Fuse calculation

[whisky-dave]

Which is the best fuse to use (3A, 5A or 13A) with a 1.15kW electric fire at a potential difference of 240v

[Graham.]

1150/240=4.79 so theoretically a 5A fuse shoud do.

[The Other Mike]

Assuming you mean at the BS1363 plug then the fuse should be sized to protect the cable.

[Harry Bloomfield]

whisky-dave pretended :

5amp ought to cover it, but it is very tight. If it fails find a 10amp.

[whisky-dave]

Nice, that's the sort of answer I'd expect :-)

But this was a question given to students doing our science foundation course where they are expected to say 5 Amps. (a student has left their marked paper (mid semester test) in the lab so I thought I'd have a look).

I have also been told that H&S state that consumers/users should only use either a 3A of 13 amp fuse in a plug and that is what we as technicians have been told to use, even though I have loads of fuses including 1A, 2A, 5A, 7A, 10A and 13A

[Martin Brown]

It isn't *all* that tight in practice but the plug will run slightly warm. A nominal 5A fuse will take nearly 20A to blow in the first second and will probably support a current of 7A almost indefinitely. eg.

It would be much more challenging with a halogen light where there is a significant inrush current when the filament is cold. An electric fire is a relatively benign load with a decently high cold resistance.

Nominally 13A fuses will support boiling two 3kW kettles for long enough to destroy the still coiled up extension cable they were plugged into.

All our village hall extension cables now have thermal overload protection to save them from the church's tea ladies.

[Dave Plowman (News)]

If you're talking a plug fuse, the load is irrelevant. As it is to protect the flex etc. In other words, a table light with a 20 watt bulb would be fine with a 13 amp fuse if the cable was rated at 13 amps.

[John Bryan]

This is due to induction heating not resistive heating by the load supplied.

[Sam]

Won't all fuses less than 13a protect the cable then!

[The Natural Philosopher]

well you could TRY a 5A. But my guess is that its pushing the limits.

13A. Remember the fuse is to protect the wiring that is downstream of it only.

[Roger Hayter]

10A . But 13A is fine.

[ARW]

I think some of that might be down to BS7671 only mentioning 3A and 13A fuses. Although I cannot find a reference to that ATM.

[tabbypurr]

5A is a better answer than 13. It would give added protection against a partial short of the heating element. Either is compliant with BS. TBH this is very basic stuff.

complete cobblers of course.

a useful rule of thumb for folks that need to be told which fuse because they have no idea. Such people normally only have 3 & 13A. That's all it is.

NT

[tabbypurr]

No, for 4 reasons.

1. Some appliances have old thin cables that come nowhere near being protected by a 10A fuse.
2. Some old appliances need low current mains fuses to give them basic safety protection.
3. Some new appliances have noncompliant CCS cables.
4. Some

[tabbypurr]

it's not, any more than a 13A load is pushing it with a 13A fuse. It's precisely what they're designed for.

that garbage is much trotted out.

NT

[tabbypurr]

Since current is the same but opposite direction in L&N conductors there is no significant induction heating.

NT

[Roger Hayter]

partial short of the heating element. Either is compliant with BS. TBH this is very basic stuff.

It depends on the heating elements temperature coefficient of resistivity and how hot it gets. Some 1.15kW heaters may draw much more than 5A for tens of seconds on starting.

they have no idea. Such people normally only have 3 & 13A. That's all it is.

[Roger Hayter]

No it is due to resistive heating and poor heat dissipation by the coiled lead.

[GB]

Does the BS require that the heating element should burn out before the cable does (say there's a voltage surge)?

And does the BS require that the element will burn out safely without setting fire to something?

[Harry Bloomfield]

on 19/03/2018, John Bryan supposed :

Rubbish, there will be no inductive heating effect. The current in live will match that in neutral and thus negate any induction. The only heating effect in a coiled lead on ac is that of resistive heating, which becomes a problem on a coiled up lead, because the heat produced is unable to escape.