Electrical: Basic Testing Procedures and Descriptions

Safe isolation 

Reason: Dead circuit testing is testing performed with the power disconnected from the circuit. The main benefit of disconnecting power supply while tests with an external energy source are performed is to eliminate hazardous risks to the environment or the person conducting the test. 

Method: The procedure for proving dead should be by use of a test lamp or two pole voltage detector as recommended in HSE Guidance Note GS38. Non-contact voltage indicators (voltage sticks) and multi-meters should not be used. The test instrument should be proved to be working on a known live source or proprietary proving unit before and after use. All phases on load side of main switch and the neutral should be tested and proved dead. 

Note: When testing live, it is always good practice to put your test leads first on the non-live conductor, that is earth / neutral and then to a live conductor. 

Test sequence and descriptions:

The following tests are carried out with the Consumers main switch isolated. 

1. External earth fault loop impedance  
[Main circuit isolated, but working in vicinity of live parts]  

Reason: To establish that a good earth exists at the origin of the installation in order for the remaining tests to go ahead. 

Method: Disconnect the main earthing conductor from the main earthing terminal. An earth fault loop impedance tester is connected at line and earth (main earthing conductor) at the supply side of the installation and a test performed. Reconnect the main earthing conductor. The result is Ze and recorded on the sheet. The prospective fault current is measured at the same time after the re-connection of the main earthing conductor. 

2. Continuity of protective and bonding conductors

Reason: To check that all circuit protective conductors (green and yellow cables) are continuous and are present at every electrical accessory on the circuit. Also to check that the main earthing conductor and main bonding conductors are continuous and correctly connected. 

Method 1: The line conductor is connected to the circuit protective conductor of the same circuit at the consumer unit and a measurement taken at ALL accessories on that circuit between line and CPC. The highest measurement obtained is recorded on the test report. Additionally, this method can be used to establish a circuit’s earth fault loop impedance.

Test result is R1 + R2. The line conductor and neutral conductor are then connected and the above repeated to obtain R1 + Rn. 

Method 2 (used for main earth and main bonding conductors): A wandering lead is connected to one end of the conductor to be tested and a measurement taken between the other end of this lead and the other end of the conductor. The reading obtained at each accessory point should be a low resistance value. Test result is R2.

During this test polarity can be checked as well. The continuity of the neutral conductor can also be checked.

3. Continuity of ring final circuit conductors 

Reason: This test ensures that all ring final circuits (sockets usually) are indeed a continuous ring with no interconnects or breaks within it.    

Method: The line, neutral and earth conductors of the circuit are identified within the distribution board and a measurement from one end to the other end of each is taken. These results are R1, R2, and Rn. 

The incoming line conductor is then connected to the outgoing earth conductor and the outgoing line conductor is connected to the incoming earth conductor. A measurement is then taken at ALL socket outlets on the ring. The highest of which is recorded on the report.  

This result is R1 + R2 for that circuit. The above is then repeated using the neutral conductor instead of the earth conductor. This test provides R1 + Rn which does not need to be recorded on the report but is essential to check the circuit correctly.

4. Insulation Resistance 

Reason: This test checks whether the insulation around a cable is still intact and has not broken down over time. It is a good indicator of the age of an installation.    

Method: An insulation resistance tester is connected across line and neutral conductors at the origin of the supply. 500V are then pumped down the conductors to see if any voltage leaks across from one conductor to the other. The same is then done for the line and earth and the neutral and earth conductors. 

Note: Ensure that all loads are disconnected, including the residual current circuit breaker, to prevent damage and influencing test results. However, where electronic or sensitive equipment may be in circuit, test to earth with line and neutral conductors connected together. 

Infinity is preferred but a minimum insulation resistance value of is 1 MΩ accepted. However, low resistances of less than 2 MΩ will require further investigation.

5. Polarity 

Reason: To check that all accessories are correctly connected to line, neutral and earth and that all switches and circuit breakers are connected in the line conductor only. 

Method: The method for this is the same as for continuity and is usually done at the same time by operating switches, sockets etc whilst conducting the test.

6. Earth electrode resistance 

[Main circuit isolated, but working in vicinity of live parts] 

Reason: Where the electrode is part of the installation and has an RCD protecting it, an electrode resistance test may be required, to make sure that any earth electrode used is of a sufficiently low impedance to allow the timely operation of the RCD protecting the installation. 

Method: An earth fault loop impedance tester is connected between line and earth at the origin of the supply and a test performed. The result of which is considered the resistance of the electrode (Ra).  

Note: Each earth rod or group of rods should be tested separately, with the test links removed and with installation isolated from the supply.   
  

The following tests are carried out with the Consumers main switch energised. 

7. Live polarity test

Reason: To verify polarity of supply authorities system.    

Method: An approved voltage indicator or test lamp shall be used. Using the approved voltage indicator, one probe shall be placed on the incoming neutral, and the other on the incoming line conductor, on the live side of main breaker. The indicator should show it is live. One probe shall now be placed on the CPC and the other on the incoming line conductor. The indicator should show it is live. A test shall be preformed between CPC & incoming neutral. The indicator should show that it is not live. 

8. Earth fault loop impedance 

Reason: This test is done at the furthest point on a circuit in order to make sure the impedance of the earth path is not too high even at the furthest point so that sufficient current will flow under fault conditions to take out the circuit breaker protecting the circuit. 

Method: An earth fault loop impedance tester is connected to line and earth at the furthest point on the circuit and the test performed to obtain the value of Zs.

9. RCD test  

Reason: To make sure RCD's trip within the correct time (30mA = 300ms max).    

Method: An RCD tester is connected and a test at 1/2 times, 1 times and 5 times the trip current is performed on each side of the cycle and a time of trip obtained. Usually milliseconds with the highest being recorded. The manual test button is then pressed as part of the functional test.

10. Functional testing

Reason: To make sure all switches, isolators, circuit breakers etc. work as they should. 

Method: Switches must be switched on and off. The circuit must be interrupted as intended.