why UK plugtop fuses are not optional

Periodically the idea arises on most UK audio fora that

'the plugtop fuse is /some sort of perceived bottleneck/
anyway my amp has a 5A fuse in the back, carnt i use a copper slug instead..?' 
or some comparable idiocy.

Well, NO. The reason is simple but subtle: the fuse in the UK plugtop is required to protect the flex cable, not the equipment.

That's due to the use of the unique 'ring main' for UK domestic socket outlets where the supply is daisy-chained around from one socket outlet to the next and then back to the 32A circuit breaker on the distribution board. The 'ring' is usually up to 10 socket outlets or a maximum of c.100 square metres of floor area served on a single level. It's not unusual to have 3-4 mains power rings in even the smallest houses as a result, each with a breaker/fuse up to 32A. The system provides a very cost-effective method of domestic wiring for very high power delivery with minimum copper use. Most socket outlets are supplied as twin 13A wall mounted sockets, but 'diversity' in use - not every socket is fully loaded - means the overall load to the 'ring' fuse/breaker is capped at 32A.

Put another way, all the socket outlets on a given ring 'see' that 32A fuse/circuit breaker protection; and each outlet is wired effectively with two inputs of 2.5sq.mm cable, or 5sq.mm equivalent at each outlet; that's about 10AWG combined, which can fully deliver such current indefinitely.

Therefore fusing-down or equivalent overcurrent protection is mandatory for any flex lead to equipment, whether plugin or fix-wired, because in case of a fault or damage to the flex lead, far more than the flex's notional limit of 13A is available. A typical 32A class B breaker will stand about 100% overload - that's over 60A - for up to a minute before opening. At 240VAC that's over 15KW of dissipation, nearly 20Bhp if you prefer.

So the BS1363 appliance plugtop incorporates a [3A up to 13A] fuse to allow the down-sizing of the appliance cable connected to that plug; usually the smallest rating that will run the load is chosen (normally 3A, 5A or 13A.)

Without the plug fuse, all appliance flex cables would need to be >32A rated, too, because if the cable got damaged it might have to withstand the full loading to blow the main ring circuit breaker. That could be 64A for a minute, or 15kW, for a normal type 'B' breaker. Hardly reasonable for a lamp flex, and a significant hazard otherwise. The equipment's own main supply fuse does not even enter this discussion.

And that's why defeating UK plugtop fuse protection is, and remains, an idiotic idea.

Postscript for this who propose using the older 15A BS546 round-pin unfused plugs: these are only legal on radial circuits with breaker protection not exceeding 16A per circuit. The reasoning is exactly the same. So is the protection afforded.

www.acoustica.org.uk (opens new window)

  • 01.11.13: page added as long overdue.