When cables are downsized without a fuse, question

In most circumstances, fuses or other OCPD must be sized to protect the cable, there are however a number of exceptions.

o.75mm flex is allowed for pendant drops on lighting circuits. This has a good record in practice, how many light fittings, even under fault or failure conditions, can draw more than the current rating of the flex.

Research has shown that 0.75 mm flex can be protected against short circuit by a fuse of up to 16 amps or an MCB of up to 20 amps. Overload protection is due to the design of the light fitting. In the exceptional case of a lighting circuit fused at more than 20 amps, then in my view 0.75mm flex should not be used.

Similar arguments apply to flexible cords on portable appliances, these are commonly 0.75mm on small appliances, and are protected against short circuit by a 13 amp plug fuse or if non fused plugs are used are protected by a circuit fuse of 16 amps or by an MCB of 20 amps, normal practice in most of the world.

As regards a 2.5mm flex or able supplying an oven on a 32 amp circuit, this should be fine since the design of the oven protects against overload, and the 32 amp MCB protects against short circuit.

In fixed installations, apparently undersized cables are allowed when the design makes overload very unlikely. For example a 2.5mm cable can supply one twin 13 amp socket from a 32 amp circuit. Overload is very unlikely since each plug is fitted with a fuse not exceeding 13 amps, and in practice the long term load is unlikely to exceed 20 amps.

From IET guide 1:

Some loads, due to the nature of the load itself, cannot present an overload and these loads can be provided with fault current protection only. As an example, for the flexible cable of a luminaire pendant, protection against fault current only is usually sufficient.

Can you give any examples of when an overload could actually happen on equipment?

I'd have thought that all plug-in equipment will be designed with the cable CCC > max load drawn by the equipment, and they already come with a fuse anyway. I'm thinking overload can't happen at the end of a circuit where equipment is connected if equipment's cable CCC > max load drawn by the equipment. Only on the main circuit supplying lots of loads (e.g. socket ring final), due to diversity being applied, giving the potential for overload. The only thing I can think of is people modifying equipment? Or damage to equipment causing higher current draw maybe?

The obvious place where we like to provide overload protection would be equipment with motors especially when doing things that may be stalled, like wood chippers or rubbish crushers because the exact mechanical load is rather indeterminate.

Mike

(and so on all but the smallest motors this is indeed required.)

The obvious place where we like to provide overload protection would be equipment with motors especially when doing things that may be stalled, like wood chippers or rubbish crushers because the exact mechanical load is rather indeterminate.

Mike

(and so on all but the smallest motors this is indeed required.)