Repeated RCBO tripping on a restaurant kitchen radial
An 80-seat restaurant in Kwun Tong was fitted out six months ago. The kitchen sub-board is fed from the landlord's intake through a 63 A MCCB and contains four 30 mA Type AC RCBOs. The RCBO protecting three 13 A radial socket outlets on the tiled wall beside the dishwashing area has tripped eleven times in three weeks - always during service, and twice within minutes of the kitchen floor being hosed down. Nothing else in the kitchen trips. The appliances are an under-counter chiller, a 2.2 kW dishwasher and a 3 kW instantaneous water heater, all plugged into those outlets and all able to run at the same time. The radial is 4 mm² on a 32 A Type C MCB. With every appliance unplugged, insulation resistance at the RCBO is 250 MΩ L–N, 250 MΩ L–E and 250 MΩ N–E. With the appliances plugged back in, N–E falls to 0.7 MΩ. A leakage clamp around line and neutral reads 4.6 mA with all three running. The kitchen manager wants it 'fixed today' and suggests fitting a 100 mA RCBO instead.
1. Using the measured insulation resistance and leakage values, describe how you would locate the cause, in order, and state what each result rules in or out.
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First, read the numbers properly. The fixed wiring is sound: 250 MΩ on all three tests is far above the 1 MΩ minimum. The 0.7 MΩ N–E that appears once appliances are plugged in is dominated by the filter capacitors and suppression components inside the chiller and dishwasher, and at 220 V it represents only about 0.3 mA - it cannot operate a 30 mA device. The total standing leakage of 4.6 mA is roughly one-sixth of the RCBO rating and well below the 30–40% of rating that would normally cause nuisance tripping. So neither a hard fault nor accumulated leakage explains eleven trips.
That points to an intermittent, environment-driven cause. Work through it as follows:
- Prove the device itself first with an RCD tester: ×½ must not trip, ×1 must trip within 300 ms, ×5 within 40 ms, on both half-cycles. A damp-sensitive or ageing RCBO can trip at a fraction of its rating.
- Isolate and remove each socket, including the back box. Look for water tracking, corrosion on the terminals, blackened or loose connections, damaged sheath at the gland, and whether the wall boxes are sealed.
- Split the radial at a joint and test each leg L–E and N–E separately at 500 V DC. A reading that collapses after the wall is wetted is a damp box or a damaged conductor in the wet zone, not a fault in the whole radial.
- Rule out a shared neutral with a neighbouring circuit by isolating one circuit at a time and re-testing; a borrowed neutral shows as a low N–E value that persists with the RCBO off.
- Measure standing leakage with the clamp on each appliance individually and watch which combination coincides with a trip.
Only after these checks would you conclude 'appliance leakage' - the evidence here does not support it.
2. The manager says the cheapest fix is to replace the 30 mA RCBO with a 100 mA device, or to disconnect the circuit protective conductor from the sockets. Explain your response and set out the compliant options you would offer.
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Both suggestions must be refused.
A 30 mA device on socket outlets rated up to 32 A (and on any final circuit where the CoP requires additional protection) is there to limit touch voltage and let-through energy so that a person contacting live metal survives. A 100 mA device will not operate before the fault current becomes lethal, and it does not remove the underlying moisture problem - it just makes the symptom reappear as a shock or a fire. Disconnecting the CPC is worse: every Class I appliance on that radial would then have a metal enclosure with no earth-fault path, and the RCBO would have nothing to sense. Both changes make the installation non-compliant with the Electricity (Wiring) Regulations and the Code of Practice, invalidate the completion certificate, and would leave you personally exposed if anything happened.
Compliant options, in order of preference:
- Remove the cause - reseal or replace the wall boxes, re-terminate with new glands, verify the socket outlets are suitable for a wet, hosed area (lidded, IP-rated accessories) and reposition any outlet that sits in the direct spray line.
- Confirm the RCBO is the correct type. Appliances with rectifier inputs and switch-mode supplies can produce pulsating DC residual current that a Type AC device handles poorly; a Type A device is the correct selection for this load mix.
- Reduce the leakage concentration by splitting the radial into two circuits, or by giving the water heater its own fixed supply, so no single 30 mA device carries the whole appliance group.
- Replace the device only with one of the same sensitivity and correct type and characteristic, and record the RCD trip-time test on the certificate.
If the client still insists, put the refusal and the reasons in writing and escalate to the building's authorised signatory.
3. The sockets are mounted 700 mm from the sink edge, have no lids or covers, and the tiled wall is hosed down by staff every night. What does this tell you about the original design, and what would you change before re-energising?
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The original selection ignored the external influences at the location. A socket outlet in a zone that receives direct water jets from a hose is not served by a standard unprotected accessory. The relevant question is not 'how far from the sink' but 'what does the environment do to the enclosure' - routine hosing is a water-jet condition, so the accessories and their entries need an enclosure rating appropriate to jets, not merely splash protection, and every conduit entry into the back box has to be sealed with the correct gland and sealant so water cannot track into the box and sit against the terminals.
What I would change before energising:
- Replace the three socket outlets with lidded, IP-rated accessories (minimum IP55, ideally IP66, with the lid closed when not in use) or, better, relocate them to a position out of the direct spray line and above any wash-down splash zone.
- Fit sealed, correctly sized cable glands or sealing entries at every box; re-terminate any conductor showing green corrosion or reduced strand count.
- Check the back boxes themselves: a tiled wall with an unsealed box behind it will collect condensate. Replace damaged boxes rather than reusing them.
- Confirm the 4 mm² radial and the 32 A Type C MCB are still appropriate for the revised arrangement, and confirm the RCBO Type A selection for the appliance mix.
- Re-test: insulation resistance on the completed circuit, RCD trip times, polarity and earth loop impedance, and record all results.
I would also advise the client in writing that nightly hosing of a wall containing electrical accessories should be stopped, or the outlets relocated outside the wet zone - the enclosure rating is the last line of defence, not a licence to spray water at electrical equipment.