Which fuse rating to use in a plug

[hllns]

Today, the vast majority are either 13A or 3A, the flex on certain items like kettles is routinely undersized due to the intermittent nature of such loads, and fused plugs in general are mostly irrelevant.
With modern circuit breakers, most short circuit faults will cause the 32A breaker to disconnect before the fuse fails.

This is something I see a work on a regular basis, a comms cab typically is fed by a PDU connected to a 16A socket on with a 16A MCB behind it, we have a model of switch which typically sees a switch mode PSU failure where the MCB will trip, we've never/rarely had to replace a plug top fuse in the flex feeding them yet.

 

[pete01]

I was brought up to us a fuse that not only is slightly smaller than the cable (in other words, the fuse protects the cable), but so the fuse is no larger than the current of the appliance.

The habit dies hard - yet we see so many low current/low energy appliances fitted with plugs containing fuses that are 'technically' oversized.

Brought up? You mean your parents told you this as you were growing up?

 

[ericmark]

The fuse in a BS 1363 plug is BS 1362, and they come as 1, 2, 3, 5, 7, 10 and 13 amp, for a fixed appliance the manufacturer can stipulate fuse size and type, but when we joined the common market, portable equipment had to be made so it could be used in other member countries, and other than the UK it is rare to have a fused plug, so if portable equipment needed a fuse, it has to be inside the equipment, so the fuse in the plug only protects the cable, so we reduced the common BS 1362 fuses to just 3 and 13 amp in the main, as they protected most of the common flex cables used, but this is not extended to fixed equipment, many boilers stipulate a fused connection unit with a 3 amp fuse. (Personally, I prefer a plug and socket, this allows the heating to be powered from an inverter or generator in a power cut)

Before we joined the EU, I tried to show my children how to select the correct fuse, and it is not as easy as it first seems, so 700 watt would with a resistive load need a 3 amp fuse, but with a motor, it could need a 5 amp, the fridge and freezer are the prime examples, the rating plate may call for a 1 amp fuse, but the start amps would rupture that, so more likely it needs a 5 amp.

Try explaining this to children, and then have them return home from school, and tell you their teacher says I got it wrong. This was before everything had to come with a 13 amp plug, so school children were taught how to fit a plug. I have had this a few times with the kids at school, the classic was there are two types of transistor can anyone in the class tell me what they are, my son, who had passes his RAE answered bipolar and field effect sir. To be told no, it is NPN and PNP, to which he burst out laughing, he did not even attempt to tell the teacher his error, and was still laughing when he told me.

Just happened it was parents - teacher night, so the teacher was told, he said "Oh field effect must be new?" to which I told him no, invented before bipolar, however missed out it was not perfected until latter.

I did google "how to select correct fuse for a uk plug" and the answer was "To select the correct fuse for a UK plug, check the appliance's power rating: if it's under 700 watts, use a 3 amp fuse; if it's over 700 watts, use a 13 amp fuse; always consult the appliance manual for the most accurate fuse size." not so different to what I was telling my children all those years ago.

 

[JohnW2]

... Other niche uses such as 3A FCUs for gas heating systems are mostly based in ancient traditions rather than actual engineering, and would be far better served by having dedicated circuits for such things.

As a a matter of interest, why do you feel that a gas heating system would "be far better off" with a dedicated circuit?

 

[ericmark]

My freezers and central heating does have a dedicated circuit, fed from the inverter, it means should I get a power cut, I will still have heating, and working freezers.

I did consider lights, but we have well over emergency requirement with lights, and a touch is likely good enough with a power cut, want the central heating to run as long as possible.

But central heating does include quite a few devices, I have two pumps, two motorised valves, a relay, two hubs for the thermostats, as well as the oil boiler, likely the lot is under 3 amps, but clearly the boiler manufacturer has no idea as to what will be fed from the same fuse and all beyond the fuse is a circuit, and so it would be hard to think of a situation where the central heating is not a dedicated circuit.

The use of the phrase ring final, seems to imply no circuits come from that ring, as it is the final, but using English it is the final ring, so no reason why a radial circuit, with its own fuse, should not come from it. However, we tend to call radials from a ring fused spurs, odd phrase, my English would have considered spurs are not fused, but the writers of BS 7671 could not have used the same "First Aid in English" book I had to use as a school child.

We have things like isolation transformers which seem as if nothing is transformed, same volts, amps, go in as come out.

But one can see someone inventing a transformer, and trying to think of a name a Zipernowsky maybe, but they don't name things after themselves, so a Bláthy, glad they didn't, my spelling is not good.

 

[Jurassicspark]

As a a matter of interest, why do you feel that a gas heating system would "be far better off" with a dedicated circuit?

Well if there’s an issue on the circuit that also has the boiler on it you won’t have heating/hot water.

 

[Jurassicspark]

Having said that, this discussion has all been about BS1362 fuses and, although you raised the issue of 'voltage rating' I presume that no such fuses (well, no BS1362-compliant ones!) are not 'rated' at less than 240 V, are they?

Yes, they are not but if you look at industrial or other equipment then the fuse rating should be adhered to.

 

[JohnW2]

Yes, they are not but if you look at industrial or other equipment then the fuse rating should be adhered to.

Fair enough. As I said, you taught me something (which I should have thought of already, but hadn't).

However, as you agree, the issue is irrelevant to the OP and this discussion thread.

 

[JohnW2]

Well if there’s an issue on the circuit that also has the boiler on it you won’t have heating/hot water.

Well yes, of course, but you could say that about anything.

I suppose the 'ideal' would be to have one circuit for each and every piece of fixed equipment and one circuit for each and every socket outlet, but ........ !

We see an awful lot of discussion about 'loss of circuits' - be they for lighting, heating systems, alarms or whatever, with all sorts of views about what should be done to 'mitigate those risks' - but they are all extremely rare events.

 

[Jurassicspark]

Well yes, of course, but you could say that about anything.

A lot of sparks even add the fridge/freezer to its own circuit. I don't really see the need of doing this considering fridges can store food after a certain period of time in a power outage.

CCTV's, burglar alarms or even electric security gates are preferred to be on their own circuits.

I've even seen the above devices before the RCD to avoid nuisance tripping. Sometimes you find that they have developed a fault so someone comes along and decides to add the breaker before the RCD and sometimes people think alarms should not even need RCD protection.

Different opinions vary.

 

[JohnW2]

A lot of sparks even add the fridge/freezer to its own circuit. I don't really see the need of doing this considering fridges can store food after a certain period of time in a power outage.

We frequently have that debate. Freezers in cellars or outhouses can go unopened for quite long periods,so ity probably makes more sense to have them ion a circuit whose loss would be rapidly noticed (maybe the one used by the TV or one's teenage daughter's hair dryer ). However, as I always say, I think it far more important to have over-temp alarms since, at least in my case, the very few occasions on which I have got close to 'losing a freezer full of food' have all been due to failure of the freezer itself, not failure of its power supply.

CCTV's, burglar alarms or even electric security gates are preferred to be on their own circuits.

I'm not sure why. Again, I would personally usually advocate putting alarms etc. on a 'instantly used' lighting circuit.

I've even seen the above devices before the RCD to avoid nuisance tripping.

Yep, that's also discussed - although it would now probably only be 'compliant' if they were hard-wired,since essentiallyvall sockets are now required to be RCD-protected.

 

[Jurassicspark]

I'm not sure why. Again, I would personally usually advocate putting alarms etc. on a 'instantly used' lighting circuit.

The lighting has a chance of developing a fault and causing everything in the circuit to turn off.

Imagine a parking place, where the CCTV's are connected from one of the lighting circuits and a fault occurs during an incident, that won't be very good.
However, the dedicated circuit for the cameras can develop a fault but the outcome is less compared to the former.

In the real world, backup batteries should be provided in case of failure but I'm just giving an example.

 

[Harry Bloomfield]

As a a matter of interest, why do you feel that a gas heating system would "be far better off" with a dedicated circuit?

You not like the words, but ease of maintenance, and improved reliability of supply. If your lights, plugs or what ever, trip on a fault, and cannot be reset due to the fault, your heating system will continue to function. If work needs to be done on your boiler circuit, then that can likewise be completely isolated, leaving your plugs/lights etc., all in working order.

 

[JohnW2]

The lighting has a chance of developing a fault and causing everything in the circuit to turn off.

True - but at least one would know that something needed to be done.

Imagine a parking place, where the CCTV's are connected from one of the lighting circuits and a fault occurs during an incident, that won't be very good.

Not 'good', but you are talking about the simultaneous occurrence of two extremely rare events, so I wouldn't personally lose any sleep over it.

In the real world, backup batteries should be provided in case of failure but I'm just giving an example.

Quite - and if we're talking alarms etc., most of them do have back-up batteries. In any event, the failure of the lighting circuit, per se, would be much more of an inconvenience than temporary loss of the alarms, even if they didn't have battery back-up.

As I've said, one can argue that 'one circuit per load' is the 'ideal' but I think a sensible approach should take into account the probability of what one is trying to protect oneself against.

 

[Deleted member 142255]

We see an awful lot of discussion about 'loss of circuits' - be they for lighting, heating systems, alarms or whatever, with all sorts of views about what should be done to 'mitigate those risks' - but they are all extremely rare events.

When we "older folk" were growing up, power cuts were a regular thing - be it thunderstorms or whatever reason - so our parents kept a torch and candles handy. It was something they were prepared for and used to. Nowadays it is rare for these events so people are not prepared. making them more dangerous events.

Yep, that's also discussed - although it would now probably only be 'compliant' if they were hard-wired,since essentiallyvall sockets are now required to be RCD-protected.

Not true. 13A sockets that may be used for outside equipment require RCD protection, but it is the installation method of the cables that requires the cable to be RCD protected, not the socket, light or switch.