and all the conductors connected to the neutral or phase conductors of the supply must also be tested, with all lamps in position and switches in the “off” position. For wiring with PVC insulated cables, the insulation resistance measured in megaohms for the conductors in this test must not be less than 12.5 megaohms, subject to a minimum variation of 1 megaohm.
When testing an entire installation, a lower value than the one provided by the formula may be acceptable, subject to a minimum variation of 1 megaohm. A similar test can be carried out before installing lamps, bulbs, etc., and in this case, the insulation resistance to earth must not be less than 25 megaohms for wiring with PVC insulated cables, subject to a minimum variation of 2 megaohms.
Insulation Resistance Test
2. Polarity Test of Switch
To verify that all switches in a two-wire installation are fitted in the same conductor throughout the supply, a test must be carried out. In a three-wire or four-wire installation, a test must be carried out to ensure that 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. In both cases, the installation must be connected to the supply for testing.
To carry out the test, a test lamp must be used to test the terminals of all switches. One lead of the test lamp should be connected to the earth. If the test lamp glows to its full intensity when the switch is in the “on” position, regardless of whether the appliance is in position or not, this indicates that the switch is connected to the right polarity.
3. Earth Continuity Test
The requirement is to perform a test on the earth continuity conductor, which includes metallic envelopes of cables and metal conduits, to ensure its electric continuity. The purpose of this test is to measure the electrical resistance of the circuit and the earthing lead from the connection with the earth electrode to any point in the earth continuity conductor. The maximum acceptable electrical resistance for this circuit and the earthing lead is 1 ohm. This limit should not be exceeded during the testing process.
4. Earth Electrode Resistance Test
The test electrode, labeled as ‘A’, is used in conjunction with two auxiliary earth electrodes, placed at a suitable distance from the test electrode as shown in the figure. A current is measured by passing it between the test electrode ‘A’ and an auxiliary current electrode labeled as ‘C’. The potential difference between the test electrode ‘A’ and the auxiliary potential labeled as ‘B’ is also measured. The resistance of the test electrode ‘A’ can be calculated using the formula R = V/I, where R represents the resistance of the test electrode in ohms, V represents the reading of the voltmeter in volts, and I represents the reading of the ammeter in amps.
Earth electrode resistance test
A hand-driven generator is used to eliminate stray currents in the soil, which can cause errors in measuring earth resistance. When the frequency of the generator coincides with the frequency of the stray current, the instrument pointer fluctuates. Adjusting the generator speed can resolve this issue. During the test, the test electrode should be disconnected from the earthing system. For testing, an auxiliary electrode with a 13 mm diameter mild steel rod is driven 1 m into the ground. It’s important to position all three electrodes so that their resistance areas are independent of each other. If the test electrode is in the form of a pipe, plate, or rod, the auxiliary current electrode ‘C’ should be placed 30 m away, and the auxiliary potential electrode ‘B’ should be placed between them. If three consecutive test electrode resistance readings do not match, 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
After an electrical installation or an extension to an existing installation has been completed, it is mandatory to provide a certificate that is countersigned by a certified supervisor who oversaw the work. The supervisor must have directly supervised the installation process. The certificate must be in the prescribed form, and it is necessary to obtain a test certificate from the local electrical supply authorities. These requirements are mandatory and must be adhered to for the installation to be considered complete.
FAQs
Which are the types of tests conducted on internal electrical installations?
There are four types of tests that are conducted on internal electrical installations. The first one is the insulation resistance test, which is used to check the insulation resistance between different parts of an electrical installation. The second type of test is the polarity test of the switch, which is used to ensure that the switch is correctly connected and the polarity of the supply is correct. The third type of test is the earth continuity test, which is used to check the continuity of the earth conductor throughout the installation. Finally, the fourth type of test is the earth electrode resistance test, which is used to measure the resistance between the earth electrode and the general mass of earth to ensure that it is within acceptable limits. These tests are essential for ensuring the safety and proper functioning of internal electrical installations.
What is the higher limit of insulation resistance of an internal electrical installation?
The insulation resistance of wiring that uses PVC insulated cables must be measured in megaohms. It is required that the insulation resistance should not be less than 12.5 megaohms. However, there can be a minimum of 1 megaohm variation. This means that the insulation resistance value can be as low as 11.5 megaohms but not lower.
What is the formula to calculate the resistance of an electrode?
The formula to calculate the resistance of the test electrode ‘A’ is R = V/I. In this formula, ‘R’ represents the resistance of the test electrode in ohms, ‘V’ represents the reading of the voltmeter in volts, and ‘I’ represents the reading of the ammeter in amps.
When it comes to installing an electrical earthing system in a building, there are several important steps to follow. First, a thorough site survey should be conducted to determine the appropriate location for the earthing system. Next, a suitable earthing electrode should be selected and installed in the designated location. Finally, the earthing system should be tested to ensure that it meets the necessary safety standards.
Similarly, installing a casing wiring system in a building involves several important steps. First, the wiring should be planned and designed to meet the electrical requirements of the building. Next, appropriate materials should be selected for the wiring system, and the wiring should be installed by a qualified electrician. Finally, the wiring system should be tested to ensure that it is safe and functional.
It is important to consider the recommended values of illumination for different structures when designing lighting systems for buildings. The recommended illumination levels vary depending on the specific type of structure, such as offices, factories, or outdoor areas. Adequate illumination is important for ensuring safety and productivity, and can also have an impact on the overall aesthetics of a space.