How much current does is take to melt a cable?

The earth core was carrying a large current. That sounds bad. (Copper melts at over 1,000 degrees Celsius, and brass at about 930 degrees Celsius)

Technical Data Conductor : Bare copper, fine wire strand in accordance to IEC 60 228 that is VDE 0295, class 5 Core insulation : PVC compound TI1 in accordance to VDE 0207 Identification : In acc. to VDE 0293, black cores with white numbers with or without green / yellow ground conductor Inner sheath : PVC compound TM2 in acc. to VDE 0207 Braid : Galvanised steel wire braid Outer sheath : Transparent PVC compound TM2 in acc. to VDE 0207 Nominal voltage : 300 / 500 V Test voltage : 3,000 V ac Temperature range : For flexible use -5 up to +70°C maximum conductor temperature Fixed installation -40 up to +80°C maximum conductor temperature Minimum bending radius : Fixed installation 12.5 × cable diameter Flame retardant : In acc. to IEC 60 332.1 resp. VDE 0472 part 804 EC directive : This cable confirms to ECD 73 / 23 / EEC (low voltage directive)

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Z.

Well if you run a cable in a way that the heat cannot escape - coiled or lagged, then the rating is reduced, if the load is on for a long time - but long is many tens of minutes to hours depending on the mass to be heated.

Melting the plastic means it has got well above 100C, as PVC will stand that all day, it just tends to stretch and tear if flexed or compressed while that hot and if you do it a lot  it loses its flexibility. In contrast normal ratings assume the copper is o hotter than 70C. That gives a predicted cable life of many decades.

Now to melt the copper smartly  is more  difficult than the plastic and needs hundreds of amps for a a single strand of 2.5mm

So was the coiled section of cable the bit that melted, or was that OK ? If that is OK we can rule out overload as the issue.

If a short section near a termination is toasted, then that tells us that termination was not connecting to all the strands well, maybe some missed the screw, or the terminal was dirty or oily.

A burnt earth suggests either it is badly wired and that is not the earth or there has been a serious fault.Even so, if only the end is damaged, there was a termination issue.

Or perhaps a whisker of stray copper bridging to another core, now blasted away, hiding the evidence.

If you can post a pic or two we can probably have a more informed best guess.

Mike.

The earth core should not carry significant current under normal conditions and should therefore not become heat damaged. During a live to earth fault, the earth core and the live core should carry the same current and be equally liable to damage.

The only likely scenario under which ONLY the earth core was damaged would be fault current from some external source such as a lightning strike, or contact with DNO equipment, or some other source of substantial current.

Or possibly some gross misuse such as improvised use on single phase with the earth conductor misused as the neutral, AND carrying the total neutral current from all three live conductors, perhaps 180 amps.

Or some other very improbable misuse.

If a cable designed for 61.5 A was loaded to 75 amps, I would expect it to get hot and have a reduced life, but prompt failure would be unlikely.

Coiling the cable is liable to result in overheating, at rated current, but would not explain ONLY the earth core being damaged.

As the cable and the connectors have obviously some form of abuse or misuse I would recommend replacement and keeping an eye on the replacement for any repeat.

"Or possibly some gross misuse such as improvised use on single phase with the earth conductor misused as the neutral, AND carrying the total neutral current from all three live conductors, perhaps 180 amps.".

Hah I automatically assumed 3 phase so the N current would be less than any of the Ph`s unless harmonics at play. I never considered single phase and doubling up

I assume a BS4343 type connector. The Earth pin would not be burnt (damaged) by a high Earth current under any normal circumstances. Damage is usually caused by plugging or unplugging under load. You have incorrectly applied the derating factor, only 3 cores will carry maximum current if the cable is 3 phase, the Earth being zero, and the neutral only the residual from the 3 phases caused by phase imbalance.

Now the cable overheat problem, the question is what was the load? As the Earth conductor is damaged I suggest that the problem was caused by a welder or similar load, being connected across the cable during use, perhaps welding structural steelwork connected firmly to Earth, although welders usually have completely isolated from Earth outputs, but perhaps some idiot has tried to connect the output work terminal to the welder Earth? The current in the Earth could easily be several hundred amps if this or some similar accident has happened, which would cause damage.

A good point (welders, unsuitable connection of leads, and resulting damaged earth conductors)!  Articles like IAEI and the alarming damage-pictures in EC&M show the risk of giving users the idea that the 'earth' clip should just connect to something apparently earthed.  In the US they've changed its name from ground to workpiece to try to encourage sensible use.  

A welder would be my thoughts too.

I've seen first-hand the mess that was made when the 1.0mm2 earth core of an inspection lamp inadvertently became the current return path for a welder! 

It would seem that securely attaching the welders' own return cable (sometimes, incorrectly called 'earth') to the work piece is not regarded as essential by some operatives........

Stray return current was my initial thought as it's mostly a welding work area .. HOWEVER a witness has stated there was no welding going on at the time

I agree that return current from welding is likely as we now know that it is a welding area.

As regards the witness stating that no welding was taking place at the time,

They might be lying especially if they caused the mishap, or they might be genuinely mistaken, or the damage might have been pre-existing and only noticed when it went bang, or was detected upon inspection.

Many years ago, a foreman at a firm of contractors that I worked for, said that a bare stranded earth wire completely vaporised on a wall when a welder was wrongly connected to it.

Z.