MCCB replacement in a trading mall: labels, back-feeds and a supervisor in a hurry
Your team is scheduled to replace a burnt-out 63 A MCCB feeding a tenant's submain in a busy shopping mall in Sha Tin. The mall is trading and the board is in a locked landlord's electrical room off the service corridor. The board chart, last updated in 2011, lists way 7 as 'SPARE'. The MCCB you must change is the third from the left, and the label beneath it reads 'Way 9 – T42 Escalator'. There is also a hand-written sticker on the panel: 'DO NOT SWITCH – FIRE'. The centre manager says the tenant's shop can lose power, but the mall's fire alarm and the escalator must stay on. The building has a 250 kVA standby generator with an automatic transfer switch and a rooftop PV array connected on the landlord's side of the intake. Your supervisor says: 'Just pull the fuse on the third from the left, put a lock on the door and get on with it, we're behind programme.' It is 11 a.m. and the mall is full.
1. Set out the isolation procedure you would follow before touching the MCCB, and explain why each step is needed given this building.
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Identify before you isolate. The board chart is fourteen years old and the label conflicts with it. Before switching anything, trace the circuit from the MCCB to the tenant's submain by inspection where possible, and confirm with the building management which tenant is which. A label is an assertion, not a test.
Isolate at the correct point. Open the upstream device that supplies the MCCB - not the MCCB itself, which is the device being replaced. Where the upstream device also supplies the fire alarm or escalator, agree a shutdown window with the building management and the fire service requirements; a fire service installation cannot simply be switched off because a contractor is behind programme.
Lock off and tag. Fit a personal padlock and a multi-lock hasp so no one else can reclose, and attach a warning notice with your name, the date and the reason. Keep the key with you. Locking the room door is not lock-off - the supervisor or a cleaner with a key can still open it.
Prove dead at the point of work, not at the board. Use an approved two-pole voltage detector of the correct category, prove it on a proving unit, test every combination of line, neutral and earth at the outgoing terminals of the MCCB, then prove the detector again on the proving unit. Test for both AC and DC, because the escalator drive and the PV inverter both contain DC sections.
Consider every back-feed. The automatic transfer switch can energise the board from the generator; the PV array will energise the landlord's side whenever the sun is out; a UPS may hold up the fire panel. Confirm the ATS is in the correct mode and has been proved not to operate, and confirm the PV inverter is isolated at its own isolator, not just at the board.
Only after all of that would I remove the MCCB.
2. When you test at the outgoing terminals of the third MCCB from the left, you still have 230 V to earth and you can hear the escalator running. What do you do?
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Stop work immediately. The circuit is not proved dead at the point of work, so nothing further happens until it is.
Establish what is actually happening before touching anything.
- Re-test with the detector proved on the proving unit, to rule out a faulty instrument.
- Clamp the conductor to see whether current is flowing, and ask the building management to cycle the escalator control to identify whether this device really is the escalator supply.
- Check whether the board has a second incoming supply, a bus-section, or a generator-backed section that has been paralleled or left closed.
- Check for a borrowed or interconnected circuit: the 2011 chart may be wrong because works after 2011 were never recorded. The escalation may be fed from this board while the 'spare' way is actually the tenant's shop.
Then act on what you find. Report to the supervisor and the building management in writing, with photographs of the labels and the test results. Do not proceed on the assumption that the labels are right, and do not pull a fuse or open a device 'to see what goes off' while the mall is trading - that could drop the fire alarm or trap someone on an escalator.
Before any further attempt: correct the board chart, agree a documented switching schedule with the building management, and re-issue the isolation with the correct device identified. If the escalator supply and the tenant's supply cannot be separated, escalate - the job may need a planned night-time shutdown with the escalator taken out of service and the fire alarm on a temporary supply.
3. The supervisor insists the MCCB change be done live to avoid a shutdown. Evaluate that instruction and state what you would do.
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Live work on a 63 A MCCB in a live distribution board with exposed busbars and with the escalator and fire alarm on adjacent ways is not justifiable. The regulations require work to be done dead unless it is unreasonable to do so, and even then only under a documented risk assessment, a permit-to-work, suitable insulated tools and PPE, and with a competent person and a second person present. None of those conditions is met here: the board can be shut down, a shutdown window can be arranged, and the consequence of an arc or a dropped spanner in a trading mall is fire, injury and evacuation.
Arguments to put to the supervisor, in writing:
- The board can be made dead with a planned shutdown; therefore working live is not 'necessary'.
- A 400 V three-phase fault at 63 A produces an arc flash and molten metal; a face shield and gloves do not make it acceptable.
- The building's fire alarm and escalator are life-safety and passenger-carrying systems; a live incident would also breach the landlord's insurance conditions and the mall's operating licence.
- Under the Electricity Ordinance and the occupational safety and health legislation, the employer and the individual both carry duties; 'everyone does it' is not a defence.
What I would do: decline the instruction, record the refusal and the reasons, and put the compliant alternative to the client - a night-time shutdown with the tenant on a temporary supply if required, the escalator taken out of service with the correct signage, the fire alarm maintained and the building management notified in advance. If the supervisor persists, escalate to the contractor's authorised signatory and, if necessary, to the building management, because they too have a duty to control work on their installation.