Part P BS 7671 IET Wiring Regulations 5 — Questions and Answers
Question 1: What is the minimum size of a main protective bonding conductor for a domestic installation with a 100 A supply?
- 10 mm² copper (or sized per BS 7671 Table 54.8 based on the main supply conductor) (Correct answer)
- 2.5 mm² copper
- 6 mm² copper
- 16 mm² copper
Correct answer: 10 mm² copper (or sized per BS 7671 Table 54.8 based on the main supply conductor)
BS 7671 Table 54.8 specifies minimum main protective bonding conductor cross-section based on the main supply conductor; for a typical 100 A domestic supply (25 mm² service cable), a 10 mm² bonding conductor is required.
Question 2: What is 'supplementary bonding' and where was it commonly required?
- Additional bonding between simultaneously accessible exposed and extraneous conductive parts (e.g., in bathrooms) — requirements relaxed in 17th/18th editions if the installation has a 30 mA RCD (Correct answer)
- Bonding required for all metal objects in every room
- Bonding the consumer unit to the gas meter
- Bonding between different circuits in the consumer unit
Correct answer: Additional bonding between simultaneously accessible exposed and extraneous conductive parts (e.g., in bathrooms) — requirements relaxed in 17th/18th editions if the installation has a 30 mA RCD
Supplementary bonding connected metalwork in bathrooms and kitchens. Under the 17th and 18th editions, it is not required if all circuits serving the location are protected by a 30 mA RCD, which is now the preferred approach.
Question 3: What is a Type B circuit breaker and what is it typically used for?
- A circuit breaker that trips at 3–5 times its rated current for magnetic operation — used for resistive loads such as lighting and socket outlets (Correct answer)
- A circuit breaker for motor loads
- A circuit breaker for high inrush current loads
- A current-limiting circuit breaker
Correct answer: A circuit breaker that trips at 3–5 times its rated current for magnetic operation — used for resistive loads such as lighting and socket outlets
A Type B MCB operates magnetically between 3–5 times its rated current; it is suitable for circuits with little or no inrush current such as domestic socket outlets, lighting and heating circuits.
Question 4: What is a Type D circuit breaker used for?
- High inrush current loads such as transformers, X-ray equipment and large motors (trips at 10–20 × rated current magnetically) (Correct answer)
- Standard domestic socket circuits
- Lighting circuits
- Low inrush current applications
Correct answer: High inrush current loads such as transformers, X-ray equipment and large motors (trips at 10–20 × rated current magnetically)
Type D MCBs have a high magnetic trip threshold (10–20 × rated current) and are used for equipment with very high inrush currents such as transformers, uninterruptible power supplies, and large motor starters.
Question 5: What does 'RCBO' stand for and what is its advantage over using a separate RCD and MCB?
- Residual Current Breaker with Overcurrent — combines RCD and MCB in a single device protecting a single circuit (Correct answer)
- Residual Circuit Breaker Output — a type of main switch
- Remote Controlled Breaker Output — operated remotely
- Rapid Circuit Breaking Output — faster than a standard MCB
Correct answer: Residual Current Breaker with Overcurrent — combines RCD and MCB in a single device protecting a single circuit
An RCBO combines residual current (earth leakage) and overcurrent (overload and short circuit) protection in a single device for a single circuit, eliminating the problem of a single RCD tripping multiple circuits.
Question 6: What is the purpose of discrimination between protective devices?
- To ensure only the device closest to the fault operates, leaving other circuits unaffected (Correct answer)
- To ensure all devices trip simultaneously for safety
- To coordinate RCD sensitivity levels
- To ensure all fuses are the same rating
Correct answer: To ensure only the device closest to the fault operates, leaving other circuits unaffected
Discrimination (selectivity) ensures that when a fault occurs, only the protective device nearest to the fault disconnects, leaving all other circuits in service — essential for maintaining supply to critical circuits.
Part P BS 7671 IET Wiring Regulations 5
What is the minimum size of a main protective bonding conductor for a domestic installation with a 100 A supply?