Fuse calculation

[Sam]

Explain what a current is, what controls the amount and how it is produced.

[Bob Eager]

Or, to give them their 'modern' name (for several years now), The Institution of Engineering and Technology.

[whisky-dave]

l {E}}} {\mathcal {E}} and measured in volts),[1] is the electrical intensi ty or "pressure" developed by a source of electrical energy such as a batte ry or generator.[2] A device that converts other forms of energy into elect rical energy (a "transducer") provides an emf at its output

EMF is measured on volts as it says, what controls the amount is the restis tance or impedence or reactance. Without a PD or EMF you don't get current.

[tabbypurr]

at the plug fuse is there to protect the cable,

115W electric fire. No mention of what lead was connected if it was a piece of 1 amp cable guess what ......

ven brought 50ma ones.

ne of them has blow.

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 c an see it blow that is the whole point.

m to test this before but then I don't write labsheets academics do)

nes I buy. We buy ~60 of these books each year I think that is a waste of m oney and we should write our own labsheets for the skills course, but the a cademics seem to like a recommened boook.

on the back of pieces of equipment.

se for everything upto 699W.

if just leads are treaded differntly because you have no idea what they wi ll be connected to power wise.

on the moulded plug it says stamped/engraved on the plug 5 amp fuse fitted.

{E}}} {\mathcal {E}} and measured in volts),[1] is the electrical intensity or "pressure" developed by a source of electrical energy such as a battery or generator.[2] A device that converts other forms of energy into electri cal energy (a "transducer") provides an emf at its output

I guess I shouldn't reply to fools.

[Sam]

You haven't defined what a current is - ie it a flow of electrons.

I always understood that the magnitude of the EMF/PD also controls the electron flow and hence the current.

Basic formula:

I=E/R

[Bob Eager]

I think that is roughly what he is trying to say. Luckily he isn't a lecturer.

[whisky-dave]

that the plug fuse is there to protect the cable,

1115W electric fire. No mention of what lead was connected if it was a pie ce of 1 amp cable guess what ......

even brought 50ma ones.

one of them has blow.

ts that even a 1.5V alkaline battery can blow a fuse, its the first labshee t 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.

hem to test this before but then I don't write labsheets academics do)

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.

n on the back of pieces of equipment.

fuse for everything upto 699W.

ch if just leads are treaded differntly because you have no idea what they will be connected to power wise.

, on the moulded plug it says stamped/engraved on the plug 5 amp fuse fitte d.

l {E}}} {\mathcal {E}} and measured in volts),[1] is the electrical intensi ty or "pressure" developed by a source of electrical energy such as a batte ry or generator.[2] A device that converts other forms of energy into elect rical energy (a "transducer") provides an emf at its output

No not when they can show you you are wrong, it make you look a bigger fool .

[whisky-dave]

cal {E}}} {\mathcal {E}} and measured in volts),[1] is the electrical inten sity or "pressure" developed by a source of electrical energy such as a bat tery or generator.[2] A device that converts other forms of energy into ele ctrical energy (a "transducer") provides an emf at its output

stistance or impedence or reactance. Without a PD or EMF you don't get curr ent.

which you don't get without a potential differnce.

Yes and what is the magnitude of the EMF/PD ?

What is E and R then ?

Electomotive Force (EMF), like pressure pushing on water, or like gravity p ulling everything on earth, is what pushes and pulls electrons to cause the m to jump from one atom to the next. Electromotive Force is measured in Vol tage.

NO VOLTAGE then NO current.

we ran a lab on this just 2-3 weeks ago.

[Martin Brown]

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.

You buy in course books for laboratory practicals? Cowboys!

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.

[Roger Hayter]

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.

[whisky-dave]

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

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.

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 recha rgable.

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

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 thing s and understanding using protoboard and stripboard amounts other basic thi ngs that those from school that have choseen computer science might need to know if they haven't studies or rememered their physics.

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

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

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

[The Natural Philosopher]

Back to the OP.

In my day you would have got 5 points for calcualtinbg what raing of fuse was theretically required and a further 5 for pointing out that this was in fact irrelevant, as the purpose of the fuse was not to protect the appliance, but its cable and therefore 13A would be a standard fitting.

>

[Martin Brown]

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.

[Dave Plowman (News)]

That would surely depend on whoever marking it had access to the regs governing such things and understood them. Not something you could guarantee.

[whisky-dave]

Yes there is because it will get warm, you shouldn't be running ampliances at 6 amps with a 5 amp fuse. Even if an aplliance might reach 5 amps you shouldn't be running it on a 5amp fuse.

yes ignore the mindless sums, that tell you the curretn will be 4.999amps so a 5 amps fuuse will be OK it won't, espeacially if you;re running it using 1amp cable.

which of course it shouldn't.

[tabbypurr]

no chance of that from you though.

[Roger Hayter]

But, of course, if they don't understand a subject they should not be setting examination questions on it. Even if the question is merely designed to beautify Ohm's law and power calculations.

[Roger Hayter]

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.

[Dave Plowman (News)]

Not really. OK to set a theoretical question which doesn't take into account practice. IIRC, the reason not to have a vast range of plug top fuses these days is that most punters simply don't understand such things and will just fit what is to hand - usually the most common one, ie 13 amp.

The original question didn't state is was dealing with a fused plug anyway.

[whisky-dave]

Already done it.