Extension Lead Fire..

flyingsparks · 29 Jun 2011

Why we shold make people more aware of the dangers of using wound up extension leads for heavy loads...

http://www.bbc.co.uk/news/uk-england-devon-13962025

xr4x4 · 29 Jun 2011

People should pay more attention. Extension leads have had 'wound' and 'unwound' amp ratings on them for years.

sparkticus · 29 Jun 2011

Yes, I agree. We have quite strict rules for installing and specifying fixed wiring yet these reels can be left simmering away carrying as much as 3KW.

It may be possible to design some sort of lock-out when the reel is fully/partially wound

Adam_151 · 29 Jun 2011

Most reels these days seem to have a maually resetable thermal overload built into them

I'm supprised this one in article actually caught on fire and burnt house down though, I would have expected the PVC to go all dripy and the conductors to migrate into each other and take out the fuse before that happened

, unless the reel was right ontop of something flamable, or very easily combustable

ban-all-sheds · 29 Jun 2011

xr4x4 said:
People should pay more attention. Extension leads have had 'wound' and 'unwound' amp ratings on them for years.

I blame Darwin.

securespark · 29 Jun 2011

Calling RF and his welding gear to re-create an extension lead fire...

sparkticus · 29 Jun 2011

Adam_151 said:
Most reels these days seem to have a maually resetable thermal overload built into them

True but that relies on excessive current. The overload may be set at 12 amps (for example) but may be simmering at 10 amps. In other words I'm not sure the heat build up due to poor dissipation will cause the overload to trip (I don't know)

Adam_151 said:
I'm supprised this one in article actually caught on fire and burnt house down though, I would have expected the PVC to go all dripy and the conductors to migrate into each other and take out the fuse before that happened , unless the reel was right ontop of something flamable, or very easily combustable

Yes, I tend to agree. If indeed this turns out to be the cause of the fire than I suspect that the circumstances must have been just right. Though if the circumstances were right on that occasion it could happen again.

JohnW2 · 29 Jun 2011

Adam_151 said:
Most reels these days seem to have a maually resetable thermal overload built into them

Many certainly have resetable 'cutouts' of some sort, but I suspect they may be current-driven. However, I see no reason why there couldn't be a proper thermally operated device.

Adam_151 said:
I'm supprised this one in article actually caught on fire and burnt house down though, I would have expected the PVC to go all dripy and the conductors to migrate into each other and take out the fuse before that happened , unless the reel was right ontop of something flamable, or very easily combustable

Indeed. I often think of that whenever I hear of fires allegedly started by overloaded cables - one would have thought that either an OPD or residual current device would usually be caused to operate before flames appeared.

Kind Regards, John.

Lectrician · 30 Jun 2011

It would be fairly hard to incorporate a thermal trip - ideally you need the thermister in the coil itself somehow....

I think we have all come across a very warm extension lead at some point, and shockingly, I did in my own home just last weekend! We are doing some building work which meant the tumble drier was out of action. As it's sunny, I didn't worry. I came home to find the misses had set the tumble drier up in a door way, and was using a wound up extension. It was only a 10m 13amp one, and was not damaged, but certainly warm.

These days you can get 50m reels which use quite small cable, still rated to 13amp. As they are so cheap, people get these rather than the shorter ones. This leads quickly to them being used without being unwound.

bernardgreen · 30 Jun 2011

A friend found the inner layers of his 30 metre extension lead had melted and welded together.

They can get hot enough to be a source of ignition before the conductors short out well enough to trip an MCB or RCD.

JohnW2 · 30 Jun 2011

Lectrician said:
It would be fairly hard to incorporate a thermal trip - ideally you need the thermister in the coil itself somehow....

It would certainly be difficult to incorporate a thermal device which was going to always be the solution. However, the nature of the problem is such that I imagine the inner part of the coiled wire is likely to be the hottest, so a thermal device at the centre of the reel would probably often result in disconnection before anything nasty happened.

Lectrician said:
I think we have all come across a very warm extension lead at some point, and shockingly, I did in my own home just last weekend!

Yes, I'm sure that most of us have been guilty. However, it's fairly unusual to use an extension reel for large sustained loads - and on the rare occasions I've done it, the fact that it was a large load has usually prompted me to unwind the cable completely. One obviously can't get much worse than what you other half did, since a tumble drier is just about the largest sustained load which exists in any household!

Kind Regards, John.

JohnW2 · 30 Jun 2011

bernardgreen said:
A friend found the inner layers of his 30 metre extension lead had melted and welded together.

Hence my suggestion that a thermal device at the centre of the reel might be a partial answer.

bernardgreen said:
They can get hot enough to be a source of ignition before the conductors short out well enough to trip an MCB or RCD.

That's the bit that somewhat surprises me. It obviously depends a fair bit on luck. If an L-E fault arises on an RCD-protected circuit, disconnection certainly should occur very quickly. As a source of ignition for very flammable things (like petrol vapour), I can see the problem, but I'm still surprised that the cable itself can ignite before there is a sufficient short to operate a fuse or MCB. However, it appears to happen, so my intuition must be wrong.

Kind Regards, John.

ericmark · 30 Jun 2011

There are a number of ways to stop this happening and the first done in this case by BBC is education.

But for it to result in "ignition of the extension reel" the reel must be made out of the wrong material. For it to melt and cause a short yes that is expected but it should not burst into flames.

I have found many leads to have overheated and melted inside but non have ignited.

JohnW2 · 30 Jun 2011

ericmark said:
But for it to result in "ignition of the extension reel" the reel must be made out of the wrong material. For it to melt and cause a short yes that is expected but it should not burst into flames. I have found many leads to have overheated and melted inside but non have ignited.

That's what I would have expected.

Kind Regards, John.

bernardgreen · 30 Jun 2011

ericmark said:
But for it to result in "ignition of the extension reel" the reel must be made out of the wrong material.

I don't yet have details but was told the reel was feeding a fan heater in an office during the cold spell when it "erupted" with flames and smoke coming out of the reel when it was moved

The mechanism may be the PVC (?) insulation in the middle layers of the coil becomes hotter than its flash point but with no air ( oxygen ) it cannot ignite. Neither can it move so it keeps the conductors apart. When air comes in contact with over heated insulation ignition of the insulation happens rapidly. This could happen if the reel is unwound or the heat reaches the layers in contact with the air. At this point the conductors probably will short out and trip protective devices but there is already enough heat to cause ignition.

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Extension Lead Fire..

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