Overloaded cables, anyone seen any really bad ones?

[Adam_151]

Just trying to do a bit of research to back up a theory, but has anyone actually seen an overloaded cable nearly start a fire (as opposed to damaged cables or poor connections which often start fires).

I'm kind of expecting that people have seen a lot that are a lot worse for the wear from being overloaded, but actually very few (if any) that were actually in danger of bursting into flames.

 

[RF Lighting]

I can only think of one that I have seen in ten years of sparking. (And I've seen some awful stuff)

It was in my apprenticeship, We were doing a full domestic rewire and found a length of rubber cable (7/.029) which had been on fire for about 18" and had also charred the floorboard above it. It was in the middle of a run, with no joints, we could see no evidence of physical damage to the cable, and we can only presume it was caused by an O/C ring. (The circuit was half removed before we found it, so don't know for sure)

 

[Job_n_knock]

Ive seen it demonstrated many times but in reality at the sharp end, yes its rare" Why? Because in this country we have always had imho a very good training environment for Apprentices. Any installations carried out by untrained persons will need very load hungry devices to cause a fire. We've all seen and felt "warm" cables that are overloaded but that's also my point, we the "trained" tend to see them before years pass by and more add-ons are included in those circuits making matters worse.
Dont be fooled by statistics, PVC and even heat resistant PVC has a flashpoint. It emits enormous quantities of gas (poisonous chlorine) and smoke before it sets fire but it will if left.

 

[RF Lighting]

I was trying to create some for a photo tonight.

I stuck a 70A load onto a 2.5mm² and it got quite warm, but there was no sign of ignition after about 5 minutes. (at this point my power supply over heated)

 

[JohnD]

Maybe the first point of failure would usually be at a terminal. I've seen a few of them crusty and burnt.

 

[Job_n_knock]

Try doing it as a dead short with no protection and step back with a fire extinguisher

 

[RF Lighting]

It was a dead short with no protection.

I connected the 2.5mm² to the output side of a 50V step down transformer, the only protection was a thermal overload on the primary transformer windings.

 

[Job_n_knock]

Lets face it, in truth its impossible to create the perfect scenario in practice unless you jam a 6" nail across your cut-out. But it does happen in reality. The cut-out was the protection.
I know for a fact that in many towns and cities, there are many cut-outs that have nothing behind them...no protection at all ! the last line of defence is the cut-out itself. It goes all the way back to the transformer windings. In spite of our pride at our Industry, a lot of it is very outdated, I know this is getting away from the original thread but this I found funny. Remember the power cuts that were happening about 5 or 6 years ago? EDF brought in their French guys to help out. They arrived and at lunchtime they set out a table with tablecloth and started to produce Ham, cheeses and bits and pieces and also a bottle of white wine. The site supervisor blew his top at the consumption of alcohol so they packed up and went home again the very same day.

 

[Adam_151]

Try doing it as a dead short with no protection and step back with a fire extinguisher

Assuming your supply impedance is low enough then one ****ed cable, one blown BS1361 100A, but no fire

Anyway, I was on about overloads, not fault currents

 

[Adam_151]

Anyway, my thinking is as follows (with quite a bit borrowed from the commentry)

The heat generated in a cable is equal to P=I²r, thus the temperature goes up roughly in line with the square of the current (I'm assuming for the purposes of this, that rate at which heat can be got rid of remains the same, which probably isn't strictly correct), I'm also assuming as the regs do that a cable run at its tabulated rating for a length of time will be at 70degress (again not strictly true, the rateings allow for worst case purity of copper, worst way you could install it and still be within the bounts of the reference, for example method 3 gives the same rating for horzontal and vertical trunking...)
Anyway if you have a broken ring thats an overload of factor 32/27 = 1.185, if we square this and multiply it by the normal termperature rise of 40 (30 to 70) we get 1.40*40 = 56, add that to the 30 ambient and you get 86 degrees

Each rise of 8degrees above 70 halves the lifetime of PVC apparently, so we have cut the life of our cable down to a quater, the starting life is assumed to be 23 years carrying the normal 27A constantly 24/7 so our cable will carry 32A for over 5 years constantly before it starts to crack, etc, and as we don't generally load our cables fully and leave it that way 24/7, it should last a lot longer (for example, if the cable is not overloaded, but carries full load for 8 hours everyday, it'll last 69 years)

This all breaks down at around 120 degrees, when the cores start melting through the PVC and it goes bang anyway (which is still quite a way from the 451F/233C flashpoint even paper has)

Actually if you work backwards from 115 degrees, you see something slightly significant, 115-30 = is a rise of 85 degrees, 85/40 = 2.125 times the normal rise, square root of that gives you 1.457 overload factor...

So to summerise...small-ish overloads should just cause the lifetime of the cable to be reduced, large overloads should end up with a nasty smell, a loud bang and a blown fuse before it actually sets light to things in the majority of cases, I trying to see if this reflects what people generally find in the real world



*I haven't got round to buying it yet, but was reading this http://www.iee.org/Publish/WireRegs/Commentary-UpdateApr04.pdf

 

[scousespark]

Adam,

Pour yourself a scotch, settle down in front of your telly and relax. All this science is going to batter your head mate.

To get cable to just burst intoa flames you need a small cable with a large breker and the juice switched on for a long time. You'll not find very many instances of this happening. You will find burnt out connections though.

Have you got yourself out on the tools yet?

 

[securespark]

I was trying to create some for a photo tonight.

I stuck a 70A load onto a 2.5mm² and it got quite warm, but there was no sign of ignition after about 5 minutes. (at this point my power supply over heated)

Should call you Dr Emmett Brown.....

Never mind your tranny. Just get a 12kW shower & wire it up to a spare way with 1.0. Switch on & wait a few minutes...

 

[blondini]

Many years ago, the parents of a friend had a fire in their bathroom airing cupboard started by cable which was feeding their electric shower.
It was a DIY installation, I think the shower was being fed by two 2.5 T&E wired in parallell which probably became one 2.5 T&E due to a loose connection. Towels etc packed in the cupboard insulating the cable and whoomf!
My friend is now a safety officer .

 

[Duncan P]

does the fact that T+E having earth in the middle add to safety (with RCD), if anything melts the first thing it should touch would be earth. Is this a reason why you use T+E rather than flexible cable?

 

[TicklyT]

I remember an overloaded telecommunications cable about 30 years ago. The system design used 48 wire PVC cable, 0.5mm dia. conductors, so that's about a 0.2 sq. mm CSA for each conductor.

The system used a 6V AC power supply capable of supplying about 15 Amps using four bunched conductors each for 'phase' and 'neutral' supplying about 300 indicator lamps (the design dated back to the days before LEDs were common). There was about 150 Metres of cable in the system.

Someone had added another distribution box to the end of the system. Everything ran as it should for a while, except the indicator lamps weren't very bright. After a couple of hours there was a strange smell in the place, and smoke pouring out the cable butts where the outer sheath was stripped back.

The cable pairs had been crossed on the new box, short-circuiting the 6V AC supply.

There was a fuse on the supply - a 1 1/4" X 1/4" cartridge, known within the firm as a 'Fuse 36' rated at 7 Amps. When the original installation went in, the fitter had apparently mis-read the spec, and fitted a 36 Amp fuse

We had to re-cable the whole system. When we cut the old cable out, we could have cut it into 3 foot lengths and sold it as walking sticks!