electrical fires

[Joe90]

can somebody settle a pub argument for me

house in our village had small fire the other day
electrical fault they reckon

but my mate reckons any electrical fault should make a fuse blow before the fault causes a fire

so what could cause a fire to happen

 

[breezer]

depends what the fault is

quite often it is said to be electrical fault when nothing else is obvious.

suppose it was an electric fire, it could have been pointing at some washing, dried it out, over heated it and set it on fire.

your mate is wrong, so he owes you a beer

suppose some one puts in a bigger fuse than is required

or for this argument runs a socket from a shower point (YOU CAN NOT DO THIS, READ ON AND FIND OUT WHY)

the socket develops a fault, the small cable feeding the socket gets hotter and hotter, it sets fire to what is surronding it, but the fuse will not go because beleive it or not, NOT enough current is being drawn yet.

To put it another way, the cable used for a socket is much smaller than that of a shower, so the smaller cable can not carry the same current, something must give, in this case the cable. not the fuse.

 

[plugwash]

firstly electrical fault is a conviniant excuse for the fire brigade

however that doesn't mean that electrical fires can't happen even on

fuses/cuircuit breakers (should) protect against significant overloads and short cuircuits

what they can't protect against is faults that draw less than the cuircuits normal operating current but dump that energy as heat in places that can't stand the heat

 

[shaggy]

...such as here Joe.

 

[Joe90]

thanks for the information guys
i'm off to get that pint now

shaggy - what's that a picture of

 

[Lectrician]

Looks like a shower - the highest risk places of fire are those that use/switch high currents.

 

[shaggy]

Spot on Lectrician; read all about it here

 

[Damocles]

Except breezer, that if someone used 2.5mm instead of 10, it just means the resistance per meter is 4x higher. So if the length used is less than 1/4 the remaining length of 10 which could have been used safely, then it will still pop the fuse.

Or put it another way again, even if the extension socket had been wired in 10mm, if it was too long the fuse would still not pop.

 

[plugwash]

depends what the fault is

quite often it is said to be electrical fault when nothing else is obvious.

suppose it was an electric fire, it could have been pointing at some washing, dried it out, over heated it and set it on fire.

your mate is wrong, so he owes you a beer

suppose some one puts in a bigger fuse than is required

or for this argument runs a socket from a shower point (YOU CAN NOT DO THIS, READ ON AND FIND OUT WHY)

the socket develops a fault, the small cable feeding the socket gets hotter and hotter, it sets fire to what is surronding it, but the fuse will not go because beleive it or not, NOT enough current is being drawn yet.

To put it another way, the cable used for a socket is much smaller than that of a shower, so the smaller cable can not carry the same current, something must give, in this case the cable. not the fuse.

it depends largely on the type of fault
int the event of a dea short unless the 2.5mm is very long the fuse/breaker should go long bfore any significant damage to the cable

overloads could do more damage but theese should be taken care of by the plug fuse

 

[Beanzy]

Another cause not related to cable size;

Class2 equipment developing an internal fault. ie; TV line output transformer windings insulation collapsing, increased heat and resistance but until it goes dead short the fuse may just cope. there are HT voltages inside a TV so there is greater potential for 'tracking' at lower currents. If someone has left flammable materials near the TV these may ignite before the fuse goes. This is especially common where someone just replaces the fuse with a 13A one when a 5A may be the proper rating. So the cables cope and the fault just shows as a higher load, with no earth leakage as no earth present.

 

[breezer]

i know all of that but you are all being technical about it, the question was should the fuse go first (to prevent a fire) , so i gave an example of what could happen

 

[kendor]

Another fault which would never blow the plugtop fuse no matter it's rating and cause a fire is an earth fault where the plug is wired up with live and neutral reversed a common problem in years past thankfully reduced somewhat by the introduction of moulded plugs on equipment.

 

[Lectrician]

Another fault which would never blow the plugtop fuse no matter it's rating and cause a fire is an earth fault where the plug is wired up with live and neutral reversed a common problem in years past thankfully reduced somewhat by the introduction of moulded plugs on equipment.

Yhink you mean reverse polarity in the socket, not plug.

 

[kendor]

No i meant the plugtop itself , maybe you misread the use of the word somewhat? where i was referring that it still happens as there are rewirable plugtop still available but it doesn't matter, a wrongly wired socket L+N reversed would achieve the same.

 

[plugwash]

actually reversal in the socket is far worse

with phase-neutral reversed in the socket then (assuming there is no rcd) the only protection from live(neutral as far as the appliance is concerned)-earth faults is the 32A breaker on the ring (or worse a 30A rewireable)

swapping live and neutral in the plug would probablly have little effect as the fuse would still be between mains live and the appliance and do not forget that appliances are designed for phase-neutral reversal nowadays (lots of european socket styles aren't even polarised)