Black cable for 110v site extension leads?

[EFLImpudence]

Your subsequent post:

You cannot or do not 'bond' with CPCs.

If not connected to an 'earthing system' then the connecting conductors would be just 'bonding' - to equalise any potential.

If connected to an earthing system to operate OPDs or RCDs then they would indeed be CPCs.
That this earthing system is at a different potential from true earth is, surely, irrelevant.
It cannot be bonded or connected to true earth as that would negate the point of the separated system.

 

[JohnW2]

Isn't that just the difference between true earth, the supply earth and CPCs which 'we' call earth but none of which is bonding?

Maybe. As I implied, I think it comes down to what one calls 'earthing'. If an exposed-c-p of a piece of Class I equipment were connected via a CPC to the N of a local tranny, or the centre-tap of a local 55-0-55 tranny, would you call that 'earthing', even if the output of the genny/tranny were 'floating', with no connection to true earth?

Whatever, as I said, if one connected a conductor between the exposed-c-ps of two pieces of equipment (to equalise potentials, hence reduce/minimise shock risk, in the event of Risteard's/erics 'improbable pair of faults'), but did not connect them to genny/tranny, I don't think one call that anything other than "equipotential bonding" (NOT "earthed equipotential bonding", as in EEBADS), could one?

Kind Regards, John

 

[JohnW2]

Your subsequent post: ... You cannot or do not 'bond' with CPCs. ... If not connected to an 'earthing system' then the connecting conductors would be just 'bonding' - to equalise any potential.

One might think so but ....

If connected to an earthing system to operate OPDs or RCDs then they would indeed be CPCs.

As I wrote in the last paragraph of my post to which you are responding, even if the bonding conductor were not connected to the 'earthing' system, it would still cause an OPD to operate in the even of a Risteard/eric pair of faults (one exposed-c-p having a fault to one live conductor, the other exposed-c-p having a fault to the other live conductor) - so the conductor in question would seemingly be performing the functions of both a bonding conductor and a CPC. I really don't know what I'd call it!

That this earthing system is at a different potential from true earth is, surely, irrelevant.

As I've said, it seems that some people are talking about 'earthing' in that sense - in the absence of any true earth connection. When Risteard, eric or the IET talk about the need for 'earthing' when two Class I items are fed from, say, a local generator, they are perhaps just talking about CPCs connecting to a local, isolated, 'earthing system', which is not connected to true earth. However, that is usually said in the same breath as 'earth rod', so I'm a bit confused.

You say that it is irrelevant whether the 'earthing system' is at true earth potential, but I think there is an important 'relevance'. If the local earthing system has no connection to (i.e. has a very high impedance to) true earth, then simultaneous contact with one of the supply's live conductors and true earth (e.g. a wet building site) is not dangerous. If the local earthing system is connected to true earth, then such simultaneous contact is potentially dangerous (although it can be mitigated by an RCD).

It cannot be bonded or connected to true earth as that would negate the point of the separated system.

You're talking about an 'isolated supply' (floating w.r.t. true earth), which is what I was also mentioning in the last paragraph. However, I thought we were talking about the pros/cons of having a connection to true earth, aren't we?

Kind Regards, John

 

[EFLImpudence]

I had better rephrase.
Whilst you can use a CPC as a bond, just bonding two parts (with no earthing) would not make the conductor a CPC.

Isn't that just the difference between true earth, the supply earth and CPCs which 'we' call earth but none of which is bonding?

Maybe. As I implied, I think it comes down to what one calls 'earthing'. If an exposed-c-p of a piece of Class I equipment were connected via a CPC to the N of a local tranny, or the centre-tap of a local 55-0-55 tranny, would you call that 'earthing', even if the output of the genny/tranny were 'floating', with no connection to true earth?

If it were to operate an OPD or RCD, it would be earthing.

Whatever, as I said, if one connected a conductor between the exposed-c-ps of two pieces of equipment (to equalise potentials, hence reduce/minimise shock risk, in the event of Risteard's/erics 'improbable pair of faults'), but did not connect them to genny/tranny, I don't think one call that anything other than "equipotential bonding" (NOT "earthed equipotential bonding", as in EEBADS), could one?

Agreed.

 

[EFLImpudence]

As I wrote in the last paragraph of my post to which you are responding, even if the bonding conductor were not connected to the 'earthing' system, it would still cause an OPD to operate in the even of a Risteard/eric pair of faults (one exposed-c-p having a fault to one live conductor, the other exposed-c-p having a fault to the other live conductor) - so the conductor in question would seemingly be performing the functions of both a bonding conductor and a CPC. I really don't know what I'd call it!

Well, it would be a CPC if that were its purpose.
It may well bond satisfactorily as well.

You say that it is irrelevant whether the 'earthing system' is at true earth potential, but I think there is an important 'relevance'. If the local earthing system has no connection to (i.e. has a very high impedance to) true earth, then simultaneous contact with one of the supply's live conductors and true earth (e.g. a wet building site) is not dangerous. If the local earthing system is connected to true earth, then such simultaneous contact is potentially dangerous (although it can be mitigated by an RCD).

Then, surely, the former would be preferred.

You're talking about an 'isolated supply' (floating w.r.t. true earth), which is what I was also mentioning in the last paragraph. However, I thought we were talking about the pros/cons of having a connection to true earth, aren't we?

Oh, I must have got confused.

Doesn't that outweigh the advantages of the transformer for little gain to cover a very unlikely occurrence?

 

[JohnW2]

I had better rephrase. Whilst you can use a CPC as a bond, just bonding two parts (with no earthing) would not make the conductor a CPC.

Agreed, but ....

If it were to operate an OPD or RCD, it would be earthing.

Do you mean "if it were put there in order to operate...."? If so, we're back to the usual problem of deciding 'why it was put there'. I was postulating that it is put their primarily to minimise the pd between the two exposed-c-ps (under incredibly unlikely fault conditions!) (i.e. bonding), but that a 'side-effect' of that is that it would also result in an OPD operating under such fault conditions (i.e. 'earthing'). Under such circumstances, it's hard to 'pick one', and, IMO, much easier to simply say that the conductor (whatever one calls it) is there both for bonding and 'earthing' ... but that approach upsets many people

Kind Regards, John

 

[JohnW2]

Well, it would be a CPC if that were its purpose. ... It may well bond satisfactorily as well.

Please see my previous post.

Then, surely, the former [totally floating, non-earth-referenced, output of tranny or genny] would be preferred.

I would have thought so.

Oh, I must have got confused. Doesn't that outweigh the advantages of the transformer for little gain to cover a very unlikely occurrence?

Again, I would have thought so - so maybe it's me that has been confused, in that when I've seen people talking/writing about 'earthing' one side (or CT) of the output of a generator or transformer, they have merely been talking about connecting it to CPCs (which I would agree with), rather than connecting it to true earth.

Kind Regards, John

 

[EFLImpudence]

Do you mean "if it were put there in order to operate...."?

I suppose so.

If so, we're back to the usual problem of deciding 'why it was put there'. I was postulating that it is put their primarily to minimise the pd between the two exposed-c-ps (under incredibly unlikely fault conditions!) (i.e. bonding), but that a 'side-effect' of that is that it would also result in an OPD operating under such fault conditions (i.e. 'earthing'). Under such circumstances, it's hard to 'pick one', and, IMO, much easier to simply say that the conductor (whatever one calls it) is there both for bonding and 'earthing' ...

But if you had OPDs then presumably you would require CPCs.

It doesn't apply to normal installations so:

if it was applied as a CPC then it's earthing but
if it was applied as a bond then it's bonding.

I can't see any other answer but if in this example it unavoidably serves both purposes then there is no way round it.

but that approach upsets many people

Surely only if related to a normal installation where they are separate.

 

[JohnW2]

But if you had OPDs then presumably you would require CPCs.

I think that we're back to that question of 'the reason' again - and, as you go on to imply, the situation we're discussing in complicated/unfamiliar, since it cannot arise within a 'normal installation'. However, in the 'non-normal installation' we're talking about, one might install an OPD purely for overload protection. Indeed, to turn your statement on its head, if one doesn't have a CPC, then the OPD can only provide overload protection - but, in this 'abnormal' situation' I suppose that could be the designer's intention.

It doesn't apply to normal installations so:
if it was applied as a CPC then it's earthing but
if it was applied as a bond then it's bonding.
I can't see any other answer but if in this example it unavoidably serves both purposes then there is no way round it.

That's how I see it.

but that approach upsets many people

Surely only if related to a normal installation where they are separate.

I know this is well-trodden ground, but are the two situations really all that different? In the case of the 'non-normal installation' you seem (above) to be accepting that if one one installs a conductor primarily to reduce/minimise pd between two parts (i.e. 'equipotential bonding') under certain fault conditions but it also 'unavoidably' facilitates operation of an OPD (i.e. 'earthing'), then it is reasonable to say that the conductor is serving both bonding and earthing functions - is that right? If so, in the case of a 'normal installation', when one installs a conductor primarily to facilitate operation of an OPD under L-E fault conditions, if it is the case (as it surely is?) that it also 'unavoidably' has the effect of reducing/minimising pds between exposed-c-ps and earthed (or 'earth-bonded'!) parts, why is it, in this case, not also reasonable to say that it is serving both functions (which, as I said, seems to upset many people)?

Kind Regards, John

 

[EFLImpudence]

I know this is well-trodden ground, but are the two situations really all that different? In the case of the 'non-normal installation' you seem (above) to be accepting that if one one installs a conductor primarily to reduce/minimise pd between two parts (i.e. 'equipotential bonding') under certain fault conditions but it also 'unavoidably' facilitates operation of an OPD (i.e. 'earthing'), then it is reasonable to say that the conductor is serving both bonding and earthing functions - is that right?

Yes, but that conductor (as in a normal installation) may run directly from one part to the other.
To be a CPC it would have to be its own earthing terminal (as it were).
Unless (in normal installation) the MET were at the location of, say, a light then there is no way a bond can be a CPC in its own right.

If so, in the case of a 'normal installation', when one installs a conductor primarily to facilitate operation of an OPD under L-E fault conditions, if it is the case (as it surely is?) that it also 'unavoidably' has the effect of reducing/minimising pds between exposed-c-ps and earthed (or 'earth-bonded'!) parts, why is it, in this case, not also reasonable to say that it is serving both functions (which, as I said, seems to upset many people)?

No, I think you are either being unfair or forgetting.

As just said, a CPC may be used as a bond.
Don't I remember some consternation recently during supplementary bonding discussions by insisting that if the CPCs impedance is such then SB is satisfied and therefore not required?

 

[JohnW2]

Yes, but that conductor (as in a normal installation) may run directly from one part to the other. ... To be a CPC it would have to be its own earthing terminal (as it were). ... Unless (in normal installation) the MET were at the location of, say, a light then there is no way a bond can be a CPC in its own right.

Are you saying that to 'be a CPC' (by which I take it you mean "to be a conductor whose purpose is to provide 'earthing' ") it has to travel 'directly' to the MET? I thought you were defining 'earthing' as simply the installation of conductor(s) which would bring about the operation of a protective device in the case of a fault (without any qualifications about the routing of the conductor{s})?

No, I think you are either being unfair or forgetting. ... As just said, a CPC may be used as a bond. ... Don't I remember some consternation recently during supplementary bonding discussions by insisting that if the CPCs impedance is such then SB is satisfied and therefore not required?

Is that not conceptually rather different? As you say, if the CPC provides a low enough impedance, then supplementary bonding is not required. That's not quite the same as saying that the CPC is providing supplementary bonding, is it?

Kind Regards, John

 

[EFLImpudence]

Are you saying that to 'be a CPC' (by which I take it you mean "to be a conductor whose purpose is to provide 'earthing' ") it has to travel 'directly' to the MET?

On a normal installation it does.
Connecting every exposed part to its neutral terminal is not allowed.
Would you not require a common point for 'earthing' a transformer supplied system?

I thought you were defining 'earthing' as simply the installation of conductor(s) which would bring about the operation of a protective device in the case of a fault (without any qualifications about the routing of the conductor{s})?

That may be the definition but surely the system would be as similar to 'normal' as posible.
I.e. a circuit protective conductor would travel with the circuit conductors and therefore 'back' to a central point.
A merely bonding conductor need not.

Is that not conceptually rather different? As you say, if the CPC provides a low enough impedance, then supplementary bonding is not required. That's not quite the same as saying that the CPC is providing supplementary bonding, is it?

Well, semantically, no, perhaps.
Can you provide that which is not required?

However, without the CPC, or were it longer, SB would be required, so ...

 

[JohnW2]

Are you saying that to 'be a CPC' (by which I take it you mean "to be a conductor whose purpose is to provide 'earthing' ") it has to travel 'directly' to the MET?

On a normal installation it does. ... Connecting every exposed part to its neutral terminal is not allowed.

No argument with that ...

Would you not require a common point for 'earthing' a transformer supplied system?

Yes, IF the 'transformer-supplied system' had an 'earthing system', then you would (probably the centre-tap terminal of the transformer), but ....

I thought you were defining 'earthing' as simply the installation of conductor(s) which would bring about the operation of a protective device in the case of a fault (without any qualifications about the routing of the conductor{s})?

That may be the definition but surely the system would be as similar to 'normal' as posible. ... I.e. a circuit protective conductor would travel with the circuit conductors and therefore 'back' to a central point. ... A merely bonding conductor need not.

I think you may have missed my point - perhaps I was not clear enough. As I explained, in a 'transformer-supplied system' which did not have an earthing system, bonding conductor(s) which simply connected the exposed-c-ps of two connected pieces of equipment (presumably, in practice, the G/Y cores in the cables supply each, joined somewhere in the vicinity of the transformer outputs), but were not connected to anything else (i.e not to the transformer) would (despite the absence of an 'earthing system') inevitably have the effect of causing an OPD to operating in the event of the incredibly unlikely pair of faults which has been postulated (since the two live sides of the supply would be connected together, via the two faults and the 'bonding conductor'. I was therefore suggesting (since people are inclined to be pedantic in this area!) that these seemingly obviously 'bonding conductors' actually fulfilled the functional definition of 'earthing' that you have agreed to above. If the conductors are achieving 'earthing' (per that definition), should they be called 'bonding', 'earthing' (hence 'CPC') or both?

Kind Regards, John

 

[JohnW2]

Are you saying that to 'be a CPC' (by which I take it you mean "to be a conductor whose purpose is to provide 'earthing' ") it has to travel 'directly' to the MET?

On a normal installation it does. ... Connecting every exposed part to its neutral terminal is not allowed.

No argument with that ...

Would you not require a common point for 'earthing' a transformer supplied system?

Yes, IF the 'transformer-supplied system' had an 'earthing system', then you would (probably the centre-tap terminal of the transformer), but ....

I thought you were defining 'earthing' as simply the installation of conductor(s) which would bring about the operation of a protective device in the case of a fault (without any qualifications about the routing of the conductor{s})?

That may be the definition but surely the system would be as similar to 'normal' as posible. ... I.e. a circuit protective conductor would travel with the circuit conductors and therefore 'back' to a central point. ... A merely bonding conductor need not.

I think you may have missed my point - perhaps I was not clear enough. As I explained, in a 'transformer-supplied system' which did not have an earthing system, bonding conductor(s) which simply connected the exposed-c-ps of two connected pieces of equipment (presumably, in practice, the G/Y cores in the cables supply each, joined somewhere in the vicinity of the transformer outputs), but were not connected to anything else (i.e not to the transformer) would (despite the absence of an 'earthing system') inevitably have the effect of causing an OPD to operating in the event of the incredibly unlikely pair of faults which has been postulated (since the two live sides of the supply would be connected together, via the two faults and the 'bonding conductor'. I was therefore suggesting (since people are inclined to be pedantic in this area!) that these seemingly obviously 'bonding conductors' actually fulfilled the functional definition of 'earthing' that you have agreed to above. If the conductors are achieving 'earthing' (per that definition), should they be called 'bonding', 'earthing' (hence 'CPC') or both?

Kind Regards, John

 

[EFLImpudence]

I think you may have missed my point

I don't think so.

As I explained, in a 'transformer-supplied system' which did not have an earthing system, bonding conductor(s) which simply connected the exposed-c-ps of two connected pieces of equipment (presumably, in practice, the G/Y cores in the cables supply each, joined somewhere in the vicinity of the transformer outputs),

Would not that be, in effect, the MET especially if there were more than two?
Can it not be called an earth even though it is very small?

but were not connected to anything else (i.e not to the transformer)would (despite the absence of an 'earthing system') inevitably have the effect of causing an OPD to operating in the event of the incredibly unlikely pair of faults which has been postulated (since the two live sides of the supply would be connected together, via the two faults and the 'bonding conductor'. I was therefore suggesting (since people are inclined to be pedantic in this area!) that these seemingly obviously 'bonding conductors' actually fulfilled the functional definition of 'earthing' that you have agreed to above. If the conductors are achieving 'earthing' (per that definition), should they be called 'bonding', 'earthing' (hence 'CPC') or both?

So, you are saying this is NOT an earthing system therefore the conductors cannot be CPCs because there IS no earth even though they perform the same function?
More just a short circuit protective system. A PSSB system???

Food for though although I'm not sure my thought was hungry.