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Melted live terminal, extension lead

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Melted live terminal, extension lead

Postby Andyp » 09 Mar 2015, 09:14

Could this be the result of not fully unwinding a 50m extension lead when in use? I have had it for about 7 years and most of the time only unwind about half of it’s maximum. Yesterday, after my chainsaw would not work, I found that live terminal in the socket had completely melted (on the right of the earth pin)

Image

Are there any other likely causes?
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Re: Melted live terminal, extension lead

Postby Deejay » 09 Mar 2015, 10:15

Morning Andy

Possibly a loose connection which has been arcing.

Is there any sign of heat damage on the cable?

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Re: Melted live terminal, extension lead

Postby TrimTheKing » 09 Mar 2015, 10:25

I've killed an extension once by not unwinding it and all that happened was the whole reel of cable melted together, but didn't affect the sockets.

Luckily I caught the temp and smell before it shorted and switched in off, but could have been very much worse!!! :shock:

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Re: Melted live terminal, extension lead

Postby Andyp » 09 Mar 2015, 10:57

Deejay wrote:Morning Andy

Possibly a loose connection which has been arcing.

Is there any sign of heat damage on the cable?

Cheers

Dave


No Dave, I ran my hands over the whole cable afterwards and saw nor felt anything. If the connection had worked loose it is impossible to see now.
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Re: Melted live terminal, extension lead

Postby Robert » 09 Mar 2015, 11:17

Resistance + current = heat.

A poor connection will create resistance and hence heat when loaded. cable itself has resistance hence the maximum current ratings for different sized cables. The ratings allow for a safe amount of heat to be generated in a cable at maximum load but they are based on the cable being in a certain application and coiled on a reel is not an expected use.

We used to make low voltage lighting kit that had 20A running through terminals. I was always double checking the assembly shops work to make sure those terminal connections were perfect. One got missed once and we spent 2 days checking lights up ladders in a john lewis store after that one overheated and failed. There were no others with problems fortunately.
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Re: Melted live terminal, extension lead

Postby Deejay » 09 Mar 2015, 13:11

Afternoon Andy

If the connection had worked loose it is impossible to see now.

I know, but screwed connections do work loose.

If I change a light switch , for example, I use my accurate torque wrench (wrist) to tighten the screws.
Come back in a couple of years when you decorate and try tightening the screws before you disconnect the wiring. The same accurate torque wrench finds them looser.

The only thing I can think of is tiny vibrations as the current flow reverses at 50 Hz. How's that for a theory?

Cheers

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Last edited by Deejay on 09 Mar 2015, 18:03, edited 1 time in total.
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Re: Melted live terminal, extension lead

Postby RogerS » 09 Mar 2015, 15:09

Dave is on the money. It won't be anything to do with you not unreeling the cable fully but simply the fact that the screw connection had loosened up over time. Enter Robert's explanation and Bingo...localised overheating.

In the old CMCR's (Colour Mobile Control Room) we had a mains control panel and one of the terminals inside that came loose and overheated much to our consternation. Checking the terminals for tightness became part of the SoP.

Which reminds me of the time we were doing a Grade 1 broadcast. Grade 1 was where there was a member of the Royal Family playing a major role...as in wedding (it was Princess Anne's IIRC). Grade 1's required a separate spare mains supply and there was a humungously huge mains changeover switch that would be used to switchover to this spare in the event of failure of the main feed.

The main feed what duly failed. To a man we leapt out of the vehicle to changeover only to find it padlocked in the main feed position. :oops: Turned out the Engineering Manager had done it as he didn't want it to me accidentally changed over'. Only he was the only person with the key. We found him eventually. Down the pub, as I recall.
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Re: Melted live terminal, extension lead

Postby Andyp » 09 Mar 2015, 18:26

Thanks at least I can be certain that my apparent misuse of the extension lead is not the cause.

Does anyone actually uncoil the whole lead ? I was considering buying a shorter one but will not bother now.

As for the screw working loose that is always possible. I am always telling the kids and the missus never to drop a plug for that very reason.

Checking the terminals in the light switch when decorating is a good tip too.
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Re: Melted live terminal, extension lead

Postby RogerS » 09 Mar 2015, 18:51

If I was using an extension lead at close to its maximum then I would unwind it (probably!). It would depend on how long I was going to use it for continuously, ambient temperature etc.
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Re: Melted live terminal, extension lead

Postby Rod » 09 Mar 2015, 22:34

Most of my extension leads I've made myself and very rarely use them coiled tight.
The coiled one I use for our Christmas Tree has a Warning engraved stating "Unwind cable completely for loads greater than 1250W (5A) Max load unwound is 3250W (13A)"

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Re: Melted live terminal, extension lead

Postby Andyp » 10 Mar 2015, 13:33

Thank you Rod. I've just read the rating sticker on the extension lead. I really ought to pay more attention to these things.

It is rated at 1725 watts wound and 3680 watts unwound. So fine for the chainsaw at 1700w but for the shredder at 2500w I really ought to unwind it fully.
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Re: Melted live terminal, extension lead

Postby StevieB » 10 Mar 2015, 15:18

I now unwind fully after having one get hot and cut out on me (fortunately it didn't melt through the cable and short circuit). Once bitten twice shy - I now unwind for pretty much everything I do with one.

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Re: Melted live terminal, extension lead

Postby Phil » 13 Mar 2015, 19:06

Agree with screws coming loose over a period of time.

I always unwind the cable especially a long one to avoid any heat build up when using the mower or any big power tool.

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