Outboard Power Distribution and Circuit Protection
What the candidate must know
This domain covers the path from source to load: main switch or isolator, distribution board, busbars, fuses and circuit breakers, residual current devices, and the discrimination between them. Candidates must understand overcurrent (overload and short circuit), earth fault protection, breaking capacity, and selectivity. Know the difference between a fuse's melting characteristic and a breaker's time-current curve, and why a device's rated current is not the same as its tripping current. Understand diversity and load balancing across phases where applicable.
Traps that appear in questions
- Confusing rated current with tripping current; a 16 A breaker holds 16 A indefinitely and trips on the curve above it.
- Choosing a protective device by cable size alone without checking fault level against breaking capacity.
- Assuming an RCD gives overload protection; it protects against earth fault current only.
- Ignoring discrimination: a downstream fault should trip the nearest device, not the main.
- Forgetting that protective devices must be accessible and that isolation must be capable of being locked off.
Worked example / rule of thumb
A 2.5 mm² circuit is protected by a 16 A breaker with a measured loop impedance giving a fault current of 180 A. The breaker's instantaneous trip is typically 5-10 times rated current, so 80-160 A; 180 A clears it, but the margin is thin. Rule of thumb: aim for fault current at least 1.5 times the top of the instantaneous band, and check the manufacturer's let-through energy against the cable's withstand.