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Shared earth conductor
- Thread starter morqthana
- Start date
Only resience I can think of.
Example - conduit runs are often the only earth (theorectically quite a robust one) and ight have items on a run that has several distinct circuits.
You often have only one "Installation earthing conductor" for an entire installation , one point of failure but again usually quite robust.
You might come across for example a lighting circuit that has no earth at its origin for some reason so you "pinch" and earth from a socket circuit to earth it (ensuring it`s extremely unlikely to become inadvertently disconnected of course).
I tend to go the other way with lighting circuits - two or more lighting circuits with a 1,.0 cpc can be a credible point of failure, ensuring the two cpc are joined at least once somewhere on those circuits might help mitigate some of the problem of one earth disconnection (also brings down the R2 value therefore effective Ze of part of circuits too).
PS - just to add that with some circuits, data and/or HiFi etc it is sometimes desireable to have one earh or grounding/shielding to stop interference but for purely electrical circuits for fault disconnecting then I reckon what I said above should be OK, of course filtering might be deliberately dumped onto the E
Example - conduit runs are often the only earth (theorectically quite a robust one) and ight have items on a run that has several distinct circuits.
You often have only one "Installation earthing conductor" for an entire installation , one point of failure but again usually quite robust.
You might come across for example a lighting circuit that has no earth at its origin for some reason so you "pinch" and earth from a socket circuit to earth it (ensuring it`s extremely unlikely to become inadvertently disconnected of course).
I tend to go the other way with lighting circuits - two or more lighting circuits with a 1,.0 cpc can be a credible point of failure, ensuring the two cpc are joined at least once somewhere on those circuits might help mitigate some of the problem of one earth disconnection (also brings down the R2 value therefore effective Ze of part of circuits too).
PS - just to add that with some circuits, data and/or HiFi etc it is sometimes desireable to have one earh or grounding/shielding to stop interference but for purely electrical circuits for fault disconnecting then I reckon what I said above should be OK, of course filtering might be deliberately dumped onto the E
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Need more information on the context of the question to be able to comment
One scenario I could imagine: consumer unit relocation using a box with DIN rail connectors where the earth connectors are bonded to the DIN rail. In that case you could theoretically run singles from the new CU location to the old for all the L and N conductors but only one 10 mm2 CPC. Would probably be easier to run T&E though.
When I previously asked, it wasn't a problem.
Just need to ensure the cpc is thick enough to cope with the biggest cable/circuit
Just need to ensure the cpc is thick enough to cope with the biggest cable/circuit
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Sounds like bad practice to me as the circuit with the original earth could be removed....maybe.....
A metal conduit or trunking system being used as en earth would be ok as the containment would likely remain if some circuits were removed.
A metal conduit or trunking system being used as en earth would be ok as the containment would likely remain if some circuits were removed.
Electrically, I can think of no real reason why not, so long as there are no 'faults' in the CPC wiring, provided that the CPC is adequate in CSA (certainly 'adequate' for the most-demanding of the circuits it serves). However, having said that, there is always an argument against putting multiple eggs in one basket, since, in the situation you envisage, a single problem in the CPC wiring could impact upon multiple circuits.
This issue was discussed in the very first thread I started here some 14 years ago. I was talking about moving a CU by a small distance, requiring extension of the cables, and the discussion was largely about the best/neatest way of effecting those extensions. At one point I suggested that, with the 'extensions' effected by connectors in some sort of enclosure, it might be possible to 'simplify' things by running just one ('combined') CPC from that enclosure to the CU. That suggestion was essentially 'shot down', primarily on the basis that it would 'complicate' subsequent testing of the installation. I didn't think (and really still don't think) that was really much of a 'complication', but it was certainly a view against the practice expressed by a number of people.
In view of:
the answer to the question must be "No".
the answer to the question must be "No".
I'm not sure what you mean by that.
If you're thinking of the (almost unbelievably improbable) situation of multiple simultaneous separate L-CPC faults, I don't think that would alter PFC - the PFC (for just one fault) is determined on the assumption of an L-CPC fault of negligible (aka 'zero') impedance, so putting additional faults in parallel with that would make no difference, would it?
Edit: what I wrote is not correct since, in the scenario envisaged (of simultaneous L-CPC faults in different circuits with a common CPC), the L's of the two circuits would effectively be in parallel, increasing PFC. However, such a scenario remains "almost unbelievably improbable'
In other words, if there were several circuits each adequately served by, say, a 1.5mm² CPC, I would think that a single 1.5mm² (for all circuits) back to the MET would be adequate, wouldn't it?
.... not, I would think, "too big" in relation to the prevailing PFC. However, if the CSA was bigger (hence lower impedance) than the minimum required for ADS in the circuit, then the PFC would rise to higher than 'it needed to be' (ADS-wise), which might possibly result in the CPC being adiabatically inadequate at that PFC.
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Indeed. That reg seems to be essentially saying the same as I have been writing.
However, I would agree with others that whether or not it is a 'desirable practice' is perhaps more questionable since (a) putting multiple eggs in one basket is rarely a brilliant idea, and (b) although we should perhaps not have to consider it, subsequent 'unthinking' changes to the installation could undermine the situation.
The Blackwall tunnel was built to cater for the foreseeable traffic too
A sort of earthing equivalent of diversity you mean? I suppose so. It could be complicated if one earth was shared, though I imagine it’s common in industrial settings. The OP hasn’t given context
Glad you agree.
I'm not so sure about that. The "question" was not about the regs but, rather, was asking "whether there was any reason" why CPCs should not be shared between circuits - and the possible downsides I mentioned could be regarded as "any reasons", even in the absence of a regulatory requirement.
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
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.
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