What has to be established first

The design current of the circuit

The cable size and its current-carrying capacity after derating

The prospective short-circuit current at the point of installation

The characteristics of the load — resistive, inductive, capacitive, or electronic

Whether earth leakage protection is required

The selection sequence

Pick the rated current from the cable it protects.

Confirm the breaking capacity exceeds the fault level at that point.

Pick the trip curve to suit the load characteristic.

Check discrimination with the device upstream.

Check that the terminals accept the cable and that the device fits the board.

Common mistakes

Choosing by price and then working backwards.

Ignoring derating for grouping and ambient temperature.

Assuming a higher-rated device is safer — it is not, if the cable is not rated for it.

Mixing devices from different manufacturers in one coordinated scheme without checking the manufacturer’s coordination tables.