How much current does is take to melt a cable?

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.

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