Fuse calculation

[whisky-dave]

He is an Associate Lecturer late 60s early 70s.

[whisky-dave]

Good point and they may not even have a fuse, so perhaps a paper clip ;-)

[Roger Hayter]

I think the answer is wrong, or at best a matter of opinion. If they had asked which of the stated fuses has a current rating nearest to and above the current drawn by an ideal theoretical 1.15kW heater which was rated at 240V then it would have been a less ambiguous question.

[Graham.]

I once noticed such a coiled up reel under the breakfast bar at a Premier Inn, it was feeding the conveyor toaster. My complaint fell on deaf ears.

[whisky-dave]

Well they should have stated whether the person deciding on what fuse to instal; was a desginier of the equipment where the fuse was and who employed you because if I were to replace the fuse I'd have to use a 13 amp.

This is why I don't think it's a good question.

Other Qs include list 3 examples of household objects that use purely AC. and 3 that use AC converted to DC.

and another I don't really like. If a light bulb was connected to a 240V AC supply would it be brighter, the same or dimmer if the light bulb was connected to 240V DC.

[Graham.]

I have seen IEC cables using 0.75mm cable supplied with 13A fuses in the mains plug. These were supplied by Cisco for the PSU of their

1801 router.

We installed a mixture of IEC cables with 5, 10 and 13 amp fuses, and there is no guarantee that they will stay paired with their original appliance.

The idea that anyone (particularly PAT testers) will correlate the faint markings on the cable sheath with the fuse fitted is vanishing small.

[Dave Plowman (News)]

An experienced PAT person should be able to tell by the outside size of the flex.

[Andrew]

I'm surprised they didn't ban the sale of rewwindable cable reels (*) when harmonised colours and part P came in.

(*) 240V ones, for domestic use.

[Martin Brown]

If it had been a 1kW halogen lamp then there would be a real risk of the initial cold filament surge current taking out the fuse.

But a 1.15kW radiant electric fire bar would have a cold resistance of around 40R and a hot one about 50R. So it would initially draw over 5A (nearly 6A) until it first started to glow red. The fuse working life will be shortened as a result but not by all that much.

You are assuming here that a 5A fuse does what it says on the tin. That is normally not the case - most mains fuses will stand a 25% overload condition almost indefinitely - but the plug will run warm as a result.

[tabbypurr]

In the real world I've seen the following in new IEC leads many times:

13A 10A 7A 5A 3A noncompliant chinese fuses of who knows what rating, some of which bear no marking at all and may well not be fuses.

NT

[tabbypurr]

as well as the markings on the plug

NT

[Graham.]

Often with examination question, the right answer is the one the examiner is expecting. I have had that dilema many times.

I hand my grandson a sheet of A4 paper and ask him what shape it is, he replies it's a rectangle. Wrong.

[Max Demian]

What is it, a parallelogram? A trapezium? A quadrilateral perhaps.

[Max Demian]

Presumably the same, near as dammit, if it's an incandescent.

[Tim+]

Gawaaan, I?ll bite. What do you call it?

Tim

[Steve Walker]

Presumably, as it does have thickness, it could be termed a cuboid. What you would call it unless it was laid flat on a table though I don't know. It could be any shape - including a hat, a broach, etc.!

SteveW

[whisky-dave]

It's a square that someone sat on :-)

[Graham.]

[The Natural Philosopher]

Well as it has roughness, no it couldn't. It is an indeterminate solid.

And thus your pedantry is exposed.

If you excuse the surface torigness to make it approximately a perfect cuboid, the thickness can be excused as well to make it a perfect rectangle.

With the ability to cherry pick properties like that you belong in politics or climate science.

[Dave Plowman (News)]

You can cut paper to provide a perfect line? Much better than any surface roughness?

Pot, kettle. HTH.