Are 3A plug fuses really necessary? Why not always 13A?

[whisky-dave]

Above what the Drivel ?

[meow2222]

do yourself a favour and plonk him. You'll never get reason from Drivel.

NT

[Doctor Drivel]

You mean I will never agree with an idiot.

[polygonum]

Hmm. Do you agree with yourself?

[geoff]

Drivel and dIMM spend their time having arguments with each other

and then kissing and making up

if only D was a dog, eh?

[Doctor Drivel]

Maxie, how are you? Are you still doing psychedelic things? Fantastic. Do you have a psychedelic Paddy band now. Amazing. Only Maxie does these sorts of things. What a man!

[SteveW]

Or Nixies :)

SteveW

[altvamp]

To "the fuse is there to protect the cable" ? Hmmm, that's rather to vague, and technically incorrect. I think you need to ask what are we protecting really, and I beg to differ that the fuse is there to protect the cable, he re are some reasons why:

Really what we are protecting is ourselves and other property from electroc ution and fire, The simple fact is that if the cable is overloaded then it will melt and cause a fire, it will then also short out the supply causing further risk. To illustrate lets consider the service fuse in your electric ity supply, without it any short or overload would probably take out at lea st part of your street. It's easy to forget that the fuse is there to protect against overload beca use in most cases it's blown due to a direct short, but if you are running a 3KW heater on a 5A lighting circuit with a 13A fuse, that wire is going t o melt sooner or later, but even assuming it doesn't, over time the excess heat will make the plastic brittle and one day it will crack, perhaps when you pick up the cable! A 13A fuse will certainly protect a direct short an a 3A cable as it won't really have time to melt before the fuse blows, but if you want a demonstration of what can happen try dropping a some 13A stra nded wire across a 12V car battery, make sure you aren't holding it and the re is nothing combustable around though as the whole length will instantly burn all it's length before it vapourises- It's quite spectacular, yo might want to wear suitable eye protection as it could be quite bright.

[tabbypurr]

You're replying to a 4 year old discussion for some reason.

e, and technically incorrect. I think you need to ask what are we protectin g really, and I beg to differ that the fuse is there to protect the cable, here are some reasons why:

ocution and fire, The simple fact is that if the cable is overloaded then i t will melt and cause a fire, it will then also short out the supply causin g further risk.

That certainly isn't fact.

without it any short or overload would probably take out at least part of y our street.

maybe for strangely forgetful folk

nning a 3KW heater on a 5A lighting circuit with a 13A fuse, that wire is g oing to melt sooner or later,

Rubbish. BTDT.

stic brittle and one day it will crack, perhaps when you pick up the cable! A 13A fuse will certainly protect a direct short an a 3A cable as it won't really have time to melt before the fuse blows, but if you want a demonstr ation of what can happen try dropping a some 13A stranded wire across a 12V car battery, make sure you aren't holding it and there is nothing combusta ble around though as the whole length will instantly burn all it's length b efore it vapourises- It's quite spectacular, yo might want to wear suitable eye protection as it could be quite bright.

It's all quite simple. 3A fuses aren't required for new goods that conform to the law, if extension leads aren't used. IOW in most cases 3A fuses are unnecessary. Now add the real life facts of old appliances, nonconforming g oods, extension leads and daisy chained extension leads and it's easy to se e why 3A fuses are still a smart idea.

NT

[John Rumm]

Yup plenty ;-)

One needs to be specific about which fuse, and in what circumstance.

There are some circumstances where the primary of job of a fuse is to provide fault protection to a cable. However there are other times it may be required to provide overload protection (to a cable or an appliance).

In this particular case it seems to be talking about the plug fuse on an appliance lead. If one ignores some very old appliances, then its true the only purpose of the fuse is for *fault* protection of the lead. If the appliance needs overload protection, then it must include its own internally.

(note this is a different situation from hard wired appliances that don't have fitted flex and plug - there a manufacturer may require a particular fuse rating in the supply)

Bit of a red herring there. We are talking about a DVR with its own internal overload protection. So overload is non issue.

The only thing we need demonstrate is that the plug fuse will clear a fault at the far end of the lead.

(and that is just a matter of establishing the Prospective Fault Current, and the applying the adiabatic equation[1] to establish if the flex has enough cross section to withstand the fault - we don't even care if it ends up with a conductor temperature north of 150 deg C in the process, so long as the thing does not vaporise or burst into flames)

[1]

More twaddle really... Even with lighting circuits wired in 1mm^2 T&E the current handling capacity of the cable will take a 3kW load indefinitely in the majority of cases (unless the cable is buried in insulation).

again, probably incorrect.

The current carrying capacity of a cable is dictated by its maximum long duration conductor temperature. For PVC insulated cables that is typically 70 deg C. So long as the cable can reach thermal equilibrium at or below that, then you can expect a long service life from it.

and that is really the end of the discussion - its purpose in life has been achieved ;-)

Not sure how the suggestion that one sticks a 0.5mm^2 CSA flex across a source capable of supplying several hundred amps for an extended period demonstrates anything other than how do dangerous stuff and abuse a battery.

ISTM the poster in question (and I have a feeling this ancient thread has been discussed here before) does not really have a clear understanding of the difference types of over current situation that one may need to protect against.

Overload protection (i.e. stopping something drawing more than its design current for a prolonged period) is different from clearing a fault where the only limit on current draw is the resistance of the wires in question (i.e. a short between live conductors, or between an line and earth conductor).

A fuse may do either or both in different circumstances.

(plug fuses are not usually called on to provide overload protection, apart from the situation where they are protecting a spur from a high capacity circuit. (you could also include multiway extension leads in that category))

Having said that, there is a slight advantage to using lower rated plug fuses when running appliances from extension leads or in other circumstances when cascaded from another BS1362 13A fuse, since they may discriminate better in the event of a fault.

i.e. put the metal leg of your ladder through your drill's battery charger flex, and a 3A fuse in its plug may blow without taking out the

13A fuse in the multiway extension lead feeding it.

[Chris B]

conform to the law, if extension leads aren't used. IOW in most cases

3A fuses are unnecessary. Now add the real life facts of old appliances, nonconforming goods, extension leads and daisy chained extension leads and it's easy to see why 3A fuses are still a smart idea.

Just a thought but do you know if they have ever been "required". I ask because, when I were a lad (late 60s early 70s) our council house was rewired from 15A Round pin to 13A square pin. The contractors who did the rewiring put new plugs on all of the appliances that were about and in use and every one of these plugs had a 13A fuse in it.

As a 14 year old who was just starting to get into engineering I thought this was wrong and changed some of the fuses appropriately. However I thought the contractors might have been doing a dodgy job and I wrote off to someone (Cant honestly remember who but BSI comes to mind - their kite-mark was everywhere at the time).

I definitely got a reply, the gist of which was "Any fuse is better than none and yes there is nothing really wrong with putting a 13A fuse in a table lamp"

I have been pretty relaxed about fuse ratings ever since.

[Tim Watts]

Now try putting your 13A rated bare wire in series with a 13A fuse across a car battery. I guarantee that the wire will be fine. Google "Let through energy" for more detail.

The fuse is designed to heat up and melt before the cable heats up to beyond its operating temperature (70C for PVC insulated).

Yes, a 13A fuse will not help a 3A cable, but you should have had a 3A fuse there in the first place.

As for your 3kW heater on a 5A lighting circuit - yes, the breaker at the source WILL protect the cable and will trip before the cable gets to

70C for an otherwise correctly designed and installed circuit.

[Brian Gaff]

Blimey 2012, almost up to date.

On an allied subject, Remember some time back an incident where a person died due to a fuse not blowing when a mains cable was taken up by an old style beater hover, mostly made of metal. The cord was not the one on the hoover but one on a table light, which only had a 13 amp fuse not a lower one so it never blew. However I'd not suggest taking up live cables in any case to be honest! Brian

[tabbypurr]

It sounds like you were wrongly advised. At that time a lot of table lamps & some other goods used what is now regarded as speaker flex for a mains le ad. This stuff is not protected by a 13A fuse, it requires a 3A to be safer . Also other appliances in use at that time could not be counted on to have inbuilt overload protection.

OTOH the safety standards of the day were a lot laxer. There was plenty of class 0 stuff in use, wires stuffed under plugs, heating run on 1930s light ing circuits, taped twisted flex joints and so on.

NT

[Roger Hayter]

I really don't think the size of the fuse on the table lamp is the issue here. It is hoover not being effectively earthed that is the immediate problem. If it had been earthed it would not have reached a dangerous voltage and either the 13A fuse or the flex to the table lamp would have melted and broken the connection.

[DerbyBorn]

I will stick to fusing with the lowest capacity fuse to suit the current of the appliance. I want the fuse to blow as soon as possible if there is a fault.

[damduck-egg]

That may be so but the early and mainly metal models of Hoover only had two round pin plugs with the plug body made of a rubber material ISTR my mother getting a new Hoover circa 1962 and while the cleaner was a considerably different to Grans 1930's one the rubber two pin

5amp plug was hardly any different in appearance. Had to use an adapter to use a 13 amp socket.

A lot of early ones were used from a lamp fitting, Hoover even supplied the bayonet connection with the rubber plug at one time.

The early Hoovers would have been used on DC supplies as well, any one know if any area had DC sockets with an earth?

G.Harman

[John Rumm]

Nothing wrong with fusing appropriately, but when dealing with fault currents (in the technical sense), it makes little difference if its a

3A or 13A fuse when you try drawing 300A through it.

[Roger Hayter]

Good point. But if the only fault current was that passing through the victim then no fuse on the table lamp could have helped.

[tabbypurr]

When you have an extension lead plugged into another, or an old appliance, or something bought from China, it certainly does.

NT