1. Why MCB / RCBO alone aren't enough
MCBs handle short-circuit and overload; RCBOs add residual-current protection. But a series arc at a loose terminal or inside a damaged cable can sit well below their trip thresholds while generating localised heat above 1000°C — more than enough to ignite insulation.
2. How AFDDs detect a fault
An AFDD analyses the high-frequency signature of the current waveform, recognises the noise pattern unique to arcing, and trips the circuit within tens of milliseconds.
3. Where they pay off most
Older flats, subdivided units, offices with extension-strip jungles, restaurant kitchens, warehouses storing flammables, and anywhere AC or dehumidifiers run 24/7.
4. Are they mandatory in Hong Kong?
EMSD does not currently mandate AFDDs across the board, but the Electricity (Wiring) Regulations and IEC 60364-4-42 recommend them in fire-risk premises. New builds and major refits should design to the latest revision.
5. Choosing a unit
Look for IEC 62606 certification — major brands include CHT, Schneider and ABB — and have a Registered Electrical Worker size and select against your load and panel type.