Part P Consumer Unit Replacement 4 — Questions and Answers
Question 1: What is the recommended height for the consumer unit main switch in a domestic installation per accessibility guidance?
- 1.35–1.5 m above finished floor level (to the centre of the main switch operating mechanism) (Correct answer)
- 0.5 m above floor
- 2.5 m above floor
- At floor level
Correct answer: 1.35–1.5 m above finished floor level (to the centre of the main switch operating mechanism)
Accessibility guidance (BS 8300 and NHBC) recommends the consumer unit main switch be at a height (typically 1.35–1.5 m to the switch centre) where it can be reached by most users, including those with limited mobility.
Question 2: What information must the circuit chart inside the consumer unit show?
- Each circuit's purpose, cable reference method, protective device type and rating, and RCD assignment (Correct answer)
- Only the fuse ratings
- Only the circuit numbers
- Only which RCD each circuit is on
Correct answer: Each circuit's purpose, cable reference method, protective device type and rating, and RCD assignment
BS 7671 Regulation 514.9.1 requires a circuit chart (schedule) to be fixed at or adjacent to the consumer unit, showing each circuit's purpose, protective device details, cable size/reference method, and RCD or RCBO assignment.
Question 3: What is the first step when replacing a consumer unit safely?
- Isolate the supply at the cut-out (service fuse) — which requires the electricity supplier's presence or a service cut-out sealing tool — and prove dead before any work begins (Correct answer)
- Switch off the main switch only
- Switch off all MCBs
- Disconnect the meter tails
Correct answer: Isolate the supply at the cut-out (service fuse) — which requires the electricity supplier's presence or a service cut-out sealing tool — and prove dead before any work begins
The supply must be isolated at the cut-out (supply fuse) to make the meter tails and main switch terminals dead before they can be safely disconnected; this requires the electricity supplier to attend or a licensed operative with a sealing tool.
Question 4: What are 'meter tails'?
- The cables connecting the electricity meter to the consumer unit main switch — typically 25 mm² or 35 mm² insulated single-core cables (Correct answer)
- The cables entering the electricity meter from the street
- The cables connecting the consumer unit to individual circuits
- The neutral and earth conductors in the consumer unit
Correct answer: The cables connecting the electricity meter to the consumer unit main switch — typically 25 mm² or 35 mm² insulated single-core cables
Meter tails are the single-core insulated cables connecting the electricity supplier's meter to the consumer unit main switch; they are live when the supply is connected and typically 25–35 mm² cross-section for a standard 100 A supply.
Question 5: Can the meter tails be extended or modified by an electrician during consumer unit replacement?
- Meter tails can be extended or modified by a DNO-registered or competent electrician, but the service fuse and supply-side equipment must not be touched without the DNO's involvement (Correct answer)
- Meter tails can be modified by any Part P registered electrician without restriction
- Meter tails cannot be modified under any circumstances
- Meter tails can only be modified by the electricity supplier
Correct answer: Meter tails can be extended or modified by a DNO-registered or competent electrician, but the service fuse and supply-side equipment must not be touched without the DNO's involvement
Meter tails (between meter and consumer unit) are within the scope of the customer's installation and can be modified by a competent electrician; however, the supply-side (service cable, cut-out and meter) must not be interfered with without the DNO's permission.
Question 6: What type of cable gland is used when a steel-armoured supply cable enters a metal consumer unit?
- An armoured cable gland that clamps the cable armour, providing both strain relief and continuity of the metal enclosure's protection (Correct answer)
- A standard PVC cable gland
- Any gland of sufficient size
- No cable gland is needed for armoured cables
Correct answer: An armoured cable gland that clamps the cable armour, providing both strain relief and continuity of the metal enclosure's protection
Armoured cables (e.g., SWA — steel wire armour) entering a metal consumer unit must use a purpose-made armoured cable gland that clamps the armour, providing mechanical protection, strain relief, and earthing of the armour through the gland.
Part P Consumer Unit Replacement 4
What is the recommended height for the consumer unit main switch in a domestic installation per accessibility guidance?