Part P Inspection & Testing 1 — Questions and Answers
Question 1: What is the purpose of initial verification (inspection and testing) of a new electrical installation?
- To confirm that the installation complies with BS 7671 and is safe for use before it is energised for the first time (Correct answer)
- To issue an invoice for the work
- To register the installation with the electricity supplier
- To obtain planning permission for the work
Correct answer: To confirm that the installation complies with BS 7671 and is safe for use before it is energised for the first time
Initial verification ensures a new installation complies with the design and with BS 7671, identifying any errors in construction before the installation is put into use, preventing danger to the occupier.
Question 2: What does 'dead testing' mean in electrical installation testing?
- Tests performed with the installation isolated from the supply (de-energised), such as continuity and insulation resistance tests (Correct answer)
- Tests performed on decommissioned circuits only
- Tests using low-voltage probes only
- Tests for the presence of voltage
Correct answer: Tests performed with the installation isolated from the supply (de-energised), such as continuity and insulation resistance tests
Dead testing refers to tests carried out with no electrical supply connected (de-energised), including continuity (CPC, bonding, ring), insulation resistance, and polarity (initial check) — all performed safely with no live parts.
Question 3: What is the minimum insulation resistance acceptable under BS 7671 for a 230 V circuit?
- 1 MΩ (1,000,000 ohms) (Correct answer)
- 100 kΩ
- 10 kΩ
- 500 kΩ
Correct answer: 1 MΩ (1,000,000 ohms)
BS 7671 Table 61 specifies a minimum insulation resistance of 1 MΩ (measured at 500 V DC) for circuits with nominal voltages up to 500 V, confirming the insulation between conductors is sound.
Question 4: What is the purpose of the ring continuity test?
- To confirm that the ring circuit is correctly wired as a ring (both conductors return to the consumer unit) and to identify crossed or broken conductors (Correct answer)
- To test insulation resistance of the ring circuit
- To measure the earth fault loop impedance
- To confirm the MCB rating is correct for the circuit
Correct answer: To confirm that the ring circuit is correctly wired as a ring (both conductors return to the consumer unit) and to identify crossed or broken conductors
The ring continuity test uses three measurements to confirm the ring is correctly formed (not broken, bridged or incorrectly cross-connected), using the phase, neutral and CPC conductors in sequence.
Question 5: What test instrument is used for insulation resistance testing?
- A dedicated insulation resistance tester (megohmmeter or 'megger') applying a DC test voltage (Correct answer)
- A multimeter on resistance range
- An RCD tester
- A loop impedance tester
Correct answer: A dedicated insulation resistance tester (megohmmeter or 'megger') applying a DC test voltage
An insulation resistance tester (megohmmeter) applies a specified DC test voltage (500 V for 230 V circuits) and measures the resistance of the insulation; a multimeter cannot apply sufficient voltage for this test.
Question 6: What is the maximum earth fault loop impedance (Zs) for a 16 A Type B MCB?
- 2.87 Ω (as per BS 7671 Table 41.3 for TN systems at 0.4 s disconnection) (Correct answer)
- 1 Ω
- 10 Ω
- 0.5 Ω
Correct answer: 2.87 Ω (as per BS 7671 Table 41.3 for TN systems at 0.4 s disconnection)
BS 7671 Table 41.3 specifies maximum Zs values for each device; for a 16 A Type B MCB on a socket circuit (0.4 s disconnection), the maximum Zs is 2.87 Ω at the standard table temperature of 20°C.
Part P Inspection & Testing 1
What is the purpose of initial verification (inspection and testing) of a new electrical installation?