On 28/03/2018 16:58, whisky-dave wrote:
On Wednesday, 28 March 2018 15:31:19 UTC+1,
wrote:
On Wednesday, 28 March 2018 14:58:30 UTC+1, whisky-dave wrote:
On Wednesday, 28 March 2018 14:25:50 UTC+1, Sam wrote:

This year I've brought over 100 1 amp QB fuses and prety much
every one of them has blow.

so again you don't know how to assess the correct fuse for the
job.

That was the correct fuse for the job as the job was to prove to
students that even a 1.5V alkaline battery can blow a fuse, its the
first labsheet the students do, that's why we provided them with a
glass QB fuse so they can see it blow that is the whole point. They
then check the fuse for continuity (I'd like the labsheet to get them
to test this before but then I don't write labsheets academics do)

You must have some really thick students then.

I suppose given that the people who seem to be teaching them can't work
out how to heat their laboratory properly this is hardly surprising.

A fresh AA cell will comfortably source about 10A into a dead short - it
gets warm quite quickly so you better have a fuse in circuit to blow.

Handing out a range of low fuse values so that some would last a bit
longer would be instructive for the students.

But being the type you appear to be you won't believe me. So
http://www.engineeringbookspdf.com/m...charles-platt/

page 15 :- How to blow a fuse. fig. 1-35 They use a 3 amp automovite
fuses which are far more expensive than the ones I buy. We buy ~60 of
these books each year I think that is a waste of money and we should
write our own labsheets for the skills course, but the academics seem
to like a recommened boook.

You buy in course books for laboratory practicals? Cowboys!

" The problem is that of "surge" or "startup" currents. These
often blow a 3A fuse.

Yes I know and yet another reason not to use the wattage
calculation on the back of pieces of equipment.

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.

--
Regards,
Martin Brown