electrical fires
- Thread starter Joe90
- Start date
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AdamW said:Surely worst of all would be if phase and neutral are reversed at the CU, where the CU is an old-style rewireable cartridge type. Then the only protection would be the 100A incoming fuse. But if that was wired in reverse...
Adam, always going one better Actually i did a job once converting an old print shop into an engineering factory part of the makeshift alarm system was bellwire wired from microswitches above doors to a bell which on closer inspection was mains rated! but worse still it was fed by a timeswitch again wired in bellwire totally bypassing the Dist board and going straight into the incomer but NOT even on the 100amp side but the incoming side!!!???!!
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I don't know how I didn't remember this earlier, but I had an electrical fire once. I was in the lab at uni. I went and got a £20K digital oscilloscope, plugged it in and thought "Hmmm, not working." then I noticed a glow coming from the inside... Small orange flames licking about in there. I whipped the plug out of the wall and turned the 'scope onto it's topside on the bench, blocking the vents and putting the fire the wrong way up. Did the trick.
What we deduced had happened was that when stored in a cupboard, a small piece of metal had found its way in through the case vents. This shorted out the "wrong" conductors, and managed to set fire to something without any internal protection circuits or fuses cutting out, or any MCBs tripping. I don't think the technician bothered opening it to find out, the 'scope was, to put it technically, "absolutely f**ked", so binned. £20K worth. Ouch! Luckily the lab technician saw the whole thing so I wasn't liable for the equipment.
What we deduced had happened was that when stored in a cupboard, a small piece of metal had found its way in through the case vents. This shorted out the "wrong" conductors, and managed to set fire to something without any internal protection circuits or fuses cutting out, or any MCBs tripping. I don't think the technician bothered opening it to find out, the 'scope was, to put it technically, "absolutely f**ked", so binned. £20K worth. Ouch! Luckily the lab technician saw the whole thing so I wasn't liable for the equipment.
kendor said:
the thing you seem to be forgetting is the fuse is connected to the live pin of the plug however the user wires the plug (connecting the fuse to the wrong pin of the plug would basically require a totally different moulding its not going to happen)
lets take the simple case of a table lamp (with BC bulb)
the first image shows correct wiring
in this case a fault between the brown core of the appliance flex and earth should blow the plug fuse
a fault between the blue core of the appliance flex and earth probablly won't blow anything unless a rcd is involved but the current from such a fault is generally low enough not to do any real damage
for the second image the wiring has been reversed in the plug.
in this case a fault between the blue core of the appliance flex and earth should blow the plug fuse
a fault between the brown core of the appliance flex and earth probablly won't blow anything unless a rcd is involved but the current from such a fault is generally low enough not to do any real damage
in the third image the wiring has been reversed in the socket
in this case a fault between the blue core of the appliance flex and earth will have significan't voltage between it and earth and in the event of a fault to earth will only be stopped by the plug fuse
a fault between the brown core and earth is unlikely to bring enough current to blow the plug fuse
in the final image the wiring has been reveresed in both the plug and the socket
in this case a fault between the brown core of the appliance flex and earth will have significan't voltage between it and earth and in the event of a fault to earth will only be stopped by the plug fuse
a fault between the blue core and earth is unlikely to bring enough current to blow the plug fuse
yUP, PLUG TOP AINT GONNA MAKE A DIFFERENCE. a REVERSAL IN THE SOCKET WOULD.
a REVERSAL IN THE cu WOULD MAKE NO DIFFERENCE EITHER, UNLESS IT WAS MAINS TAILS.
a REVERSAL IN THE cu WOULD MAKE NO DIFFERENCE EITHER, UNLESS IT WAS MAINS TAILS.
If mains tails where reversed, N everywhere would be live - no fuses in plug tops, or CU would operate with a '''n''' to E fault.
Bit boring - this 'what if' scenario.
Bit boring - this 'what if' scenario.
But, yea, if the circuit cables where reversed, MCB would operate, but not plug top fuses.
breezer said: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
And answer said it in one specific example ....no. a fire can start from as little as 80mA tracking along a cable. Most fuses will not blow with just 80mA unless you also manage to get a decent increase in the load, beyond the fuse rating. So although fuses may help they won't prevent a fire if the fault has already ignited something. As for being technical....yes .. that's how you work things out isn't it?
B
breezer
Beanzy said:
you missed the point with your attempt at sarcasm, the origonal question was from a bloke who wanted a question answered as basicaly can or can not,
breezer said:
and you read that in which part of his question?
I'm not going to assume anything about someones desire for info from the fact it's an arguement down the pub. Surely he needs as much info as he can use to outwit his pub mate?
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