When cables are downsized without a fuse, question

On plugged in appliance it's different as there's the fuse to create a new Ib<In<It (or max load<fuse<It).

Also overload protection (i.e. heavier than anticipated currents flowing in an otherwise healthy circuit) may be provided downstream of the conductor it's protecting as the same current will be flowing in the protective device as in the conductor. That's often the case with industrial motor circuits where the fuse/MCB in the distribution board is rated far higher than the cable and overload protection is provided next to the motor by a special motor overload device (that was it can be more precisely matched to the motor's characteristics and have the convenience of being able to reset it locally). You also see the same principle on domestic ring final circuits where 20A cable (usually 2.5mm²) may be used on a spur from a 32A circuit as long as it's to a solitary socket - the assumption being that the total load on even a double socket won't exceed 20A for any significant length of time.

The fused 13A plug is itself rather an oddity - in most of the rest of the world domestic plugs are entirely unfused - so 0.75mm² even on a 16A or 20A protective device is very common. Generally it's up to the appliance to prevent overload so the upstream protective device only has to protect the flex from short-circuits - which generally it will do very successfully - at least as far as preventing gross overheating or fire - even if occasionally the flex isn't serviceable afterwards - but given that the appliance it belongs to is already faulty, that isn't a huge loss.

We only really need fused plugs because we like 30A or 32A protection for socket circuits - and those larger devices are a lot worse at protecting small flexes from faults.

   - Andy.

On plugged in appliance it's different as there's the fuse to create a new Ib<In<It (or max load<fuse<It).

Also overload protection (i.e. heavier than anticipated currents flowing in an otherwise healthy circuit) may be provided downstream of the conductor it's protecting as the same current will be flowing in the protective device as in the conductor. That's often the case with industrial motor circuits where the fuse/MCB in the distribution board is rated far higher than the cable and overload protection is provided next to the motor by a special motor overload device (that was it can be more precisely matched to the motor's characteristics and have the convenience of being able to reset it locally). You also see the same principle on domestic ring final circuits where 20A cable (usually 2.5mm²) may be used on a spur from a 32A circuit as long as it's to a solitary socket - the assumption being that the total load on even a double socket won't exceed 20A for any significant length of time.

The fused 13A plug is itself rather an oddity - in most of the rest of the world domestic plugs are entirely unfused - so 0.75mm² even on a 16A or 20A protective device is very common. Generally it's up to the appliance to prevent overload so the upstream protective device only has to protect the flex from short-circuits - which generally it will do very successfully - at least as far as preventing gross overheating or fire - even if occasionally the flex isn't serviceable afterwards - but given that the appliance it belongs to is already faulty, that isn't a huge loss.

We only really need fused plugs because we like 30A or 32A protection for socket circuits - and those larger devices are a lot worse at protecting small flexes from faults.

   - Andy.