Part P Consumer Unit Replacement 3 — Questions and Answers
Question 1: What is a 'split-load' consumer unit?
- A consumer unit with two RCDs, each protecting a separate group of circuits, so a fault tripping one RCD does not affect the other group (Correct answer)
- A consumer unit split into two separate enclosures
- A consumer unit where the main fuse is split between two phases
- A consumer unit where lights and sockets are on separate MCBs
Correct answer: A consumer unit with two RCDs, each protecting a separate group of circuits, so a fault tripping one RCD does not affect the other group
A split-load consumer unit has two RCDs in the main unit; circuits are divided between the two RCDs so that if one trips (due to fault or nuisance trip), only half the circuits lose supply, maintaining some services.
Question 2: What is the advantage of a fully RCBO-equipped consumer unit over a split-load RCD unit?
- Each circuit has its own RCBO so a fault or nuisance trip on one circuit only affects that circuit — all other circuits remain live (Correct answer)
- RCBOs provide better overcurrent protection than MCBs
- RCBOs are cheaper than MCBs combined with RCDs
- A fully RCBO board requires a smaller enclosure
Correct answer: Each circuit has its own RCBO so a fault or nuisance trip on one circuit only affects that circuit — all other circuits remain live
With a dedicated RCBO per circuit, any trip only affects the single faulted circuit; a split-load RCD board can lose half the installation's circuits when one RCD trips, which can be inconvenient or dangerous (e.g., freezer loss).
Question 3: What is a 'high integrity' consumer unit?
- A consumer unit where essential circuits (smoke alarms, boiler, freezer) are individually protected by RCBOs while other circuits are on split-load RCDs, ensuring critical circuits are never lost (Correct answer)
- A consumer unit rated for commercial use only
- A consumer unit with a fire-resistant enclosure
- A consumer unit with a higher current main switch
Correct answer: A consumer unit where essential circuits (smoke alarms, boiler, freezer) are individually protected by RCBOs while other circuits are on split-load RCDs, ensuring critical circuits are never lost
A high integrity (or high availability) consumer unit protects critical circuits (smoke alarms, refrigerator, security) with individual RCBOs, while less critical circuits are on split-load RCDs, combining flexibility with security for essential loads.
Question 4: Where must the consumer unit be located in a domestic property to comply with BS 7671?
- In a readily accessible location, not in a bathroom, not in a room with an explosive atmosphere, and where the main switch can be reached without obstruction — typically the hall, utility room or kitchen (Correct answer)
- Only in the kitchen
- Only in an understairs cupboard
- Only at the point of entry of the supply cable
Correct answer: In a readily accessible location, not in a bathroom, not in a room with an explosive atmosphere, and where the main switch can be reached without obstruction — typically the hall, utility room or kitchen
The consumer unit must be readily accessible for operation in an emergency; it must not be in a bathroom and should be at a height convenient for operation (typically 1.5–2 m to the top of the unit per NHBC guidance).
Question 5: What is the purpose of a 100 mA time-delayed RCD sometimes used at the origin of an installation?
- To provide discrimination against downstream 30 mA RCDs — it trips only if the downstream RCD fails to clear a fault, ensuring the main supply is maintained unless all other protection fails (Correct answer)
- To protect the main supply cable from overload
- To provide additional protection for the whole installation
- To replace the 30 mA RCDs in the consumer unit
Correct answer: To provide discrimination against downstream 30 mA RCDs — it trips only if the downstream RCD fails to clear a fault, ensuring the main supply is maintained unless all other protection fails
A 100 mA time-delayed RCD at the origin provides a last-resort protection level while discriminating against the 30 mA RCDs on individual circuits — it won't trip on a 30 mA fault (cleared by the circuit RCD) but will trip if the downstream device fails.
Question 6: What is a dual function RCD/main switch combined device?
- A main switch that also incorporates a whole-installation RCD function, combining both in a single device at the head of the consumer unit (Correct answer)
- A main switch with an integral MCB
- A device combining RCD with the supply meter
- A device that combines the service fuse and main switch
Correct answer: A main switch that also incorporates a whole-installation RCD function, combining both in a single device at the head of the consumer unit
Some consumer units use a combined RCD/main switch at the head, providing both isolation and whole-installation residual current protection in a single device, simplifying the design and reducing component count.
Part P Consumer Unit Replacement 3
What is a 'split-load' consumer unit?