On Thursday, 29 March 2018 10:48:55 UTC+1, Martin Brown wrote:
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.

Yep, I've been talking to a couple this morning they admit it, in school most practiacal aren't even demostrated and they don't do practicals at school which is why most of teh oversea studetns are better practically because they still do practicals while here in the UK everything is academical accessed , we as a country don;t value practical skills anymore, it's why we have a shortage of engineers and lots of admin.


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.

Yes I agree, and as theb weathe rgot worse the administration team emailed everyone to tell us they had decided to go home at 3pm leaving the rest of us techs, academics, students, researchers and everyone else still here.


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.

And even at 10Amps we can't use these to heat the lab.

Yes well done which is why we run this lab to show such things as students don't even need to use batteries in this modern world most things are rechargable.

You see this is what real teaching is about, its not just about giving them marks out of 10 on a multi-choice sheet.


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

and that is why we use 1a QB I;ve already suggested we could you ant-surge too but this course unit isn lt about fuses it;s about how to connect things and understanding using protoboard and stripboard amounts other basic things that those from school that have choseen computer science might need to know if they haven't studies or rememered their physics.


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!

Yes I agree, but it does seem thnat some of our academics can't write practicals like they used to, perhaps they do did their degrees in just theory.


" 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 are used as heaters, they were very popular a few years ago less so now it seems. So why advise people to use this sort of thing to calculate fuse values. ?
and why is c question in a mid-term test for science foundation students.


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.

and he is wrong to state that the fuse should be 5amp.