Part P BS 7671 IET Wiring Regulations 5

Question 1

What is the minimum size of a main protective bonding conductor for a domestic installation with a 100 A supply?

Accepted Answer

10 mm² copper (or sized per BS 7671 Table 54.8 based on the main supply conductor)

Answer

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?

Accepted 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

Answer

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?

Accepted 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

Answer

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?

Accepted Answer

High inrush current loads such as transformers, X-ray equipment and large motors (trips at 10–20 × rated current magnetically)

Answer

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?

Accepted Answer

Residual Current Breaker with Overcurrent — combines RCD and MCB in a single device protecting a single circuit

Answer

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.

Part P Electrical Practice Test

Part P Electrical Practice Test

Part P BS 7671 IET Wiring Regulations 5