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What causes short circuit, why it can cause fire?


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The maximum current between any two points in any circuit flows along the path of least resistance.

When an unintentional low resistance path occurs between two conductors (perhaps one being earth) a very large unintentional current can flow.

This develops a large amount of heat as the heating effect is proprotional to the square of current.

So (metal) conductors can get very hot and if theya re touching something flammable they can start a fire.

 

Does this help, ask for more if you did not follow any part of it ?

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Posted (edited)

As studiot said ^^^

There are applications where a controlled "short circuit" to create heat or light is useful. For example electric fires, light bulbs, oven grills... These items tend to use high melting point and low resistance materials so that enough current flows to heat the elements without them melting, tungsten filament light bulbs are an example of this.

This is why, as mathematic says, circuit breakers are designed into electrical systems to prevent unintended over current which could result in cables heating up beyond their design threshold. 

In most modern domestic premises you will generally find 2 types of circuit breaker (fuse), or a combination of both. There are  MCB's which stands for -  miniature circuit breaker, these are the "over current" protection devices. You will also find RCD's which stands for - residual current device, these are shock protection devices. 

Both devices are designed with rated operating times to cut off the current flow in the event of a fault,

Other types of fuses that are actually designed to "over heat" and melt to cut off current flow are common in things like plug tops, old electrical systems, large industrial applications...  

It's very important, and on point to your OP, that fuses and circuit breakers are carefully selected so that the correct current rating is used!!  For example it would not be a good idea to use a 32 amp rated fuse on a circuit that was designed to operate at 6 amps. A fault in this scenario could possibly result in cable conductors over heating to such a degree that they are hot enough to set on fire any combustible material near to them, before the circuit breaker or fuse operated.   

Edited by Intoscience
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10 minutes ago, Intoscience said:

It's very important, and on point to your OP, that fuses and circuit breakers are carefully selected so that the correct current rating is used!!  For example it would not be a good idea to use a 32 amp rated fuse on a circuit that was designed to operate at 6 amps. A fault in this scenario could possibly result in cable conductors over heating to such a degree that they are hot enough to set on fire any combustible material near to them, before the circuit breaker or fuse operated.   

 

This would not be a short circuit however. It is an example of an overload condition, nor is the intentional use of the heating effect of cuurent in electric heaters, light bulbs etc.

 

Peter has asked specificially about the relationship of short circuits, which are unintentional connections, to heating.

There are other undesirable effects of short circuits (without necessarily causing large currents) but which can cause danger to life.
This happens when an exposed conductor becomes unintentionally connected to a live supply and then a danger if touched. Metal cases, knobs suffer from this.

 

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6 minutes ago, studiot said:

 

This would not be a short circuit however. It is an example of an overload condition, nor is the intentional use of the heating effect of cuurent in electric heaters, light bulbs etc.

 

Peter has asked specificially about the relationship of short circuits, which are unintentional connections, to heating.

There are other undesirable effects of short circuits (without necessarily causing large currents) but which can cause danger to life.
This happens when an exposed conductor becomes unintentionally connected to a live supply and then a danger if touched. Metal cases, knobs suffer from this.

 

Yes, I'm well aware of all this, I'm an electrical inspector and tester by trade. 

I was assuming from his post that he wanted a layman's explanation on how a fire could result from a short circuit. So was focusing on how conductors might heat up due to current flow which is what causes the fires. The examples I'd given were (though not technically short circuits per say) aimed at this rather than going into great technical detail.   

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16 minutes ago, Intoscience said:

Yes, I'm well aware of all this, I'm an electrical inspector and tester by trade. 

Rewarding job, though I don't know where you are it can be interesting to swop regulatory stories.

Did you know that in the UK it is illegal for the consumer to connect the neutral to earth, whereas in the US it is illegal not to ?

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Posted (edited)
23 minutes ago, studiot said:

Rewarding job, though I don't know where you are it can be interesting to swop regulatory stories.

Did you know that in the UK it is illegal for the consumer to connect the neutral to earth, whereas in the US it is illegal not to ?

I'm UK based like your good self. My wife is from Bath originally, I'm from Stoke on Trent. 

I'm only familiar with English regs, BS7671, so no, I didn't realise this was the case in the US. 

It can be a rewarding job but also boring at times, to be honest. The most rewarding part was when doing an inspection a dangerous fault was identified, before somebody got injured or worse. I've seen some terribly shocking (excuse the pun) installs over the years. One I can recall was at a horse stables, where the owner had nailed some exposed conductors (stripped back T&E) to a stable door and connected it to a 230v plug top, plugged into an old outdoor socket with no rcd protection. This was his attempt to stop the horse from chewing the door by shocking it! Unbelievably stupid, people can be sometimes, let alone cruel in this case!

Another one was a call out job I attended where a single mother and her new born baby had "no electric". It was an old Wyle rewireable fuse board with no other protection, on a TT earth system. The cables were VIR and though obviously old, looked ok going into the CCU the socket and lighting circuit fuses had blown. Anyhow, to cut a long story short, the house had no carpets and on the landing area I could see through the floorboard cracks some cables that looked trapped over some pipes. As I lifted a board that was partially nailed there was a smell of gas and a quite hiss. The lead gas pipe run the length of the landing, and some idiot had nailed through it! even more alarming was that the old VIR cables that had perished exposing the bare conductors were routed in many place over the lead gas pipe! Scary situation to say the least!

To be honest I've just recently taken a new job which is more project management than anything. So my inspection and testing days will now be limited to favours for friends etc rather than my job.

Thanks   

Edited by Intoscience
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Posted (edited)
On 7/8/2021 at 11:17 AM, studiot said:

The maximum current between any two points in any circuit flows along the path of least resistance.

When an unintentional low resistance path occurs between two conductors (perhaps one being earth) a very large unintentional current can flow.

This develops a large amount of heat as the heating effect is proprotional to the square of current.

So (metal) conductors can get very hot and if theya re touching something flammable they can start a fire.

 

Does this help, ask for more if you did not follow any part of it ?

IMO, brilliant explanation, Studiot.

Edited by joigus
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2 hours ago, Intoscience said:

Another one was a call out job I attended where a single mother and her new born baby had "no electric". It was an old Wyle rewireable fuse board with no other protection, on a TT earth system. The cables were VIR and though obviously old, looked ok going into the CCU the socket and lighting circuit fuses had blown. Anyhow, to cut a long story short, the house had no carpets and on the landing area I could see through the floorboard cracks some cables that looked trapped over some pipes. As I lifted a board that was partially nailed there was a smell of gas and a quite hiss. The lead gas pipe run the length of the landing, and some idiot had nailed through it! even more alarming was that the old VIR cables that had perished exposing the bare conductors were routed in many place over the lead gas pipe! Scary situation to say the least!

Not being a day to day electrical inspector, I have only occasional experience of this in my work, and under rather specialised circumstances as well.

However when we moved into our present house (built in 1938/39) I did face a somewhat similar situation.
The previous owner had advertised the house as 'rewired'.
This meant that someone had replaced almost all the old round pin standard sockets with square pin types.
But on the original power circuitry wiring.
Many of the old cables entering the back of the new sockets were in the same bare condition you describe above. Really scary.
Conversely the lighting circuit cables had been replaced but the owner did not like the look of modern switches and fittings so fitted the new cables into the existing fittings, but clipped the earth conductor short as the old lighting system had no earth provision.

I was sort of glad to have the opportunity to rip it all out and start again.

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On 7/9/2021 at 11:56 AM, studiot said:

Not being a day to day electrical inspector, I have only occasional experience of this in my work, and under rather specialised circumstances as well.

However when we moved into our present house (built in 1938/39) I did face a somewhat similar situation.
The previous owner had advertised the house as 'rewired'.
This meant that someone had replaced almost all the old round pin standard sockets with square pin types.
But on the original power circuitry wiring.
Many of the old cables entering the back of the new sockets were in the same bare condition you describe above. Really scary.
Conversely the lighting circuit cables had been replaced but the owner did not like the look of modern switches and fittings so fitted the new cables into the existing fittings, but clipped the earth conductor short as the old lighting system had no earth provision.

I was sort of glad to have the opportunity to rip it all out and start again.

Yeah, you see this sort of thing often. Especially when they use decorative metal switches with no cpc present.

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