Shared earth conductor

[blup]

Are you suggesting that multiple separate simultaneous L-CPC faults is 'foreseeable'? Don't forget that they really would have to be essentially 'simultaneous' in onset, since each L-CPC fault would be cleared very rapidly (<1 second) by it's circuit's OPD.

I suppose one can invoke "never say never", but you're not going to get much closer to 'never' than the probability of two separate L-CPC faults, on different circuits, arising within 1 second of one another

Certainly forseeable in an industrial context

Not really. However, I've just realised that what I previously wrote was not correct. If two or more simultaneous L-CPC faults were to arise on different circuits with a common CPC, then that would increase the PFC, since the L conductors of the two circuits would then effectively be in parallel. The argument for not needing a bigger CPC (than for a single circuit) therefore relies on the incredible improbability of two or more faults, on different circuits, arising within 1 second of one another.

POssible in theory

 

[morqthana]

The OP hasn’t given context

Sorry.

The context is 4 x 25A & 1 x 40A radials using, respectively 4mm & 6mm single core cables in surface mounted trunking. Probably 3x25A in one run of trunking and 1x25A & the 40A in another. I'd be looking to use a shared 4mm in each group.

 

[JohnW2]

Certainly forseeable in an industrial context

I just find it more than a little 'far-fetched' to envisage a situation in which an L-CPC fault of negligible impedance in one circuit would arise before a similar fault in a different circuit had been cleared by that circuit's OPD (or an RCD). Two such faults occurring in, say, the same month would probably be very unlikely (even in an industrial context), but within less than one second of one another ..... ?!!

POssible in theory

Very little is 'impossible', but .... !!

I think you must be one of those 'very risk-averse' folk

 

[JohnW2]

The context is 4 x 25A & 1 x 40A radials using, respectively 4mm & 6mm single core cables in surface mounted trunking. Probably 3x25A in one run of trunking and 1x25A & the 40A in another. I'd be looking to use a shared 4mm in each group.

Yes, tis question is only really going to arise in relation to wiring in singles, since otherwise one would presumably be using T+E, rendering the question moot.

Give or take the two minor caveats which I (and others) have mentioned, I think I would be fairly comfortable doing as you suggest - rather than running 3 or 4 G/Y singles side-by-side in the trunking.

 

[blup]

I just find it more than a little 'far-fetched' to envisage a situation in which an L-CPC fault of negligible impedance in one circuit would arise before a similar fault in a different circuit had been cleared by that circuit's OPD (or an RCD). Two such faults occurring in, say, the same month would probably be very unlikely (even in an industrial context), but within less than one second of one another ..... ?!!

Very little is 'impossible', but .... !!

I think you must be one of those 'very risk-averse' folk

Exploring possibilities isn’t risk averse, it actually identifies the daft

 

[plugwash]

I just find it more than a little 'far-fetched' to envisage a situation in which an L-CPC fault of negligible impedance in one circuit would arise before a similar fault in a different circuit had been cleared by that circuit's OPD (or an RCD). Two such faults occurring in, say, the same month would probably be very unlikely (even in an industrial context), but within less than one second of one another ..... ?!!

Two independent faults in two different circuits within a second of each other would indeed be extremely unlikely.

The harder question is how plausible is it for there to be two "live-cpc" faults on different circuits with a "common cause". I would think the only remotely plausible situations in which this would happen would be those in which some massively destructive event has happened. Even digging through a cable is not an instant process.,

And that "massively destructive event" may well be equally likely to cause faults other than live-CPC faults, for example a fault from one circuit's live to another circuits neutral.

 

[JohnW2]

Two independent faults in two different circuits within a second of each other would indeed be extremely unlikely.

Quite so. I would struggle to find sufficiently extreme superlatives to express 'how unlikely' I consider it.

The harder question is how plausible is it for there to be two "live-cpc" faults on different circuits with a "common cause". I would think the only remotely plausible situations in which this would happen would be those in which some massively destructive event has happened. Even digging through a cable is not an instant process., .... And that "massively destructive event" may well be equally likely to cause faults other than live-CPC faults, for example a fault from one circuit's live to another circuits neutral.

Yes, I almost mentioned that in an attempt to pre-empt the 'extremely risk-averse' amongst us. Once can, indeed, imagine catastrophic events, such as buildings exploding or being seriously on fire that might result in multiple simultaneous faults in an electrical installation - but in such a situation I feel sure that one would have much more important things than the adequacy of CPCs to worry about

 

[JohnW2]

Exploring possibilities isn’t risk averse,

... indeed it isn't, provided that, when appropriate, the exploration results in the conclusion that the risk in question is so low as to not warrant any concern or 'action'.

it actually identifies the daft

It would, indeed, be seemingly pretty daft if, in a situation such as we are discussing, the exploration resulted in a conclusion different from that mentioned above.

 

[morqthana]

I hereby promise not to use any digging apparatus of any sort, be it mechanically powered or hand operated, on the trunking containing these cables.

 

[JohnW2]

I hereby promise not to use any digging apparatus of any sort, be it mechanically powered or hand operated, on the trunking containing these cables.

Fair enough, but it seems that some of those around here would probably be concerned that your promise would not necessarily be transferred to future owners of the installation (or, indeed, other workers within the building whilst it is still yours)

 

[SUNRAY]

I have been questioned several times on jobs why I've not used individual earth wires on each circuit, in fact these sort of installations

are a typical example.
The grey enclosure is part of a commercially produced item and as can be seen the internal earth wiring is to the earth bolt. Frequently the instructions (spec) is for individual earth per circuit, indeed many installers would automatically do that anyway - but personally I see little or no advantage to link the earth bolt in this item to the earth bar in the dimmer pack or CU with 6 wires when one is significantly cheaper. This job we used 6mm² but TBH realistically it really doesn't need to be bigger than the live conductors, as has already been mentioned - that would need more than one simultaneous fault to have any effect.

 

[morqthana]

TBH realistically it really doesn't need to be bigger than the live conductors, as has already been mentioned - that would need more than one simultaneous fault to have any effect.

6mm twin&earth has a 2.5mm cpc

 

[SUNRAY]

6mm twin&earth has a 2.5mm cpc

Indeed

 

[ebee]

I do , sometimes, have a passing thought for cpc size reduction compared to L size for circuits with conductors over 1.0.
Will it be a possible/credible great benefit to effectively increase the cpc size by commoning some together to some extent or building in a little auto redundancy.
Usually the answer is not much so not really needed although no harm done either.
with the 1.0mm (say T & E for example) then sometimes connections can come adrift by peoples heavy handedness on modifying circuit parts so a termination can pull out or fracture. I have seen the effects of sloppy works where this has happend and ditto where someones DIY attempts in a building have caused some havocs with gas/electrics/water supplies etc.
In the main not really required but if you have an inkling that a "roughneck" might meddle in the future then maybe so, worth a little extra thought.

 

[JohnW2]

I do , sometimes, have a passing thought for cpc size reduction compared to L size for circuits with conductors over 1.0.

Provided there are no'downsides', it's never really fair to criticise someone who wants to exceed 'minimum requirements', even though the CPC sizes in T+E we have are generally adiabatically adequate and large enough enough to allow the required 'maximum Zs' (for ADS) to be achieved

However, as for 'downsides', I would have to do some sums to ascertain whether this could ever be an issue, but it's just possible that increasing CPC size could sometimes be somewhat of a 'two-edged sword' - whilst that would decrease Zs (to a level lower than needed for ADS), the consequential increase in PFC might possibly render the CPC adiabatically inadequate. Watch this space whilst I 'investigate'

Kind Regards, John