Fault Finding With Insulation Resistance Testing

At this point the City and Guilds 2394 qualification requires this part of the old City and Guilds 2391 exam to be undertaken. For those of you who would like to know about this part of the old Inspection and Testing exam this guide is here to be studied.

At this stage of the practical exam the delegate will be required to perform a fault finding procedure on a “black box” simulating a circuit under various and adjustable fault conditions.

The delegate will be required to select the appropriate tester (insulation resistance tester) and after visually inspecting the tester for faults, setting it up for the correct voltage.
Since the simulated circuit will be a single phase low voltage circuit, the required test voltage with which the tests must be undertaken is 500V with accordance of BS 7671.

Test sequence is as follows:

• Test between L-N
• Test between L-E
• Test between N-E

Possible test results:

• 2 Mohm - open circuit (no electrical connection between conductors which is as it should be)
• 2 Mohm but not 0 Ohm - possible electrical connection between conductors (faulty condition which needs further investigation)
• 0 ohm - short circuit between conductors (faulty condition)

The examiner will “insert” one or more of these faults into the “black box” and the delegate will be required to identify and explain the possible causes and solutions.

Read more about Insulation Resistance testing