Regulation 433.3.1

" 433.1 Every circuit shall be designed so that a small overload of long duration is unlikely to occur."


Basically we  must design with some slack - a 32 A breaker will carry 32A all day, and perhaps nearly 40A for many hours.

A 13A fuse will carry 20A long enough to get the pins on a 13A plug to skin burning temperature.


We should not design in a way that tries to use this extra - so to deliberately put a 35A load on a 32A breaker may not trip, but it would break this rule, and it is not to be done.


There is a problem with sockets, as we have no control over what total load is plugged in, so instead the standard ring and radial configurations use recommended cable sizes that mean they are generally under-run even at the full load,except in some corner cases that luckily arise very rarely.

16A plugs do not need fusing, as they are only ever wired  on 16 or 20A radials in 2.5mm² or larger cable , so the protection at the origin is enough, and would not discriminate correctly if you added another 16A fuse or breaker at the load end.

I am not so sure that the plugtop fuse provides overload protection. I would have thought it was there to provide the lead with fault protection. Hence the reason a 0.75mm2 flex can be supplied directly from a 20A PD when it’s Iz is only around 6A.

Didn't the OP ask about 433.3.1 (A device for protection against overload need not be provided...) rather than 433.1?


(i) is only needed because 433.2.1 is so badly worded.

(iii) is only for the origin,


so I'm guessing you're thinking about (ii) - characteristics of the load or supply mean it's not likely to overload. Generally that's when you know maximum possible current a load can draw and have cables can supply that. Logically if a overload can't happen, there's no real need to provide a protective device to prevent it (but normally you'd still have to provide protection from faults (e.g. short-circuits).


You should apply a little care when using that get-out though as some devices can draw a larger current, but less than a typical short circuit current, when certain internal faults occur - say a short part way along a heating element or transformer winding. Often the design prevents that or would produce an earth fault current that an upstream RCD would detect, so would fail safe as far as not being able to overload is concerned, but not always.


We haven't specifically fused sockets for a long time (although it was occasionally done long before the days of 13A plugs) - the wiring to sockets is generally sized according to the protective device supplying it - although there are a few exceptions - such as a spur from a 32A ring supplying a single or double socket where the cable needs only be rated at 20A - that's certainly one example where (ii) applies.


The lack of fuses in plugs, is normally not a BS 7671 matter (as the installation would typically stop at the socket) - but as others have already said the principle for protecting the flex is the same - generally the load would be limited by the appliance. The same logic applies where the fuse is rated higher than the flex's continuous rating (sometimes seen with fridges/freezers were a 13A fuse is fitted with a 0.75mm2 flex - a 3A fuse isn't used as it wouldn't reliably survive the compressor starting currents).

 

   - Andy.