Martin Brown wrote:

On 29/03/2018 11:23, Roger Hayter wrote:
Martin Brown wrote:

On 28/03/2018 16:58, whisky-dave wrote:
On Wednesday, 28 March 2018 15:31:19 UTC+1,
wrote:

absolutely the wrong conclusion. It's just not quite as simple as
3A fuse for everything upto 699W.

Exactly so don't use caculations for this sort of thing, especailly
in a test.

The only major exceptions have a very low initial resistance like quartz
halogen lamps which really do have an aggressive switch on surge
current. An 1kW electric fire would work OK with a 5A fuse and is
clearly the answer that the examiner was expecting.

I don't doubt that that was what the examiner was expecting. If the
fire is 1.15kW at 230V (if from EU it could even be 220V) then it would
draw 6A at 253V.

A nominally 5A fuse will sustain a 7A current almost indefinitely so
there is actually no problem in practice. This is the difference between
engineering and just doing the sums mindlessly. You might get extra
points for pointing out that the plug will run warm as a result.

This might be true if plugs were not subject to considerable
accelerations in daily use, being removed and banged on hard things when
hot. I don't think your 5A fuse will last long.

--

Roger Hayter