Part P Consumer Unit Replacement 3

Question 1

What is a 'split-load' consumer unit?

Accepted 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

Answer

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?

Accepted 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

Answer

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?

Accepted 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

Answer

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?

Accepted 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

Answer

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?

Accepted 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

Answer

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.

Part P Electrical Practice Test

Part P Electrical Practice Test

Part P Consumer Unit Replacement 3