Consumer Unit Replacement Flashcards
6 cards from real PART P practice questions. Tap to flip, then mark Knew It or Still Learning — missed cards come back until you master them.
Read the first 6 Consumer Unit Replacement flashcards as text
What is a 'split-load' consumer unit?
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.
What is the advantage of a fully RCBO-equipped consumer unit over a split-load RCD unit?
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).
What is a 'high integrity' consumer unit?
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.
Where must the consumer unit be located in a domestic property to comply with BS 7671?
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).
What is the purpose of a 100 mA time-delayed RCD sometimes used at the origin of an installation?
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.
What is a dual function RCD/main switch combined device?
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.