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Posted By halesowen Baggie
An employee has filed a hazard report relating to a extension lead.
The lead has been used without it being fully unwound and has hence melted where the coils are still on the barrel.
This is obviously due to the resistance the current comes against in the coils.
My questions are;
Why did the fuse not trip the extension lead?
The extension lead has 4 sockets to plug electrical items into and is rated at 13amps, what does this mean if I were to run 4 items (eg, radio,drill,grinder,light) off this extension lead.
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Posted By Gff
The current is not affected by the coiling but by the load. The heat generated will be higher if the load or current is greater but the fuse won't open if the current is not greater than it's rating hence the reason for it ending up melting.
In other words the current does not need to exceed 13A to cause the build up in heat, you only have to use the power for long enough to allow it to build up
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Posted By Descarte
Is it not just the fact that this was a coiled electrical wire the reason it generated heat? or am I barking u pthe wrong tree
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Posted By Gff
Yes
Heat is generated when the current flows anyway but coiling the cable helps it build up, try running 32a through 1.5mm cable it will melt with out it being coiled, you can see this with thermographics.
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Posted By allan wood
the extension lead should be unwound fully before use, if not the remianing cables acts as a coil so generating heat and increased resistance
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Posted By halesowen Baggie
Sorry to be thick, i just have a mental block with Electricity.
Would a 30ma RCD have prevented the cable overheating?
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Posted By Gff
No RCD's only protect againts faults involving current leakage from phase to earth not over current or phase to phase or phase to neutral short circuit
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Posted By Alan Nicholls
Unless the insulation breaks down and allows the conductors to touch each other then the fuse will remain intact. A good one for your Fire RA.
Regards Alan N
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Posted By Gff
The question was in relation to the RCD which only protect against current leakage.
if the conductors shorted, phase to neutral or phase to phase (3phase) then the Cicuit breaker would have operated. The fuse would have blown in a high current fault
At the end of the day fully unwind your drum before use
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Posted By Chas
With regard to the second part of the original query, I was always told that the four plugs fitted into a four socket extension lead should have a combined fuse rating of no more than that of the plug on the extension lead. ie four plugs each of 13A fuse rating in a 13A extension lead should not be permitted. Is this just an urban myth or fact?
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Posted By Gff
You can draw your own conclusions.
Your likley to be plugging into a 32amp ring main the fuse for the extension will be rated at 13amp. Overloading this the extention plug fuse will open (the weakest link), if it doesn't and you don't unwind your coil, it would heat up very quickly.
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Posted By David J Jones
Simple rule - no matter how many sockets are on any 13 amp extension lead, coil or cable, be there two, four, six sockets, or however many, the total load plugged into that extension must never exceed 13 amps.
Put it another way - add up the current rating of each item as you plug them in then stop when you reach a total of 13 amps. A common mistake is to add "just one more" if that last item goes a little over 13 amps total. Don't.
David
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Posted By Descarte
For that reason, most offices and companies I have worked for have banned extension leads and multi plugs to prevent "daisy chaining"
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Posted By Tabs
Just because an electrical item has a 13A plug, or even a 13A fuse, doesn't mean it will draw 13A.
In your example the radio and light (sic) are unlikely to draw very much, whereas a drill or grinder under heavy loading (i.e. doing a tough job) may well approach 13A each.
Five-unit hi-fi will happily run through a single 5A fuse - has done for years in my house.
The reason a coiled lead will heat up has more to do with the lack of air cooling. A lead laid on the floor has lots of air taking the heat away. Not so in a multi-layer coil.
Easiest solution - get a lead to suite the situation. A 30 metre lead to cover a 3 metre distance is way too much, use a 3 meter and you will naturally have to have it fully unwound.
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Posted By Charles Robinson Tech IOSH
When you coil a wire it becomes an inductance in effect acts like a transformer therefor when ever using an extension lead you should uncoil it before use look up inductance to for more technical input and calculations etc.
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Posted By Tabs
As far as my memory goes back though, it will only induce current, and therefore heat, in a ferrous material inside the diameter of the coil - not within the wire itself.
but my memory is struggling to remember 1978-1983 :-) (my eng study years)
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Posted By Gff
this is not the case.
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Posted By stevehaigh
WOW !!!!!!!!!!
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Posted By Garry Homer
Steady everyone.
The cable gets hot only because it has no cooling when it is all wrapped tightly. The same effect ocurs when cables are surrounded in insulation, ie, cables running through insulated walls and loft spaces. Similarly, bunched cables in conduits and cable-ways must be de-rated, even cables running side by side will effect the cable rating of each cable. The IET regs, BS7671, show cables may have to be de-rated to 50% of their otherwise normal rating. Cables running through hot zones are also de-rated for the same reason.
As previously said, without the conductors coming together and touching, there is no electrical fault and the protective devices will not operate. So there is nothing to stop a coiled extension cable bursting into flame if it is feeding a 13 amp load.
My 30m extension cable has two ratings, 4 amps coiled, 13 amps unwound, but this still relies upon me to think and act accordingly.
Best to keep extensions as short as possible as they are easy to damage and are generally a trip hazard.
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