Interesting.

[EFLImpudence]

Not sure what the point of the exercise was but the numbers are interesting.

 

[jj4091]

I remember a few years ago someone on here connected a length of 2.5 mm copper wire across his welding machine and ran it up to 80A without blowing it ( securespark I think but not sure ) I think he was demonstrating that the rated current carrying capacity of cables is underrated.
It's strange what happened there though and above my paygrade but in actual fact neither of them failed as such.

 

[JohnW2]

I remember a few years ago someone on here connected a length of 2.5 mm copper wire across his welding machine and ran it up to 80A without blowing it ( securespark I think but not sure ) I think he was demonstrating that the rated current carrying capacity of cables is underrated.

I think we all do (or should) accept that the "current carrying capacities" we work with are very conservative., They presumably represent the current which will, with the installation method used, raise the conductor temperature to its 'rated' level (usually 70°C or 90°C) - but I imagine that it would require much higher temps than that before the cable suffered any significant harm, let alone became a 'fire risk'.

 

[chivers67]

I guess the current takes the path of least resistance

 

[JohnW2]

I guess the current takes the path of least resistance

Perhaps a bit 'pedantic' - but that oft-written statement is clearly not true.

As I'm sure you know, if there are two or more 'paths', it is not the case that all of the current will 'take the path of least resistance'. Rather, the total current will be shared ('divided') across all the paths, inversely proportional to the resistance of each path, so that the path through which the greatest amount of current will flow will be the one 'of least resistance'.

 

[plugwash]

It's strange what happened there though

Not sure what you think was "strange"

The sharing of current is proportional to the cross-sectional area of the conductor but the surface area of a conductor is proportional to the sqaure root of the CSA. So with conductors in paralell the larger conductor will tend to overheat first.

and above my paygrade but in actual fact neither of them failed as such.

I would consider a wire smoking and the insulation falling off to be "failed".

I think we all do (or should) accept that the "current carrying capacities" we work with are very conservative., They presumably represent the current which will, with the installation method used, raise the conductor temperature to its 'rated' level (usually 70°C or 90°C) - but I imagine that it would require much higher temps than that before the cable suffered any significant harm, let alone became a 'fire risk'.

While it's probablly true that ratings of the cables we work with day to day are conservative, I think it's also true that this video presents what is very much a "best case scenario" and could lull people into a false sense of security.

1. The conductors in the video were in free air and were some distance from any other conductors.
2. There was no mechanical loading on the insulation except it's own weight. Thermoplastics don't go suddenly from solid to liquid, they go soft and gooey.

In the real world, we run multi-conductor cables where at least two conductors are carrying current at any given time. We support, restrain and even handle those cables via their insulation. So the insulation has to resist not only it's own weight, but the forces placed on it by the installation and use of the cable.

And of course we protect them with fuses and breakers that don't trip the instant the current goes over their rating. There needs to be enough margin in the conductor ratings that the protective device trips before the insulation loses it's integrity.

 

[JohnW2]

While it's probablly true that ratings of the cables we work with day to day are conservative, I think it's also true that this video presents what is very much a "best case scenario" and could lull people into a false sense of security.

Yes, that's quite possible, particularly given that it takes very little to 'lull some people unto a false sense of security', in a similar fashion that it can also take very little to unnecessarily 'frighten' some people.

.... And of course we protect them with fuses and breakers that don't trip the instant the current goes over their rating. There needs to be enough margin in the conductor ratings that the protective device trips before the insulation loses it's integrity.

Probably long before "the insulation loses it's integrity". From the characteristics of MCBs, we know that our cables are deemed to be 'safe' with a current up to 1.45 times their 'rated' current-carrying-capacity for about an hour, and I suspect that all that happens at 1 hour of 1.45In is that the conductor reaches its 'rated max temp' (which, if true, would be well below a temp that would do significant harm).