Part P BS 7671 IET Wiring Regulations 2 — Questions and Answers
Question 1: What is the fundamental protection against electric shock in BS 7671?
- Basic protection (preventing contact with live parts) and fault protection (stopping dangerous current in the event of a fault) (Correct answer)
- Only fuses
- Only earthing
- Only RCDs
Correct answer: Basic protection (preventing contact with live parts) and fault protection (stopping dangerous current in the event of a fault)
BS 7671 requires two levels of shock protection: basic protection (preventing access to live parts under normal conditions, e.g., insulation) and fault protection (limiting shock current if a fault occurs, e.g., earthing and protective devices).
Question 2: What does the term 'protective bonding' mean in BS 7671?
- Connecting extraneous conductive parts to the main earthing terminal to prevent dangerous potential differences (Correct answer)
- Connecting live conductors together
- Bonding the consumer unit to the meter
- Connecting the neutral to the earth at the consumer unit
Correct answer: Connecting extraneous conductive parts to the main earthing terminal to prevent dangerous potential differences
Protective bonding (main and supplementary) connects extraneous metallic parts (such as gas pipes, water pipes, structural steelwork) to the main earthing terminal, preventing dangerous voltage differences during a fault.
Question 3: What is the minimum disconnection time for a circuit with a fault to earth in a TN system supplying a socket outlet circuit not exceeding 32 A?
- 0.4 seconds (Correct answer)
- 5 seconds
- 1 second
- 0.1 seconds
Correct answer: 0.4 seconds
BS 7671 Table 41.1 requires a maximum disconnection time of 0.4 seconds for socket outlet circuits up to 32 A in TN systems, to limit the duration of dangerous touch voltages in the event of a fault.
Question 4: What does 'Zs' represent in electrical installation testing?
- The total earth fault loop impedance of the circuit (Correct answer)
- The supply impedance only
- The resistance of the cable
- The impedance of the RCD
Correct answer: The total earth fault loop impedance of the circuit
Zs is the total earth fault loop impedance, comprising the supply source impedance (Ze) plus the impedance of the phase and protective conductors of the circuit (Zs = Ze + (R1 + R2)).
Question 5: What test instrument is used to measure earth fault loop impedance?
- An earth fault loop impedance tester (Correct answer)
- A multimeter
- An insulation resistance tester
- A voltage tester
Correct answer: An earth fault loop impedance tester
An earth fault loop impedance tester injects a current pulse around the fault loop path and measures the impedance (Zs), which is compared to the maximum values in BS 7671 to verify protective device operation.
Question 6: What is the purpose of the prospective fault current (PFC) test?
- To measure the maximum short-circuit and earth fault current available at a point in the installation to ensure protective devices can safely interrupt it (Correct answer)
- To test RCD operation time
- To measure insulation resistance
- To test continuity of the earth conductor
Correct answer: To measure the maximum short-circuit and earth fault current available at a point in the installation to ensure protective devices can safely interrupt it
The PFC test measures the maximum fault current available at any point in the installation; protective devices (MCBs, fuses) must have a breaking capacity equal to or greater than the PFC to safely interrupt a fault current.
Part P BS 7671 IET Wiring Regulations 2
What is the fundamental protection against electric shock in BS 7671?