New 32 A Circuit in an Ageing Commercial Switchboard
A restaurant renovation in a 30-year-old Hong Kong commercial building adds a 32 A single-phase circuit for a 7 kW combi oven. The existing 400 A three-phase, four-wire switchboard has a spare 63 A MCCB way. The proposed cable is 6 mm² PVC-insulated copper in 25 mm conduit, 30 m long, run with three other loaded circuits in a 40 °C plant room. The measured earth-fault loop impedance at the new socket is 0.9 Ω; the 32 A Type C MCB is rated 10 kA. The neutral bar shows heat discolouration, and a 30 mA RCD trips intermittently when the oven and a nearby coffee machine operate together. The building owner wants the circuit energised for a soft opening in three days. The registered electrical worker says the existing neutral is 'probably fine' and proposes to replace the MCB with a 40 A one to stop tripping.
1. Identify the non-compliances and statutory duties, and explain why the proposed 40 A MCB is unacceptable.
Read the worked answer
Several issues must be resolved before energisation. The 6 mm² PVC cable in a 40 °C plant room with four grouped circuits is likely below 32 A after derating; verify against the Hong Kong Code of Practice for the Electricity (Wiring) Regulations. The 0.9 Ω earth-fault loop impedance is too high for a 32 A Type C MCB: the maximum is about 0.72 Ω for 0.4 s disconnection, so the circuit may not disconnect safely. The neutral bar discolouration suggests a loose or overloaded neutral; the intermittent RCD trip with the coffee machine suggests a shared neutral, leakage or harmonic current. Replacing the 32 A MCB with a 40 A device is wrong: it reduces protection and may exceed cable capacity. Under the Electricity Ordinance (Cap. 406), the work must be carried out by a registered electrical contractor and registered electrical worker, tested, and a work completion certificate issued. Do not energise for the soft opening.
2. Set out the design verification, inspection and testing required before you would support energisation, and how you would handle the owner's opening date.
Read the worked answer
Carry out a design check: calculate design current, apply ambient, grouping and thermal-insulation correction factors, check voltage drop over 30 m, and verify the protective device rating. Measure insulation resistance, polarity, earth continuity and RCD trip time. Isolate and inspect the neutral bar; re-torque or replace it, and measure neutral current with the oven and coffee machine running. If the neutral is shared, separate the circuits. Improve the earth-fault loop by adding a supplementary earth electrode or using a 30 mA RCBO with a disconnection time that meets the adiabatic check; do not rely on the RCD for fault protection alone. Only after tests and a signed certificate should you request energisation. Tell the owner in writing that the opening date cannot be met safely, give the specific defects and the date you can re-test, and offer a temporary alternative only if it is separately protected and certified.