What are fuses for?

Kilty_2

We were taught (Standard Grade Physics) that a fuse protects the plug's flex from over-current and subsequent melting and fire, NOT the appliance itself

YorkshireBoy

Kilty wrote:

We were taught (Standard Grade Physics) that a fuse protects the plug's flex from over-current and subsequent melting and fire, NOT the appliance itself

You see, that's what you get when you let a schoolteacher loose with a subject like electrical engineering!

The fuse (or any protective device) protects everything downstream of the fuse - including yourself when you're leaning on the washer just at the very moment it develops an earth fault (and somebody didn't terminate the earth wire in the plug). :eek:

djohn2002uk

kat21 wrote:

djohn2002uk
I suggest you back track the thread and actually read what rygon has posted he quiet rightly as you do and I have too, pointed out that their is indeed a ring main fuse and also the boards fuse which as you say is between one hundered amperies and sixty amperies

please re read what rygon says and read towards the end of his posting. My own explination is alot simpler
kat21

But using Ohms Law to state a bit of theory is totally irrelevant. It can never happen.
My house has a supply cable with copper conductors of .0225sq" cross section (My house is totally Imperial) and even that would not carry 16000A (Amperes not Amperies);)
My street is fed with a cable with conductors of .1sq" and this wont carry 16000 amps either and is protected at the substation by a cartridge fuse of 300A rating.
He could have used as an example, some cooker cable of 6 or 8mm and come up with an even higher current which would in his words be even more "scary" but just as irrelevant because the situation will never arise.

Now in his latest post he says "all that current will cause a lot of heat damage". Again he seems to have a little amount of theory in his head but no practical experience. e.g. You get a short circuit on a cooker which is only protected by a 30A fuse back at the consumer unit. Acording to his theory, a current of more than 16000A will flow (because the cable is bigger than in his example and therefore lower resistance). Where do we see all this heat damage because the fuse has taken milliseconds to blow? The normal scenario is that the element on the cooker that caused the short will be replaced, the fuse replaced and jobs done.
I did an old fashioned apprenticeship where you learned theory and practical, and at the end of it I realised that you can get by with learning the practical side with no theory but not the other way round.

shockingmoment

A fuse reacts within an envelope called a fuse curve. Basically, the larger the fault current, the quicker the fuse will blow. And yes, many hundreds or thousands of amps will flow, depending on the fault level (the capacity of the sytem). This fault current will be for a very very short period of time (determined by the arcing time (called I squared t)! Fuses provide protection of the circuit downstream of the fuse - protecting both the cable and the apparatus as well as providing protection against electric shock and harm from arcing.
Hope this helps!

tomstickland

A fuse can't blow until some excess current has flowed. Then again, the current draw won't instantaneously be that deduced form Ohms law. What will happen will be somewhere in the middle and it's quite conceivable that quite a large current will flow before the first fuse blows.

YorkshireBoy

djohn2002uk wrote:

16000 amps will never flow through the cable because even if a nail were put in the plug, the ringmain would be protected by either a 30A fuse or a 32A breaker so that would be the maximum current to flow, and only for milliseconds.

The maximum current that will flow into a L-N fault on a circuit is called the 'prospective short circuit current', or PSCC. This can range from several hundred to several thousand amps in a domestic installation, depending on how far you are from the supply transformer and the size of the supply cable feeding your property. By contrast, PSCC's in industrial substations can be as high as 27,000 amps on a standard 1,000KVA transformer terminals.

It's called 'prospective' because that's the maximum current that could theoretically flow. In practice this value is never reached because the protective device cuts off the supply. However, I can assure you that, because no (domestic) protective device can operate within one half cycle, many hundreds of amps can flow into your domestic fault before the protective device has time to operate. Fuses work on 'let through energy' - or I squared T (I x I x T). Therefore, the higher the current the less time to blow.

tomstickland

That'd be because I^2 R is a heating value and therefore I^2 t would suggest a total heat input, I guess the fuse requires a certain heat input to melt.

baldelectrician

To put it simply a fuse is a safe (specially designed) weak point in an electrical system.
A fuse in a plug top protects the appliance connected to it and it's flex.

A fuse (and a MCB) in a consumer unit protects the circuit it powers.

Do not confuse a fuse (or MCB) with a RCD (a RCD usually has a press to test button on it).

A RCD works on a totally different principle.

When a fuse blows, the larger the overload the quicker it blows, so a 30A fuse may happliy let 38 A pass for upwards of 15-25 mins.

A RCD works on a balance principle- think of it like the radiator in your lounge- the water that goes in the pipe on one side returns from the pipe on the other side, if there is a leak there is a difference between what is going in and what is going out.
A RCD usually trips with an imbalance to earth of more than 0.03 Amps, where a standard fuse won't. This is why RCD's are soooooo good at saving people.

djohn2002uk

Y B. Again you are quoting theory but at least you qualify it. You, being an electrician, know that a fault on a circuit protected by, say, a 30A fuse, rarely gets blown by a slight overload and is usually caused by a serious fault with the potential of drawing a high current. This then will blow the fuse/trip the breaker rapidly, in milliseconds, certainly before reaching anywhere near 1000s of amps.

In my experience, the only time I saw any "Heat damage" due to excessive current was when lightening struck overhead wires and all the insulation in an underground cable connected to it, normally carrying 11000volts, was burnt totally over 100yds. But that was more due to the high voltage I would think.

tomstickland

Heat input will be proportional to I^2 t, so you could have 1000000A for an infinitesimely small time period and you'd not see any heat damage. I agree, the peak current is unlikely to be as high as simple theory would say, but it might be a good bit higher than you expect.