very hot 13 amp plugs

[JohnW2]

do the conductors of 2.5mm T+E 'clipped direct' really reach a temp of 70 °C with 27A flowing?

They can't do, can they?

I certainly wouldn't have thought so.

The 27A would have to be the absolute maximum CCC but, as we have been discussing, a significant safety margin is built in.

Indeed, and that's where these margins start causing confusion. At first sight, one could be excused for believing that "conductor operating temperature 70 °C" at the top of the CCC tables in the regs meant what it says, but it presumable doesn't. I guess it probably means that the temperature would rise to 70 °C if all of the margins ever 'got used up' by the prevailing situation, and that the temperature would get nowhere near that high at 27A. Hence, maybe they meant (and therefore should have written) "maximum permissible conductor operating temperature 70 °C". However, in that other discussion to which westie referred (the one about a 300A supply), people seemed to be talking as if the 70 °C (or 90 °C) temperatures would actually be seen with a current corresponding to the tabulated 'CCC'.

Kind Regards, John

 

[ricicle]

As far as I am aware a 2.5mm² cable, clipped direct, running constantly at 27A in an ambient temperature of 30c will eventually reach 70c. Obviously the surrounding temperature and load cycling will affect this.
There was a table somewhere (can't remember where) that gave the approximate lifespan of pvc cable for various temperature/time usage.

 

[RF Lighting]

Remind me at the weekend and I'll load up some cable as per the maximum rating and installation method in BS7671 and see what happens...

 

[JohnW2]

As far as I am aware a 2.5mm² cable, clipped direct, running constantly at 27A in an ambient temperature of 30c will eventually reach 70c. Obviously the surrounding temperature and load cycling will affect this.

I very much doubt it, because of the 'margins' to which EFLI has referred. We know that these are such that cables must be able to safely carry a lot more than their tabulated CCCs for appreciable periods of time, which would result in temps considerably in excess of 70 °C if that temp were reached at the tabulated CCC. At the very least, that 2.5mm² cable might have to carry over 39A for an hour whilst wait for a 27A Type B MCB (if one existed!) to operate. That current of 1.45 times the tabulated CCC corresponds to a heat production of about 2.1 times that at 27A.

Kind Regards, John

 

[JohnW2]

Remind me at the weekend and I'll load up some cable as per the maximum rating and installation method in BS7671 and see what happens...

Excellent idea. Thanks. ....'baited breath' as always!

Kind Regards, John

 

[EFLImpudence]

As far as I am aware a 2.5mm² cable, clipped direct, running constantly at 27A in an ambient temperature of 30c will eventually reach 70c. Obviously the surrounding temperature and load cycling will affect this.

Notwithstanding what has already been said, that could only happen if the - presumably 25A - MCB were overloaded.

There was a table somewhere (can't remember where) that gave the approximate lifespan of pvc cable for various temperature/time usage.

The table may be correct but it would not apply unless the conductors actually attained that temperature.



To RF - Likewise.

 

[JohnW2]

As far as I am aware a 2.5mm² cable, clipped direct, running constantly at 27A in an ambient temperature of 30c will eventually reach 70c. Obviously the surrounding temperature and load cycling will affect this.

Notwithstanding what has already been said, that could only happen if the - presumably 25A - MCB were overloaded.

As said,for the purpose of this discussion I think we should assume a hypothetical 27A MCB (hence not overloaded) - but, for the reasons we've discussed, I find it very hard to believe that the conductors would ever reach anything like 70 °C at 27A.

Kind Regards, John

 

[DetlefSchmitz]

BS1363 includes a temperature rise test for plugs/sockets. but I forget what the acceptable temperature rise (at 20A through a double socket) is. I'm sure someone will be able to remind us.

At a quick glance it looks like 52° K rise on pins and accessible surfaces. However I seem to remember that this is inside a box after 8 hours so probably not representative of real life.

 

[ricicle]

I think you will find that what I have posted is correct. Any rise in current (and hence temp.) above that is inversely proportional to the time allowed for that rise in temp. to occur. That is why we look at K values when designing non-standard circuits.
I think the maximum temp. for pvc without causing degradation is around 140c (off the top of my head) I'm not quite sad enough yet to sit here with my bible !

 

[EFLImpudence]

Nobody is disputing what you say ...

but if the conductors do not actually reach the supposed temperature because of safety margins then the table will not apply.

 

[JohnW2]

I think you will find that what I have posted is correct. Any rise in current (and hence temp.) above that is inversely proportional to the time allowed for that rise in temp. to occur.

I'm not too sure what you're trying to say there. With any constant current flowing, the temperature will gradually rise, progressively more slowly as time progresses, until thermal equilibrium is achieved (which I would have thought would be achieved quite quickly with PVC T+E cables) after which there will be no further rise.

That is why we look at K values when designing non-standard circuits.

I'm not too sure what you're saying here, either. Are you thinking of the 'k' factor used in the infamous 'adiabatic equation'? If so, what we're talking about here is the antithesis of an adiabatic process.

I think the maximum temp. for pvc without causing degradation is around 140c (off the top of my head) I'm not quite sad enough yet to sit here with my bible !

That sounds like a credible figure, but we're not talking just about immediate degradation. As has already been mentioned in this thread, 'life expectancy' of PVC cable is related to operating temperature, over a wide range of temperatures (not just 'immediately damaging' ones).

Whatever, as far as I am concerned, I think this is all probably a bit moot, since I do find it hard to believe that a 2.5mm² PVC cable carrying 27A will ever get anywhere near 70 °C, let alone higher.

Kind Regards, John