Part P Consumer Unit Replacement 2 — Questions and Answers
Question 1: What must be checked about the existing earthing arrangement before replacing a consumer unit?
- The type of earthing system (TN-S, TN-C-S, TT), the condition and size of the earthing conductor, and compliance with the current requirements of BS 7671 (Correct answer)
- Only the visual condition of the earthing conductor
- Only the earthing system type
- Earthing does not need to be checked during consumer unit replacement
Correct answer: The type of earthing system (TN-S, TN-C-S, TT), the condition and size of the earthing conductor, and compliance with the current requirements of BS 7671
The earthing system type determines permissible protections (e.g., TT requires RCDs); the earthing conductor must be verified as correctly sized and connected; deficiencies must be remedied before the new consumer unit is commissioned.
Question 2: What is the recommended approach if the existing main bonding conductors are found to be undersized during a CU replacement?
- They should be upgraded to comply with the current requirements of BS 7671 Table 54.8 as part of the consumer unit replacement work (Correct answer)
- Leave them as they are if they are in good condition
- Only upgrade them if the customer specifically requests it
- Oversized bonding conductors are never required regardless of the supply size
Correct answer: They should be upgraded to comply with the current requirements of BS 7671 Table 54.8 as part of the consumer unit replacement work
Upgrading undersized bonding conductors to current requirements is best practice and should be recommended or carried out as part of CU replacement; deficiencies that affect safety must be addressed.
Question 3: After fitting a new consumer unit, which test must be performed to check the consumer unit itself?
- Verification that all connections are secure, correct polarity, insulation resistance of busbars, and functional testing of all protective devices (RCD tests, MCB tests) (Correct answer)
- Only a visual inspection
- Only an RCD test
- Only an insulation resistance test of the whole installation
Correct answer: Verification that all connections are secure, correct polarity, insulation resistance of busbars, and functional testing of all protective devices (RCD tests, MCB tests)
After installation, the new consumer unit should be verified: all connections checked for security and polarity, insulation resistance tested, and all RCDs tested for correct operation before the installation is handed back to the occupier.
Question 4: What is the purpose of the neutral bar in a consumer unit with RCDs?
- In an RCD-equipped consumer unit, each RCD has its own neutral bar for the circuits it protects — neutral conductors must be connected to the correct bar matching their circuit's RCD to ensure correct operation (Correct answer)
- All neutral conductors go to a single common neutral bar regardless of RCD
- Neutral conductors are connected to the earth bar
- Neutral bars are not used in RCD consumer units
Correct answer: In an RCD-equipped consumer unit, each RCD has its own neutral bar for the circuits it protects — neutral conductors must be connected to the correct bar matching their circuit's RCD to ensure correct operation
Correct neutral grouping is critical in split-load RCD consumer units; each circuit's neutral must be connected to the neutral bar associated with its RCD, otherwise the RCD will not sense the correct residual current.
Question 5: Why is it important to identify all circuit cables before removing the old consumer unit?
- To ensure each circuit is reconnected to the correct protective device in the new consumer unit, maintaining the circuit's labelling and protection level (Correct answer)
- Because cables cannot be disconnected without labelling
- To comply with Part P only
- Because unlabelled cables cannot be inspected
Correct answer: To ensure each circuit is reconnected to the correct protective device in the new consumer unit, maintaining the circuit's labelling and protection level
Correct identification ensures each circuit is reconnected to a protective device of the right type and rating in the new unit, maintaining circuit protection and preventing mis-wiring that could leave circuits unprotected or on incorrect RCDs.
Question 6: What documentation must be issued upon completion of a consumer unit replacement?
- An Electrical Installation Certificate with Schedule of Inspections and Schedule of Test Results, plus a Building Regulations compliance notification (Correct answer)
- Only a receipt for the work
- Only a handover note to the occupier
- Only the consumer unit manufacturer's installation documentation
Correct answer: An Electrical Installation Certificate with Schedule of Inspections and Schedule of Test Results, plus a Building Regulations compliance notification
Consumer unit replacement is notifiable work under Part P, requiring an EIC with full schedules and Building Regulations compliance notification (either self-certified via a Competent Person scheme or through Building Control).
Part P Consumer Unit Replacement 2
What must be checked about the existing earthing arrangement before replacing a consumer unit?