Protective devices exist to interrupt fault current before it damages conductors, equipment or people. A fuse melts, a circuit breaker trips and resets, and a residual-current device detects a current imbalance to earth. Each device has a defined role and a defined discrimination behaviour relative to the devices around it.
Matching load, device and conductor
A sound design keeps the design current of the circuit at or below the device rating, and the device rating at or below the current-carrying capacity of the conductor. The actual values come from the installation standard, the cable data and the load characteristics.
Checking a circuit condition
Problem. A circuit carries a design load of 25 A. The protective device is rated 32 A and the installed conductor has a tabulated capacity of 40 A. Does the general condition hold?
Solution. 25 A ≤ 32 A ≤ 40 A, so the condition holds. Other requirements — starting current, disconnection time, ambient derating and voltage drop — must still be checked.
Discrimination
Discrimination means that the protective device nearest the fault operates first, so the rest of the installation stays energised. Achieving it requires compatible device characteristics and is verified from the manufacturer’s selectivity data, not from ratings alone.
- A fuse is a one-shot device that interrupts by melting.
- A circuit breaker trips and can be reset after the fault is cleared.
- A residual-current device responds to current imbalance to earth.
- Discrimination keeps a fault local to the affected circuit.
- Ratings and disconnection times must be verified against the local standard.
- Protective devices interrupt fault current safely.
- Keep I_load ≤ I_device ≤ I_conductor.
- Selectivity is proven from manufacturer data.
- Never upsize a device to mask nuisance tripping.
Worked example
A 13 A design load with a 16 A device and a conductor rated 20 A satisfies 13 ≤ 16 ≤ 20. A 20 A device on the same circuit would not, because it would leave the conductor unprotected at its limit.