Part P BS 7671 Wiring Regulations 2 — Questions and Answers
Question 1: According to BS 7671, what is the maximum earth fault loop impedance (Zs) for a Type B 32A MCB to disconnect within 0.4 seconds on a 230V circuit?
- 0.72Ω
- 1.15Ω
- 1.44Ω (Correct answer)
- 2.30Ω
Correct answer: 1.44Ω
For a Type B 32A MCB to trip within 0.4 seconds, the fault current must reach 5 times the rated current (5 × 32A = 160A). Using Zs = Uo/Ia = 230/160 = 1.44Ω. This is the maximum tabulated value in BS 7671 Table 41.3. In practice, the measured value should not exceed 80% of this (1.15Ω) to account for temperature increase during fault conditions.
Question 2: BS 7671 Regulation 314.1 requires that every installation is divided into circuits. What is the primary reason for this?
- To increase the number of socket outlets
- To avoid danger and minimise inconvenience in the event of a fault (Correct answer)
- To make the installation more expensive
- To use more cable
Correct answer: To avoid danger and minimise inconvenience in the event of a fault
Dividing installations into circuits limits the effect of a fault to a single circuit, preventing total loss of supply to the entire installation. It also allows circuits to be matched to the specific loads they serve, enables appropriate protective device selection, and reduces the risk of fire from overloaded cables.
Question 3: What is the recommended maximum number of current-using points on a domestic ring final circuit according to BS 7671 guidance?
- 10 points
- 20 points
- No specific limit, but the floor area served should not exceed 100m² (Correct answer)
- 50 points
Correct answer: No specific limit, but the floor area served should not exceed 100m²
BS 7671 does not specify a maximum number of socket outlets on a ring final circuit. Instead, the guidance note recommends that the floor area served by a single ring circuit should not exceed 100m². The assumption is that diversity of use means not all outlets will be loaded simultaneously. Non-fused spurs are limited to one per point.
Question 4: Under BS 7671, what type of RCD is required to protect circuits supplying equipment that may produce DC fault currents (such as EV chargers and solar inverters)?
- Type AC RCD
- Type A RCD
- Type B or Type F RCD (Correct answer)
- Type S (selective/time-delayed) RCD
Correct answer: Type B or Type F RCD
Equipment containing rectifier circuits (EV chargers, solar inverters, VFDs) can produce smooth DC fault currents that Type A RCDs cannot detect. BS 7671 requires Type B RCDs (which detect AC, pulsating DC, and smooth DC fault currents) or Type F RCDs (for frequency-controlled equipment). This is increasingly relevant with the growth of EV charging installations.
Question 5: What does BS 7671 require regarding surge protection devices (SPDs) in domestic installations?
- SPDs are optional and never required
- A risk assessment must be carried out, and SPDs installed where the consequence of an overvoltage could result in serious injury, damage to valuable equipment, or disruption of critical systems (Correct answer)
- SPDs are required on every circuit
- SPDs are only required in commercial installations
Correct answer: A risk assessment must be carried out, and SPDs installed where the consequence of an overvoltage could result in serious injury, damage to valuable equipment, or disruption of critical systems
Regulation 443.4 of BS 7671 (18th Edition) requires a risk assessment for transient overvoltages. SPDs must be installed where the consequence of overvoltage could result in serious injury or loss of human life, interruption of public services, or damage to cultural heritage, commercial or industrial installations. For dwellings, SPDs are strongly recommended and increasingly standard.
Question 6: What is the purpose of the supplementary bonding requirement in BS 7671 for locations containing a bath or shower?
- To increase the voltage available for heating
- To ensure all simultaneously accessible extraneous-conductive-parts are at the same potential, reducing touch voltage (Correct answer)
- To provide a better earth return path for the lighting circuit
- To bond plastic pipes together
Correct answer: To ensure all simultaneously accessible extraneous-conductive-parts are at the same potential, reducing touch voltage
Supplementary bonding in bathrooms connects all simultaneously accessible extraneous-conductive-parts (metal pipes, radiators, baths) to ensure they are at the same electrical potential. This eliminates dangerous touch voltages between metal parts if a fault occurs. However, under the 18th Edition, supplementary bonding may be omitted if the main bonding and automatic disconnection requirements are met.
Part P BS 7671 Wiring Regulations 2
According to BS 7671, what is the maximum earth fault loop impedance (Zs) for a Type B 32A MCB to disconnect within 0.4 seconds on a 230V circuit?