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 does 'cable reference method' mean on a consumer unit circuit schedule?
Answer: The installation method for the cable (e.g., clipped direct, in conduit, embedded in wall, under floor insulation) which determines the current-carrying capacity of the cable
The reference method describes how the cable is installed; BS 7671 specifies different current ratings for the same cable cross-section depending on the installation method (thermal conditions affect heat dissipation and thus current capacity).
What is the consequence of using a cable with an insufficient current-carrying capacity for its circuit load?
Answer: The cable insulation overheats and degrades, potentially causing a fire — eventually the protective device will operate but only after dangerous heating has already occurred
An undersized cable carrying excess current will overheat, degrading and potentially igniting the insulation; the protective device (MCB or fuse) may not trip until the cable is already damaged because it is sized for the protective device, not the actual load.
What does 'ambient temperature correction factor' mean for cable current rating?
Answer: A factor applied to reduce the cable's current-carrying capacity when it is installed in an environment hotter than the reference temperature (typically 30°C)
Cable current ratings in BS 7671 are based on a reference ambient temperature; if the ambient temperature is higher (e.g., in a roof space in summer), the cable carries less current for the same temperature rise, and a correction factor is applied.
What is a 'grouping factor' for cables?
Answer: A derating factor applied when multiple cables are bunched together, reducing each cable's current-carrying capacity due to mutual heating
When cables are grouped (touching or in a bundle), they cannot dissipate heat as effectively as a single cable in free air; a grouping (or bunching) derating factor reduces each cable's permitted current-carrying capacity.
How is a cable's current-carrying capacity (Iz) selected for circuit design?
Answer: Iz must be greater than or equal to the design current (Ib) and the selected protective device's nominal current (In), applying correction factors for temperature, grouping and thermal insulation
Cable selection requires: Iz ≥ In ≥ Ib (after applying all derating factors for temperature, grouping, thermal insulation), and the protective device's effective operating current must not exceed 1.45 × Iz of the cable.
What is the purpose of adding circuit breaker lock-offs or CU isolators during consumer unit replacement?
Answer: To prevent accidental re-energisation of circuits while work is in progress, ensuring safe working conditions throughout the replacement process
Locking off individual MCBs or applying circuit-specific isolators prevents accidental re-energisation by others while the electrician is working on the installation, a fundamental safe working practice under Electricity at Work Regulations.