Electrical Testing Quiz

Test probes and leads used to apply or measure voltages over 50 V a.c. should comply with:

• BS 7671 Requirements for Electrical Installations

• Health & Safety Executive Guidance notes GS 38

• BS 5490 Specification for Classification of Protection

• International Electrotechnical Commission Publication 479

Answer explanation

Test probes and leads for voltages over 50 V a.c. must comply with Health & Safety Executive Guidance notes GS 38, which specifically addresses safety standards for electrical testing equipment.

Which one of the following tests can only be carried out after the supply is connected?

• Polarity

• Insulation resistance

• Continuity of protective conductors

• Earth loop impedance

Answer explanation

Earth loop impedance testing requires the supply to be connected to measure the impedance of the earth connection accurately. The other tests can be performed without the supply being connected.

BS 7671 requires that the resistance of each conductor of a ring circuit shall be measured. A suitable instrument for this purpose would be

• An earth loop impedance tester

• A low reading ohmmeter

• An insulation resistance tester

• An audible type continuity tester

Answer explanation

A low reading ohmmeter is specifically designed to measure low resistances accurately, making it suitable for checking the resistance of each conductor in a ring circuit as required by BS 7671.

Which one of the following is MOST suitable to conduct a polarity test?

• A low resistance ohmmeter

• An earth loop impedance tester

• An insulation resistance tester

• Residual current device tester

Answer explanation

A residual current device tester is designed to check the polarity of circuits by measuring the current flow and ensuring proper operation of RCDs, making it the most suitable choice for conducting a polarity test.

Insulation resistance testers are designed to measure

• Low resistance using a.c. voltage

• Low resistance using d.c. voltage

• High resistance using a.c. voltage

• High resistance using d.c. voltage

Answer explanation

Insulation resistance testers measure high resistance, typically using d.c. voltage. This is because d.c. voltage provides a more accurate assessment of insulation quality compared to a.c. voltage.

Instrument test leads should comply with HSE Guidance Notes GS 38, when used to measure or apply voltage exceeding

• 25 V a.c. or 50 V d.c.

• 25 V d.c. or 50 V a.c.

• 50 V a.c. or 100 V d.c.

• 50 V d.c. or 100 V a.c.

Answer explanation

Instrument test leads must comply with HSE Guidance Notes GS 38 for voltages exceeding 50 V a.c. or 100 V d.c., making this the correct choice.

A test of continuity at each socket-outlet of a ring final circuit is carried out to ensure that:

• All cables are of the same size

• All cables are of the same construction

• No radial branches exist

• The polarity of sockets is correct

Answer explanation

Testing continuity at each socket-outlet ensures that no radial branches exist in a ring final circuit, which is crucial for maintaining proper circuit integrity and safety.

Operating the test button on a r.c.d. (residual current device) will indicate the:

• Correct mechanical operation of the device

• Earth continuity of CPC

• Continuity of earthing conductor

• r.c.d. trips within 300 milliseconds

Answer explanation

Operating the test button on an r.c.d. checks its functionality by ensuring it trips within 300 milliseconds, indicating proper operation and safety against electrical faults.

The test voltage that should be applied when testing the insulation resistance of an electrical installation

• 230 V a.c.

• 230 V d.c.

• 500 V a.c.

• 500 V d.c.

Answer explanation

The correct test voltage for insulation resistance testing is 500 V d.c. This voltage is sufficient to identify insulation failures without causing damage, making it the standard choice for such tests.

An ohmmeter used to verify the continuity of a protective earthing conductor indicates a value of 0.20 ohms. If the resistance of the instrument test leads is 10 mΩ, the true resistance of the protective earthing conductor is

• 0.201 ohm

• 0.21 ohm

• 0.19 ohm

• 0.18 ohm

Answer explanation

The ohmmeter reading includes the resistance of the test leads. To find the true resistance of the earthing conductor, subtract the lead resistance (0.01 ohms) from the reading (0.20 ohms), resulting in 0.201 ohms.