Part P Consumer Unit Replacement 1 — Questions and Answers
Question 1: Why was consumer unit replacement made notifiable work under Part P?
- Because a consumer unit is the main protection and control device for the entire installation, and incorrect replacement could leave the whole property unsafe (Correct answer)
- Because it requires planning permission
- Because only the electricity supplier can replace consumer units
- Because consumer units must be approved by the local authority
Correct answer: Because a consumer unit is the main protection and control device for the entire installation, and incorrect replacement could leave the whole property unsafe
The consumer unit contains all the protective devices for the installation; incorrect installation could compromise protection for every circuit, so replacement is notifiable to ensure compliance with current regulations is verified.
Question 2: What does BS 7671 18th Edition require all new domestic consumer units to be made from?
- Non-combustible material (metal enclosures) (Correct answer)
- Plastic (to prevent shock hazard)
- Any material rated to 850°C
- Any material if the consumer unit is less than 100 A
Correct answer: Non-combustible material (metal enclosures)
Following Amendment 3 to the 17th Edition (and carried into the 18th), domestic consumer units must have enclosures manufactured from non-combustible material (metal), to prevent fires from arcing faults inside the unit spreading to plastic.
Question 3: When replacing a consumer unit, what additional work is required under current regulations?
- The existing installation must be inspected and tested; the earthing and bonding must be verified (and upgraded if deficient); an EIC must be issued (Correct answer)
- No additional work is required beyond fitting the new consumer unit
- Only the new consumer unit needs to be tested, not the existing installation
- The entire installation must be rewired before a new consumer unit can be fitted
Correct answer: The existing installation must be inspected and tested; the earthing and bonding must be verified (and upgraded if deficient); an EIC must be issued
Consumer unit replacement includes a partial inspection and test of the existing installation (particularly earthing, bonding, and assessment of circuits) to confirm the installation is safe before the new unit is connected.
Question 4: What type of consumer unit is now considered best practice for new domestic installations?
- A fully RCBO-equipped consumer unit (one RCBO per circuit) or a consumer unit with at least two RCDs (split-load) protecting all circuits (Correct answer)
- A rewirable fuse board with no RCDs
- A single-RCD consumer unit with all circuits on one RCD
- A consumer unit with only MCBs and no RCDs
Correct answer: A fully RCBO-equipped consumer unit (one RCBO per circuit) or a consumer unit with at least two RCDs (split-load) protecting all circuits
Current best practice (and required by BS 7671 for additional protection) uses either a fully RCBO-fitted consumer unit or a split-load dual-RCD board; all socket outlet circuits and bath/shower circuits must have 30 mA RCD protection.
Question 5: What is the purpose of the main switch in a new consumer unit?
- To provide a means of isolating the entire installation from the supply for maintenance and emergency isolation (Correct answer)
- To control all RCDs simultaneously
- To protect the installation from overloads
- To control the electricity supply to the meter
Correct answer: To provide a means of isolating the entire installation from the supply for maintenance and emergency isolation
The main switch (rated as a double-pole isolator) allows the entire installation to be safely de-energised for maintenance or in an emergency, disconnecting both line and neutral conductors simultaneously.
Question 6: What is the typical rated current for the main switch in a single-phase domestic consumer unit?
- 100 A (Correct answer)
- 32 A
- 63 A
- 230 A
Correct answer: 100 A
The standard main switch in a domestic consumer unit is rated at 100 A, matching the standard domestic supply fuse rating; this allows for the maximum domestic load without nuisance operation of the main switch.
Part P Consumer Unit Replacement 1
Why was consumer unit replacement made notifiable work under Part P?