Separate lives and neutrals through conduit

[Harry Bloomfield]

Well for when the cpc gets used for something else

Eh? What 'else' might you have in mind?

 

[AndyPRK]

Plumbing controls

it’s probably for robustness

 

[JohnW2]

Plumbing controls

... and timer fans - but put on a flak jacket before you mention it, even though it is 'regs compliant' (if appropriately sleeved at terminations)

Kind Regards, John

 

[EFLImpudence]

Yep, that too.
Are you asking whether a 1.5mm² CPC is always adequate for a circuit whose live conductors were 2.5mm² (or 4mm²)?

I am.

If so, then that's an interesting question, which I doubt that any of us ever really consider - if the CSA of the live conductors is adequate for the design load, then we simply 'assume' that the CPC that comes in T+E of appropriate size will always be adequate.

We do assume that.

IF it is adequate then that must mean that the CPCs in other cables [and if using the same size singles CPC (as live conductors) because we do (not sure if that is just habit or normal practice to avoid calculation)] is always over-sized.

I don't know whether it ever happens in practice but I suppose that, in theory, the CPC of T+E is not always (adiabatically) adequate. The required CSA depends upon I²t and, as far as I am aware, I²t continues to rise ('indefinitely') as PFC rises. Since there is 'almost no limit' to how high PFC can be (one sometimes sees figures of 'thousands of amps mentioned), I presume that it is 'not impossible' (with a very short circuit in an installation next to a substation) that it could be so high that the CSA of the standard CPC was inadequate. However, I cannot recall having even seen/heard anyone even considering that possibility.

Quite.

That's easy, but you're perhaps asking the question the wrong way around. The point is that, in adiabatic terms, the live conductors are 'oversized' - but, for those live conductors, there is also the consideration of the ('continuous') current-carrying-capacity (the antithesis of 'adiabatic'). In other words, considering 2.5/1.5mm² T+E, 1.5mm² would be adiabatically adequate for the live conductors, but the cable would then only have the CCC of 1.5mm² cable, not that of "2.5mm²" cable.

Not sure I follow that.
Why would the live conductors' CCC be lower?

Plus, the PSCC would be higher than the PEFC.

You mean flex?

Yes.

If so, that's another 'good question' - since theory would indicate that the same 'conventional relationship' between CPC and live conductor CSAs would be just the same as in T+E.

Yes. As above, they must always be larger than actually necessary.

 

[EFLImpudence]

Does anyone know why when T&E was invented it was decided to have a smaller CPC?

Then was it decided that 2.5/1.0 was wrong and increased to 2.5/1.5? It does not seem likely as 4.0/1.5 continued.

 

[winston1]

Goes back a long way. 7/029, 3/029CPC.

 

[JohnW2]

I am. ... We do assume that. .... IF it is adequate then that must mean that the CPCs in other cables [and if using the same size singles CPC (as live conductors) because we do (not sure if that is just habit or normal practice to avoid calculation)] is always over-sized.

Indeed - but, as I said, at least in theory (I don't know about 'in practice') the CPC of T+E is not always going to be adequate.

Not sure I follow that. 1

"CCC" and the maximum current a conductor can safely cope with under adiabatic conditions are two very different things. "CCC" (Iz - or, in fact, at least 1.13 x 1z) is the maximum current that a a cable is deemed to be able to carry safely for a continuous/indefinite period of time - and will inevitably be less than the current that it is deemed to be able to 'safely' carry under adiabatic conditions (i.e. conventionally for no more than 5 seconds - and for no more than 0.1 seconds if one goes by 434.5.2).

Plus, the PSCC would be higher than the PEFC.

That might sometimes be a small consideration - but with a short final circuit in a TN-C-S installation there would be very little difference between the two.

Yes. As above, they must always be larger than actually necessary.

Indeed. If the CSA of CPCs in T+E is deemed to be 'adequate', then those in all flexible cables >1.0mm² must be appreciably 'more than adequate' (aka 'over-sized').

Kind Regards, John

 

[SUNRAY]

You mean flex? If so, that's another 'good question' - since theory would indicate that the same 'conventional relationship' between CPC and live conductor CSAs would be just the same as in T+E.

Kind Regards, John

I have a drum of flex here, for a long time, which does indeed have a smaller CPC.

 

[JohnW2]

I have a drum of flex here, for a long time, which does indeed have a smaller CPC.

Interesting. In some senses 'consistent' (with T+E), but I must say that I have never seen, nor even heard of, such an animal.

Does the cable in question have harmonised colours?

Kind Regards, John

 

[SUNRAY]

Interesting. In some senses 'consistent' (with T+E), but I must say that I have never seen, nor even heard of, such an animal.

Does the cable in question have harmonised colours?

Kind Regards, John

I purchased since 1974 as I remember the car I had when I got them. Originally a dozen reels of which I fitted plug & socket (XLR) on one, the rest have been used for installation work. I have every expectation it is harmonised but over 40 years I hay be wrong.

 

[amatureleccy]

No green/yellow?

Yes indeed - I also purchased the green and yellow.

 

[amatureleccy]

This converstation has gotten very technincal. I ahve another question but will ask it in a separate thread.

Thanks all