Why don't lamps come with a 1 amp fuse in the plugs?

[EFLImpudence]

I was referring to the amount of power a 3/13 amp fuse can take at 230v, not the fact that an appliances power and current consumption decreases as the voltage goes down to the point of where the current goes below the fuses threshold.

Yes that is true but the 690W will not relate to the figures on the appliances.

I don't think it takes a genius to understand that a appliance that is rated at 240v will consume less than it's rated current and thus less power at 230v - Ohms lawe

.
Not a genius, no, but a lot do not realise.

I simply took the fuse rating and multiplied it by the official UK voltage. whether BS1362 fuses rated are 230 or 240v should not matter as they are current dependent and not power dependent right?

Yes.

The cable size being ok for 1kW was not what I was referring to, 1Kw is more suited to a 5 amp fuse than a 13 amp fuse - that was my point.

No, it depends on the cable.

 

[EFLImpudence]

That would be true but what is this 1kW load which could do that (in the home?)?

A vacuum cleaner or similar, something like a 1kW fan heater, a good few DIY power tools etc. etc. (the common feature in all those examples being the presence of a motor which could jam)?

Yes, fair enough with the vacuum.

I don't think the fan in a heater is likely to be the same problem.

 

[eveares]

It doesn't. However, it might require a little bit more 'genius' to understand that Ohm's Law only applies to 'dumb resistive' loads. With the more intelligent loads that we are increasingly seeing (e.g. those powered by a SMPSU), a decrease in supply voltage will actually lead to an increase in current (and little change in power).

Very True, but one should should only use ohms law for resistive loads and not things like switched mode power supply's. After all, ohms law is based on the constant of proportionality (resistance) , that is the current and voltage across two points are directly proportional to each other.

 

[JohnW2]

A vacuum cleaner or similar, something like a 1kW fan heater, a good few DIY power tools etc. etc. (the common feature in all those examples being the presence of a motor which could jam)?

Yes, fair enough with the vacuum.

... and the power tools?

I don't think the fan in a heater is likely to be the same problem.

Possibly not, but it's surprising how much current can sometimes be drawn by even a small motor if it jams. A while ago the (quite small) discharge pump in our WM got jammed (by a coin that somehow got through the filters!). That resulted in a lot of smoke and eventually (took some time before we detected the smoke!) tripped a B32A.

Kind Regards, John

 

[JohnW2]

Very True, but one should should only use ohms law for resistive loads and not things like switched mode power supply's. After all, ohms law is based on the constant of proportionality (resistance) , that is the current and voltage across two points are directly proportional to each other.

Indeed - but, as I said, to know that requires a bit of knowledge/ understanding (aka 'genius' ). With some appliances, it's not necessarily immediately obvious whether or not they represent a 'dumb resistive load'.

Kind Regards, John

 

[EFLImpudence]

Ok. sorry. I forgot all the motors. With 1kW mentioned I was thinking of an electric fire.

HOWEVER, Is this not the case with power tools?
That is, they have 13A fuses with relatively small flex.

 

[eveares]

No, it depends on the cable.

While it true that plug fuses are primarily designed to protect the cable feeding the appliance, having a fuse suitable to the appliances power rating may help protect the appliance its self from damage should a fault develop.

i.e. A 1 kw fan heater with a 13 amp fuse in it, while the 1mm cable will be be able to handle 10 amps and likely enough as well for the 13 amp fuse to blow before damaging the cable, the fan heater it's self may not survive.

Thus using the lowest fuse possible relative to the devices power is a good idea for devices that are not internally fused.

 

[JohnW2]

Ok. sorry. I forgot all the motors. With 1kW mentioned I was thinking of an electric fire.

Fair enough.

HOWEVER, Is this not the case with power tools? That is, they have 13A fuses with relatively small flex.

I'm not sure. 1kW is more than the average 'small power tool', and it wouldn't surprise me if ~1kW ones had at least 1.25mm² flex.

Kind Regards, John

 

[JohnW2]

While it true that plug fuses are primarily designed to protect the cable feeding the appliance, have a fuse suitable to the appliances power rating may help protect the appliance its self from damage should a fault develop. ... Thus using the lowest fuse possible relative to the devices power is a good idea for devices that are not internally fused.

As I said at the start, I essentially agree with that view - which is why, for example, my 'small electronic loads' have 1A fuses in their plugs. I doubt that the fuses would often 'protect' the devices very much, but they just might on occasions.

Kind Regards, John

 

[SimonH2]

Why was this, surely 17 amps through the fuse should of caused it to blow!

It takes quite a bit more than it's rated capacity to blow a fuse.

I recall many years ago at college we had a visiting theatre production. They brought their own lighting and controller - easier than using the host's facilities which means relearning the lighting at each venue. The tech guy told me he'd worked out (by trial and error) what he could run on each of the 13A plugs the lighting ran from - it was "somewhat more" than 2x13A

 

[winston1]

Flexible cables sizes.

0.5mm² = 3A
0.75mm² = 6A
1mm² = 10A
1.25mm² = 13A

Since only require a fuse to protect a cable only 3, 6, 10, and 13 amp fuses are required. Oddly you can buy 1, 2, 3, 5, 7, 10 and 13 seems some one forgot 6A was rating for 0.75mm² flex?

When the fuse sizes were selected we didn't use metric size cables. 14/0076, 23/0076, and even 7/0076 were used on small appliances. Don't know what the current ratings were but I guess 7/0076 was what required 1A fuses.

 

[eveares]

It takes quite a bit more than it's rated capacity to blow a fuse.

It seems your right by this graph.

And these websites

http://pat-testing-fss.blogspot.co.uk/2014/02/whay-13-amp-fuse-does-not-seem-to-blow.html

http://www.electriciansforums.co.uk...m/23212-bs1363-fuses-dont-mean-what-they.html

It seems this a dangerous flaw/situation with our plug fuses! I think I will be using 10 amp fuses in place of 13 amp fuses from now on for appliances/devices what do not have a large inrush current!

 

[JohnW2]

It seems this a dangerous flaw/situation with our plug fuses! I think I will be using 10 amp fuses in place of 13 amp fuses from now on for appliances/devices what do not have a large inrush current!

It's not a 'flaw' which is peculiar to plug fuses - it applies equally to any fuse or any MCB, RCBO or similar. However, it's not really a 'flaw', in as much as those characteristics of over-current protective devices (OPDs) are taken into account in the design calculations we undertake. Hence, for example, since we know that a Type B MCB is allowed to take as long as an hour to operate at 1.45 times it's 'rated current' (In), the 'maximum current-carrying-capacity' (CCC) of cables we get from our tables are actually 1.45 times less than (i.. about 0.69 of) the current which the cable is considered able to safely carry for one hour. In other words, if the tables say that the CCC of a particular cable is, say, 20A, that means that it is considered that it can safely carry 29A for at least an hour - so that it is 'safe' if protected by a 20A Type B MCB.

It would obviously be impractical, and also generally undesirable, to have an OPD which always allowed currents up to 12.99A to flow indefinitely without operating, but operated very quickly if the current was 13.01 or above.

In terms of what you suggest, I really don't see that dropping from a 13A fuse to a 10A one (obviously for a load no greater than 10A) would really achieve much. Although your graph shows only 3A and 13A fuses,one can imagine roughly where the curve for a 10A fuse would be - and the 10A fuse would still allow considerably more than 13A to flow for quite some time before blowing.

Kind Regards, John

 

[Risteard]

Would it not be more safer to have a 1 amp fuse in the plug instead?

A3 amp fuse is probably safer than the method used elsewhere in the world including the rest of Europe and the USA. They don't have any fuses at all in the plugs and rely on a 10A or 15A MCB in the supply to the sockets.

It obviously hasn't dawned on you that it is the differences in our wiring practices which necessitate them, so I think it's ridiculous to suggest that almost all of the rest of the world is somehow less safe.

And for the record I don't see any benefit in fitting a non-standard 1A fuse to an appliance whereby a 3A (or 13A) or no BS 1362 fuse will still see the cable protected.

 

[eveares]

I really don't see that dropping from a 13A fuse to a 10A one (obviously for a load no greater than 10A) would really achieve much

Lets say you have a 3Kw appliance with no or little inrush current, I would rather the fuse blow at the current required to blow a 10 amp fuse and not a 13 amp fuse. Thus using a 10 amp fuse in a 3kw appliance seems a little safer than using a 3kw.

I also hope that a 13 amp fuse would blow on a over loaded extension lead before a fire started from the wires melting and thus arcing due to the heat.

i.e. 3 core 1.5mm flex may be rated at 16 amps, but I assume a 20 amps load indefinitely, while it may get hot it would not get to hot to cause a fire as it has been designed with this in mind.

Also if 13 amp fuses don't blow until over 20.8 amps, why are they called and labeled as 13 amps?

I assume fuses are designed like this to allow for high inrush currents, other wise why not start using 10 amp fuses in 3kw appliances.