Conduit for earth & PIR

[Paul_C]

Unless it's some recent amendment. It's always been considered perfectly acceptable in the past.

 

[JohnW2]

Do you mean that you've got two circuits in the same conduit, sharing the same conduit as their cpcs?

Nothing wrong with that.

Or with using a single earth conductor for two circuits with a PVC conduit installation, so long as it is sized for the largest protective device.

Are you sure? I don't think that is correct.
Edit - Seems you're correct, but I would consider it extremely bad practice.

It's not really any different from having a sub-main with a single CPC ultimately serving a number of final circuits, is it?

Kind Regards, John.

 

[ColJack]

well yes it would..
that has a suitable sized cpc for the sub main and then radiates out to the smaller sub circuits with smaller cpc's..
the above scenario has a single sized cpc that can and does serve several points of several circuits along it's length..

 

[Paul_C]

Edit - Seems you're correct, but I would consider it extremely bad practice.

Why? Electrically speaking, how is it any different from using metallic conduit as the earth for two or more circuits?

 

[JohnW2]

well yes it would..
that has a suitable sized cpc for the sub main and then radiates out to the smaller sub circuits with smaller cpc's..
the above scenario has a single sized cpc that can and does serve several points of several circuits along it's length..

I must say that I still don't really see the conceptual difference, provided the 'shared CPC' has adequate CSA. Indeed, the sub-main (and its CPC) could theoretically serve more than one daisy-chained CUs.

Kind Regards, John.

 

[ColJack]

yes but that sub main CPC is sized for the size of the sub main and not several sub mains in one location..

so even though a 95mm sub main serves several points, the CPC is sized correctly for that 95mm..

you don't have several 95mm sub mains with a single CPC..

 

[holmslaw]

..

 

[JohnW2]

yes but that sub main CPC is sized for the size of the sub main and not several sub mains in one location..
so even though a 95mm sub main serves several points, the CPC is sized correctly for that 95mm..
you don't have several 95mm sub mains with a single CPC..

I still don't really follow. As I keep saying, I am talking about a situation in which the CPC is appropriately sized for what it's doing. I'm not suggesting that if a circuit requires an X mm² CPC, that one can use a single Xmm² CPC for two or more such circuits. In fact, I might suggest that would be OK, were it not for the fact that I know jolly well that someone would then wheel out the hypothetical (and incredibly unlikely) scenario of two separate circuits simultaneously and independently developing L-E faults! Having said all that, I don't imagine that anyone would consider have circuits sharing a CPC, unless it was conduit or perhaps (very rarely) the CPC of a multi-core, multi-circuit cable.

Kind Regards, John.

 

[Paul_C]

In the earth wire scenario I would disconnect LN&E, leaving whatever also used that cpc unearthed.

Why disconnect the earth? And wouldn't the presence of extra circuit conductors with only the single earth conductor coming out of the same conduit alert you to the fact that the earth is shared between circuits?

I don't think they meant a cable cpc, and if you did allow cable cpc sharing there should be an in your face warning label. But the BRB doesn't call for one.

Again, if you were using a multiway cable with a single, shared earth core, wouldn't the fact that there is only a single earth connection but multiple circuits tell you something?

 

[Paul_C]

I'm not suggesting that if a circuit requires an X mm² CPC, that one can use a single Xmm² CPC for two or more such circuits. In fact, I might suggest that would be OK,

That's been considered acceptable for a very long time, not just in the British Wiring Regs. but in other national codes too, and for precisely the reasons you've mentioned already - The chances of two independent and simultaneous faults on two or more circuits is extremely remote.

So if you have a 5A circuit and a 15A circuit running together, it's permissible to use a shared earth conductor so long as it's sized for the 15A circuit.

 

[JohnW2]

That's been considered acceptable for a very long time, not just in the British Wiring Regs. but in other national codes too, and for precisely the reasons you've mentioned already - The chances of two independent and simultaneous faults on two or more circuits is extremely remote.

Maybe - but, as I said, I wrote what I did because I knew jolly well that someone would probably invoke the 'two independaent and simultaneous faults' if I hadn't

So if you have a 5A circuit and a 15A circuit running together, it's permissible to use a shared earth conductor so long as it's sized for the 15A circuit.

Indeed - but, as I recently wrote, I don't think many people would contemplate doing that unless we were talking about conduit or a multicore cable - apart from anything else, if one isn't using singles in conduit or multi-core (and assuming one is talking about final, not distribution, circuits), there's not much left for fixed wiring other than T&E - so one might as well use the separate CPCs.

Kind Regards, John.

 

[holmslaw]

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