Fuse calculation

No, he's right. For practical purposes. Just as, if someone asked you the shape of a table-cloth, you'd answer circular or as it might be square and so on. Even if it was folded up to go in a drawer.

Any other answer risks you being shipped off to the funny farm by your nearest and dearest. Who would also get power of attorney over you and sell your toy train layout that cost you £5k and hours of time, for tuppence-ha'penny.

and that the earth is flat because it curves less than a 1/4 wavelenght of light, which for opticians means Earth is optically flat :-)

Only if it happens to be flat at the time.

In geometry, a cuboid is a convex polyhedron bounded by six quadrilateral faces, whose polyhedral graph is the same as that of a cube. While mathematical literature refers to any such polyhedron as a cuboid,[1] other sources use "cuboid" to refer to a shape of this type in which each of the faces is a rectangle (and so each pair of adjacent faces meets in a right angle); this more restrictive type of cuboid is also known as a rectangular cuboid, right cuboid, rectangular box, rectangular hexahedron, right rectangular prism, or rectangular parallelepiped

{sorry couldn't resist it}

May be 5A will do, as "cold" heater element resistance would be higher than normal heating.

Surely it's the other way round.

But according to regulations in the UK on changing fuses it should be either a 3A for below 720W or a 13amp for above 720W .

ROSPA doesn't represent "regulations", it's just advice, based on the ratings of fuses they expect people to have at hand, and even that page suggests 5A for some appliances ...

As I said it's a bad question and in this university we have the regulations which tell us we must only use 3 & 13amp fuses, about 10 years ago I and others had the job of checking all fuses and replacving them with the 'correct ones' i.e 3 or 5, this is one reason why I have dozens of 5, 7 and 10 amp fuses, I even have 1 amp and 2 amp that I used in angle poised lamps with 40 or 60w bulbs. ANd don;t forget the FUSE is to protect the cable NOT the item of equipent so it really doesnlt matter what current rating the electric fire is, you put the fuse in that is correct for the cable.

AIUI the type of heater that is in an incandescent lamp - filament, of course - has low initial resistance so that it comes on 'instantaneously' whereas an electric fire/kettle/shower is pretty well constant. Cold inrush to a 3kW kettle+bad idea. The 2 elements in my electric shower, measured when cold of course, have the resistance corellating to the rated power. The electric kettle stays the same, although the meter won't show the power for the first cycle or two.

Nonsense!

Fact.

Your view is too simplistic. Any fuse value below the rating of the cable will protect it. Therefore a "univeral" cable rated at say 5amps feeding an appliance rated at say 2 amps will still be protected by using a fuse suitable for the appliance.

If an appliance needs a fuse for protection it should be in the appliance. Plug fuses are not there to protect the appliance and can't be relied on to do so.

So?

So a 13A fuse is perfect protection if the cable is thick enough and a

OOps! I stand corrected. Surely fuse rating shall be higher than % Amps.

Makes sense absolutely.

incorrect.

ROSPA is not regulations.