Fuse calculation

[newshound]

Could not have put it better!

[ARW]

I don't think that BS761 has an opinion on it. Overload is not relevant (the load is set by the heaters wattage) so in this case only fault current applies.

If you have such a voltage surge then the least of your worries would be a 1.15kW heater!

[Mike Humphrey]

Hmm, a strict reading of the rules would suggest that your electrical installation is defective - not fit for purpose as you should have sockets available where the kettles need to go, rather than using extension leads. Of course, how to provide sockets in the middle of the floor for kettles which don't then become a trip hazard when the same area is used as a dance floor is an interesting challenge...

Mike

[Andrew Gabriel]

A 5A fuse passing 4A will give off 0.64W, generating some heating and deterioration of the plug over time.

A 13A fuse passing 4A will give off 0.09W, resulting in less heating in the plug and longer life.

Unless the heater has a longer flex than is allowed by EU regulations (which requires the flex is protected against fault current when fused at 16A), a 13A fuse will protect the cable OK.

(A BS1362 fuse gives off about 1W at full load current.)

[tabbypurr]

a) which ones? b) fuses are pretty tolerant of that. Look up the curves.

NT

[Johnny B Good]

+1

Nicely put. That saved me having to stick *my* oar in. :-)

[Johnny B Good]

Yes. Provided the load doesn't demand the use of a higher fuse rating, the lower the better the protection. The only issue would that of the heating effect inside the plug when running close to the chosen fuse rating current limit[1]. With a table lamp, even a 1A fuse will be safe from this unless you fit a 250W GLS lamp (in which case, choose a 2 or 3 amp fuse. :-).

[1] All fuses rely upon the heating effect to perform their function so the plug's fuse holder must be designed to cope with the circa 1 to 2 watt's worth of dissipation whenever the fuse is carrying its rated current. The lower the fuse rating, the lower the dissipation at the rated current. However, this isn't a linear variation since the thicker fuse wire inside a 13A fuse cartridge will conduct the heat away more rapidly via its end caps into the Live pin and the live wire termination than the thinner wire in a 1A fuse cartridge.

[Johnny B Good]

The fuse rating is chosen either to protect the flex or else, with low wattage loads, reduce the risk of conflagration within the 'appliance' by using the lowest rated fuse required to support the normal current demanded by the appliance.

The flex used by a 60W table lamp might well be protected sufficiently with a 10A fuse but even better, a 1A fuse will provide yet better protection against damage to its in line switch which could later become a fire hazard after the fault has been repaired.

Something less trivial than a 60W table lamp such as a mains powered radio or TV set will have its own safety fuse built in to protect against the risk of conflagration, often these days, non-user serviceable soldered in fuses since overloads on the low voltage side of the smpsus used in modern devices are protected by the smpsu's own over-current protection, leaving the only type of mains voltage fuse blowing event as a fault in the smpsu itself which are often catastrophic cascade faults involving more than just a single component failure. In this case, the plugtop fuse will be chosen to protect the mains flex, typically a 5 or

10 amp fuse might be specified by the manufacturer.

You can even apply this principle of "The lower the fuse rating, the better the protection." to Wylex consumer units when you know the occupancy of the the top floor (by your, as yet unfledged, 35 yo "Twenty something" lad no longer needs a 30A fuse in that ring main circuit when a 15A fuse will still more than suffice for the job. :-)

[Bill Wright]

13A.

Bill

[Martin Brown]

There is power where the kettles should go, but the tea ladies sometimes do strange things to avoid walking about 3m to the kitchen bench. The extension was coiled up and being used essentially as a 2 way adaptor so they could plug in multiple kettles at once on a nearby table instead.

I have concluded that you have to assume that people will do crazy things and defend against them. Rules only go so far and get ignored.

[PeterC]

With the v. few items with old, thin flex I have fuses of 1, 2, 3, 5, 7, 10 & 13A semi-sorted in 3 boxen. I suppose, with a bit of modding, I could also look at sub-1A fuses (fast, delay, slow), but nowadays they're pretty well redundant. We had an oven at work that drew about 15 - 16A peak but with odd solenoids banging in at times. The v. old 13A fuse went and the replacements would last only days - not good in military testing. Had to hard-wire it to a 20A fused breaker in the end.

[Brian Gaff]

Unless there is a surge on start up which can happen on heaters. I'm very surprised we do not get 7.5 amp fuses for this sort of thing. 13 amp ones go with a heck of a bang. You can get very slow blow fuses of course with a lot of that heat conducting stuff in them, but I'd guess a lot of fuses one sees in pound shops are dodgy in the extreme. Brian

[Brian Gaff]

Yes most people who set questions in exams have not experienced the real world very much has been my suspicion for a while now. They are steeped in the maths but know little about the real world. Brian

[Dave Plowman (News)]

And that is the correct way to protect the appliance if needed. Not using the plug fuse.

In the appliance you can choose the most suitable type of fuse. Fast blow, anti-surge, etc.

Hence there not being the same range of plug fuses as once was the case. Most won't understand the differences. So best to make things for a single one.

[Dave Plowman (News)]

I'd say you shouldn't make such an extension available in a public place. Use single outlet ones only. And not cable reels as they tend not to get fully unwound.

[Dave Plowman (News)]

It is the real world which says no new appliance should require a less than 13 amp fuse in a plug. Because if a lower value blows, many will just replace it with what is to hand, so likely a 13 amp one.

[whisky-dave]

But that wasn't the question was it. This was a mid-term test, so you would got this question wrong.

[whisky-dave]

So we both got it wrong then because the answer is 5A.

[whisky-dave]

Which is why it's a question in the science foundation course I supose.

The same fuses our PAT tester person uses then.

So it really depends on your job then and little to do with the applienace ? So as I would think not a very good quetion to ask.

[The Other Mike]

With a BS1363 plug top to IEC 60320 C13 connector with both the cable (BS EN

50525 sized at 1mm^2) and the end connector rated at 10A which fuse would be fitted by the manufacturer?

With a BS1363 plug top to IEC 60320 C13 connector with the cable BS EN 50525 sized at 0.75mm^2) rated at 5A and the end connector rated at 10A which fuse would be fitted by the manufacturer?

The limit of supply by the manufacturer in both cases is the BS1363 plug top with a BS1362 fuse, the cable to BS EN 50525 and the IEC 60320 C13 connector as a tested and complete assembly to be offered for sale in the UK.