Consumer unit change in a block of flats: borrowed neutral and a sealed supply
You are booked to replace the consumer unit in Flat 3 of a converted four-storey terrace. Each flat is metered separately in a locked communal cupboard; Flat 3 has a 100 A DP main switch with a sealed meter and no lockable isolator on the flat side. The landlord's supply in the same cupboard feeds communal lighting and a fire alarm panel. The tenant is out and the letting agent has signed your permit but is now uncontactable, and only the agent holds the cupboard key. While identifying circuits, you open the lighting MCB and the communal landing light outside the flat goes out. The flat below is occupied by a family with a young child, and the communal stair is the only escape route from the upper floors.
1. Set out the isolation you would carry out to change this consumer unit, including your tester checks. Explain precisely what the 100 A main switch does and does not isolate.
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What the main switch does not do
The 100 A main switch isolates the outgoing ways and the busbar. It does not isolate the meter tails between the meter and the main switch, and it does not isolate the sealed cut-out. Those tails stay live throughout, so a board change cannot be done on the strength of the main switch alone.
The cut-out fuse is sealed and belongs to the DNO/meter operator. You do not pull it. Isolation at the origin must be arranged with the supplier, which needs access to the communal cupboard you have been refused. The job is not safe to start.
The sequence, once genuine isolation is available
- Identify every source: the flat's supply, the landlord's circuits in the same cupboard, and confirm no PV, battery or generator.
- Risk assessment and method statement; inform the tenant and other residents.
- Operate the point of isolation, fit a lock-off, attach a warning notice, keep the key on your person. Where the isolation is remote or not under your control, lock that point too.
- Prove the voltage indicator on a proving unit (GS38 compliant, fused probes), test L-N, L-E and N-E at the point of work on every conductor, then prove the indicator again.
- Repeat the prove-test-prove sequence if you leave and return.
- Treat the tails as live until you have personally proved them dead - which in this case you cannot do.
Judgement
Proving dead is a claim about a specific point, not about the installation. Any answer that says 'main switch off, proved dead at the outgoing terminals, proceed' misses the tails entirely.
2. The landing light goes out when the flat's lighting MCB is opened. What does that tell you, and what are the dangers for you, the tenant and the other residents?
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Diagnosis
A circuit on one origin losing its return when a circuit on another origin is switched is the classic signature of a borrowed neutral: the communal lighting neutral has been landed in Flat 3's neutral bar, and probably the line taken from a flat way too, so the landlord's lighting is effectively wired through the flat's board.
Dangers to you
- Isolating a Flat 3 circuit at a single-pole MCB leaves that shared neutral connected and carrying current from the landlord's supply. Undo it at the bar and you are working on a live conductor.
- Insulation resistance results for the flat become meaningless - you are testing two installations joined together.
- An RCD on either side can trip for faults on the other, and any test result you record may be false.
Dangers to residents
- The landlord's lighting now depends on a tenant's main switch. Anyone restoring the communal light, or the tenant switching off at the main switch, is switching someone else's circuit.
- If the fire alarm shares that neutral, a flat's fault can disable a life-safety system.
- The tenant is paying for communal lighting through their meter.
Judgement
Opening the flat's main switch to 'make the landing light safe' is not a fix - it is you operating a circuit you do not own and cannot isolate. Record, report, do not rewire the landlord's installation on a tenant's authority.
3. What do you do now, before you leave site, and what goes in writing?
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On site
- Do not pull the sealed fuse, do not work on the tails, do not start the board change.
- Leave the installation as you found it, energised and safe, with the borrowed neutral still connected. Removing it would leave the communal lighting and possibly the fire alarm dead with no warning to residents.
- Attach a durable warning notice at the communal cupboard identifying the borrowed neutral and the two origins.
In writing
A short report to the letting agent and the landlord, copied to the tenant, stating:
- The consumer unit change cannot proceed: the origin cannot be isolated because the meter cupboard is not accessible, and a sealed DNO cut-out will not be interfered with.
- A borrowed neutral exists between the landlord's lighting and Flat 3. This is a potentially dangerous defect (C2 in EICR terms) on the communal installation and it invalidates testing of the flat.
- It must be rectified by the landlord's contractor before any board change, and a separate circuit with its own neutral run from the landlord's origin.
- The energy supplier should be told if the supply arrangement cannot be confirmed.
Judgement
The exam question is not 'can you swap a board?' but 'do you refuse, and can you justify refusing in writing?' An uncontactable client plus a sealed supply plus a cross-installation neutral is three independent reasons to walk away, and the fire alarm makes it urgent rather than merely inconvenient.