Under this clause there are exceptions for the MEN to not need be located on the main switchboard
If using such exception, it doesn't mention any change of location for the MEC or other parts of the earthing system
Would you still do everything else as normal in such a situation? Doesn't seem right having a MEC still run to the main earth bar but not having the MEN connection there
5.3.5.1 MEN location exceptions
5.3.5.1 MEN location exceptions
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Re: 5.3.5.1 MEN location exceptions
Fig 5.2 illustrates the configuration when the MEN is located i.a.w these Exceptions.
Note that the MEC still has one end connected to installation main earth bar.
The other end is connected to the substation earth bar; to which the electrode is also connected.
Basically the whole "main earthing system" (as defined in ESR 4) gets relocated;
but the only component part of it that has a specific location stipulated by "3000" is the MEN link; so that's the only requirement that needs an Exception.
All the things required to be connected together are still connected together; so we have effective protective earthing and effective equipotenial bonding.
The installation's earthing system is still connected to mass of earth.
And there's still a connection between distribution neutral and earth for purposes of having an 'MEN" distribution system.
Note that the MEC still has one end connected to installation main earth bar.
The other end is connected to the substation earth bar; to which the electrode is also connected.
Basically the whole "main earthing system" (as defined in ESR 4) gets relocated;
but the only component part of it that has a specific location stipulated by "3000" is the MEN link; so that's the only requirement that needs an Exception.
All the things required to be connected together are still connected together; so we have effective protective earthing and effective equipotenial bonding.
The installation's earthing system is still connected to mass of earth.
And there's still a connection between distribution neutral and earth for purposes of having an 'MEN" distribution system.
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Re: 5.3.5.1 MEN location exceptions
Just for confirmation
The the switchboard the MEN has moved from is still the main switchboard, it just becomes a main switchboard without a MEN connection inside and still meets all other requirements?
The the switchboard the MEN has moved from is still the main switchboard, it just becomes a main switchboard without a MEN connection inside and still meets all other requirements?
Re: 5.3.5.1 MEN location exceptions
Yes; it still meets the definition of "main switchboard" in ESR 4.
However, because ESRs in several cases refer to "the MEN swbd closest to supply (or similar wording), this relocation of the MEN creates problems.
EG definition of mains depends in part on them supplying an MEN swbd - but since the MSB is no longer an MEN swbd; then the wiring supplying it can't be "mains", and work on that wiring can't be "mains work".
It's probably best to just accept / ignore such anomalies, as while it could develop onto an interesting line of argument / discussion there's nothing to be gained by it. Eg not being "mains work" means no inspection required - but what sort of fool would try to avoid the cost of an inspection on that sort of technicality?
What we're really getting into here is the fact that our "MEN system" is a particular variant of what IEC calls TNC-S system of earthing.
With MEN we use TNC for the distribution and up to the MSB (and sometimes beyond); and TNS for the installation beyond MSB.
"C" meaning that the same "common" conductor is used for the two functions of return current(neitral) and protective earthing; ans "S" meaning that we use "separate" conductors for these functions.
In the TN-C part, the distribution N is earthed at origin (transformer); and also at "multiple" points along the N. Hence the term MEN standing for multiple earthed neutral. With the standard, defined, MEN system; these points are generally within the installations supplied by the tx.
The MEN connection is the point where TNC changes to TNS.
When considering cases where the tx supplies a single installation; in effect "multiple" earths means only two - and the one for the tx is likely to be more effective than the installation electrode.
In terms of providing protection there's not a lot of difference between that and just using the good one alone; so moving the MEN link into the substation and removing the lesser electrode makes sense. So "3000" allows this as an acceptable alternative within Part 2.
However, because ESRs in several cases refer to "the MEN swbd closest to supply (or similar wording), this relocation of the MEN creates problems.
EG definition of mains depends in part on them supplying an MEN swbd - but since the MSB is no longer an MEN swbd; then the wiring supplying it can't be "mains", and work on that wiring can't be "mains work".
It's probably best to just accept / ignore such anomalies, as while it could develop onto an interesting line of argument / discussion there's nothing to be gained by it. Eg not being "mains work" means no inspection required - but what sort of fool would try to avoid the cost of an inspection on that sort of technicality?
What we're really getting into here is the fact that our "MEN system" is a particular variant of what IEC calls TNC-S system of earthing.
With MEN we use TNC for the distribution and up to the MSB (and sometimes beyond); and TNS for the installation beyond MSB.
"C" meaning that the same "common" conductor is used for the two functions of return current(neitral) and protective earthing; ans "S" meaning that we use "separate" conductors for these functions.
In the TN-C part, the distribution N is earthed at origin (transformer); and also at "multiple" points along the N. Hence the term MEN standing for multiple earthed neutral. With the standard, defined, MEN system; these points are generally within the installations supplied by the tx.
The MEN connection is the point where TNC changes to TNS.
When considering cases where the tx supplies a single installation; in effect "multiple" earths means only two - and the one for the tx is likely to be more effective than the installation electrode.
In terms of providing protection there's not a lot of difference between that and just using the good one alone; so moving the MEN link into the substation and removing the lesser electrode makes sense. So "3000" allows this as an acceptable alternative within Part 2.
Re: 5.3.5.1 MEN location exceptions
Understood, thank you Alec
Further question, can we supply a separate MEN installation in an outbuilding from a non-linked subboard? I always thought this was ok but does that conflict with the TN-C-S way of thinking going back to a combined PEN after they have already been split?
For example, mains to main switchboard, submain from MSB to distribution board in same building, submain from distribution board to separate MEN installation main switchboard in outbuilding
Further question, can we supply a separate MEN installation in an outbuilding from a non-linked subboard? I always thought this was ok but does that conflict with the TN-C-S way of thinking going back to a combined PEN after they have already been split?
For example, mains to main switchboard, submain from MSB to distribution board in same building, submain from distribution board to separate MEN installation main switchboard in outbuilding
Re: 5.3.5.1 MEN location exceptions
A good question.
As you've noted; the logic of TNC-S seems to require that once you've separated, from a PEN conductor (TNC) to separate N & PE conductors (TNS), you don't change back.
However that logic has never? been reflected in NZ Regs / Wiring Rules. Certainly not since 1993.
----------------
History lesson (because we still find unusual combinations that complied with these rules but wouldn't comply now).
Under 1993 & 1997 Regs We had MEN switchboards, "linked busbar switchboards" (with a N-E link but no MEC to electrode) , and distribution switchboards (no link, mandatory PEC in supplying submain).
A DB had to have a PEC in the submain supplying it. For MEN & linked, it was optional.
An MEN could only be supplied from another MEN (except for supply from network or a generating source)
It was prohibited to supply a linked swbd from a DB; but they could be supplied from either an MEN or another linked swbd.
And a DB (no link) could only be supplied from an MEN or another DB.
A complicated set of rules, but clearly spelled out.
So in effect; you could have an endless cascade of MEN swbds, either with (TNS) or without (TNC) a PEC in the various submains.
That allowed switching back to TNC after converting to TNS.
From any of them you could run a submain to a DB, but the submain always had to include a PEC (ie, TNS)
Or you could run to a linked swbd; and on to another linked swbd; always with a PEC.
But you couldn't feed a (linkless) DB from a liked swbd; only from an MEN
-----------------
The change in 2010 ESRs removed the middle-ground of a swbd with a link but no PEC and no MEC+electrode.
And it removed the options of having links in parallel at both ends of any submain.
Which is important, because we don't want load current flowing on the PECs under normal conditions.
So we've been gradually moving closer to international practice.
But not simply adopting it unchanged.
Another aspect is that under "3000" we only use PEN submains to supply outbuildings;
whereas this 1993 / 1997 Regs allowed cascaded MEN swbds within the same building .
The only other case where we use PEN (TNC) within an installation is for mains.
--------------
Back to question:
can we supply a separate MEN installation in an outbuilding from a non-linked subboard?
While there's no such example in fig 5.3; I can see nothing in Section 5 that prohibits this.
In terms of earthing, the governing clause is 5.5.3.
5.5.3.1(a) allows for only 2 options for an (individual) outbuilding; the earthing system in the outbuilding must be either
(i) connected by PEC to supplying swbd; or
(ii) connected as a separate MEN installation.
If we choose option (ii); then we have to comply with both (c) and 5.5.3.2.
(c) (iv) says the PEN submain must run either direct;ly from MSB; or via one or more DBs in outbuildings).
If directly from MSB, the MSB will be an MEN swbd.
If via intermediate swbd(s); then (v) requires that continuity of the PEN conductor can't rely on any terminals within the intermediate swbd(s).
Ie no looping in & out again, but we can tap off the PEN to connect to the intermediate N-bar(s).
Any such intermediate swbds must be in (other) outbuildings; but nothing to require them to be MEN.
I suspect that's more because nobody thought anyone would try to feed a PEN from a standard DB, rather than because anyone thinks it's a good idea. But on the other hand, prohibiting something just 'cos it's not in our comfort zone isn't a good enough reason. We should only introduce prohibitions for sound reasons of improved safety.
And I'm struggling to think of any reduction of safety from doing as you've suggested.
On some cases it may even be the logical option.
PEN submains should only be used where there's a significant separation between the structures; reducing risk of being able to simultaneously contact items connected to the two earthing systems - which may be at different potentials.
So if the submain to 1st outbuilding is short, then it should have a PEC.
Then if submain to 2nd outbuilding is long enough, and separation large enough, to justify using PEN; plus running 2nd submain directly from MSB is impracticable; you'd end up
As you've noted; the logic of TNC-S seems to require that once you've separated, from a PEN conductor (TNC) to separate N & PE conductors (TNS), you don't change back.
However that logic has never? been reflected in NZ Regs / Wiring Rules. Certainly not since 1993.
----------------
History lesson (because we still find unusual combinations that complied with these rules but wouldn't comply now).
Under 1993 & 1997 Regs We had MEN switchboards, "linked busbar switchboards" (with a N-E link but no MEC to electrode) , and distribution switchboards (no link, mandatory PEC in supplying submain).
A DB had to have a PEC in the submain supplying it. For MEN & linked, it was optional.
An MEN could only be supplied from another MEN (except for supply from network or a generating source)
It was prohibited to supply a linked swbd from a DB; but they could be supplied from either an MEN or another linked swbd.
And a DB (no link) could only be supplied from an MEN or another DB.
A complicated set of rules, but clearly spelled out.
So in effect; you could have an endless cascade of MEN swbds, either with (TNS) or without (TNC) a PEC in the various submains.
That allowed switching back to TNC after converting to TNS.
From any of them you could run a submain to a DB, but the submain always had to include a PEC (ie, TNS)
Or you could run to a linked swbd; and on to another linked swbd; always with a PEC.
But you couldn't feed a (linkless) DB from a liked swbd; only from an MEN
-----------------
The change in 2010 ESRs removed the middle-ground of a swbd with a link but no PEC and no MEC+electrode.
And it removed the options of having links in parallel at both ends of any submain.
Which is important, because we don't want load current flowing on the PECs under normal conditions.
So we've been gradually moving closer to international practice.
But not simply adopting it unchanged.
Another aspect is that under "3000" we only use PEN submains to supply outbuildings;
whereas this 1993 / 1997 Regs allowed cascaded MEN swbds within the same building .
The only other case where we use PEN (TNC) within an installation is for mains.
--------------
Back to question:
can we supply a separate MEN installation in an outbuilding from a non-linked subboard?
While there's no such example in fig 5.3; I can see nothing in Section 5 that prohibits this.
In terms of earthing, the governing clause is 5.5.3.
5.5.3.1(a) allows for only 2 options for an (individual) outbuilding; the earthing system in the outbuilding must be either
(i) connected by PEC to supplying swbd; or
(ii) connected as a separate MEN installation.
If we choose option (ii); then we have to comply with both (c) and 5.5.3.2.
(c) (iv) says the PEN submain must run either direct;ly from MSB; or via one or more DBs in outbuildings).
If directly from MSB, the MSB will be an MEN swbd.
If via intermediate swbd(s); then (v) requires that continuity of the PEN conductor can't rely on any terminals within the intermediate swbd(s).
Ie no looping in & out again, but we can tap off the PEN to connect to the intermediate N-bar(s).
Any such intermediate swbds must be in (other) outbuildings; but nothing to require them to be MEN.
I suspect that's more because nobody thought anyone would try to feed a PEN from a standard DB, rather than because anyone thinks it's a good idea. But on the other hand, prohibiting something just 'cos it's not in our comfort zone isn't a good enough reason. We should only introduce prohibitions for sound reasons of improved safety.
And I'm struggling to think of any reduction of safety from doing as you've suggested.
On some cases it may even be the logical option.
PEN submains should only be used where there's a significant separation between the structures; reducing risk of being able to simultaneously contact items connected to the two earthing systems - which may be at different potentials.
So if the submain to 1st outbuilding is short, then it should have a PEC.
Then if submain to 2nd outbuilding is long enough, and separation large enough, to justify using PEN; plus running 2nd submain directly from MSB is impracticable; you'd end up
- Rating: 16.67%
Re: 5.3.5.1 MEN location exceptions
Hi,
I have been asked by an engineer to attach a 'bonding earth' from a pumpshed to the transformer to reduce the harmonics due to some large vsd's. I have said that this would provide a parallel path for the neutral current and instead we would need to omit the link from the installation and installed in the tx, am I correct in my thinking?
Also in regards to figure 5.2, If a installation was to run a mec to the network transformer however the transformer only provides a combined neutral earth bar, would a reconfiguration be required to seperate the two and provide the necessary men link? I could be miss reading 5.5.1.1 exception 2, but can you just take the mec to the neutral bar is required by the distributor.
.
I have been asked by an engineer to attach a 'bonding earth' from a pumpshed to the transformer to reduce the harmonics due to some large vsd's. I have said that this would provide a parallel path for the neutral current and instead we would need to omit the link from the installation and installed in the tx, am I correct in my thinking?
Also in regards to figure 5.2, If a installation was to run a mec to the network transformer however the transformer only provides a combined neutral earth bar, would a reconfiguration be required to seperate the two and provide the necessary men link? I could be miss reading 5.5.1.1 exception 2, but can you just take the mec to the neutral bar is required by the distributor.
.
Re: 5.3.5.1 MEN location exceptions
I do not believe the proposed action will do anything to reduce the harmonics. To get rid of harmonics requires frequency filters.
Questions that affect how the Wiring Rules apply is whose transformer it is, and whether or not it supplies any other installation.
Also where is the point of supply?
Assuming that the conductors from tx to swbd are "consumer mains"; then "3000" allows only two options.
The normal option is as per Fig 5.1; with the mains neutral being actually not just a neutral, but a PEN conductor.
Option 2 is illustrated in Fig 5.2; with no MEN link, MEC, or electrode within the installation, the mains N is just an N.
This option is only allowed under the Exceptions to particular clauses.
5.5.1.1; which allows for the electrode end of the installation's MEC to be connected to something other than an installation earth electrode .
5.3.5.1; which allows the installation MEN connection / link to be somewhere other than at the installation's MSB.
Those Exceptions cover 2 of the 3 components that make up the installations "main earthing system" (as defined in ESR 4).
The 3rd component is the electrode [5.3.6], and there's no Exception needed for that because the clause doesn't dictate the precise location of the installation's electrode.
While these Exceptions allow for several sub-options as regards the detail (eg exactly what the MEC gets connected to);
there are really only the two options; making it an "either / or" situation and you have to choose one or the other.
Therefore having both a full main earthing system within the installation (as per normal), and an additional earthing conductor back to the substation, would be non-compliant with Part 2.
In particular, it would, as you've noted, result in load current being carried in a protective earthing conductor; contrary to 8.3.8.1 (a) (also, in 2018 edition, 5.5.2.1).
Therefore, if the engineer insists, he will have to produce a certified design for work to Part 1; i.a.w ESRs 58 & 59.
Required documentation for Part 1 work is set out in 1.9.4.
Calling it a "bonding conductor" doesn't make it one. Equipotential bonding is about dealing with voltage difference between exposed conductive parts and extraneous conductive parts.
Touch voltages can be cased by the high fault currents carried by the protective earthing system; resulting in voltage along the length of the PEC.
This effect is mitigated by using one of the 3 methods of "fault protection" permitted by 2.4.1. In this case almost certainly using automatic disconnection of supply, with the mandated time limits for operation mitigating the risk of anyone receiving a shock from simultaneously accessible parts.
The other main case of touch voltage is differences on potential between parts connected to different buts of land. There are many causes, but these are what equipotential bonding is designed to deal with.
Harmonics cause a lot of problems, but not touch noted for producing touch voltage. Since that's not the problem being addressed, calling this proposed conductor 'bonding" is just playing with words.
You don't state what the symptoms are; but since there are large VFDs involved it's likely that there are induced HF currents (as distinct from "harmonics"), that may well be causing HF touch voltages. Clause 5.4.8 refers (though not in any detail).
This is another form of touch voltage, and as for EPR, the answer is ensuring that all simultaneously accessible conductive parts are at similar potential. Fact is that normal earthing, designed for 50 Hz fault currents, is often a high impedance to HF. Therefore to deal with HF induced touch voltage, we need an earth path that has low impedance to HF
And if this is the problem, then again the proposed additional earthing conductor between MSB & TX won't do anything to help.
First step would be to ensure that the advice in the code of practice for VFDs has been followed.
Power Drive Systems (PIB 49); published by Radio Spectrum Management:
https://www.rsm.govt.nz/about/publications/pibs/pib-49/
------------
With regard to the detail of where the MEC can be connected, you can choose either Exception to 5.5.1.1.
Exception 2 allows for connection to a "neutral" bar if so required by the distributor.
However using a combined N&E bar would be non-compliant, as 5.3.5.1 requires an "MEN link".
The Exceptions allow it to be at the sub, but you can't not have one.
Questions that affect how the Wiring Rules apply is whose transformer it is, and whether or not it supplies any other installation.
Also where is the point of supply?
Assuming that the conductors from tx to swbd are "consumer mains"; then "3000" allows only two options.
The normal option is as per Fig 5.1; with the mains neutral being actually not just a neutral, but a PEN conductor.
Option 2 is illustrated in Fig 5.2; with no MEN link, MEC, or electrode within the installation, the mains N is just an N.
This option is only allowed under the Exceptions to particular clauses.
5.5.1.1; which allows for the electrode end of the installation's MEC to be connected to something other than an installation earth electrode .
5.3.5.1; which allows the installation MEN connection / link to be somewhere other than at the installation's MSB.
Those Exceptions cover 2 of the 3 components that make up the installations "main earthing system" (as defined in ESR 4).
The 3rd component is the electrode [5.3.6], and there's no Exception needed for that because the clause doesn't dictate the precise location of the installation's electrode.
While these Exceptions allow for several sub-options as regards the detail (eg exactly what the MEC gets connected to);
there are really only the two options; making it an "either / or" situation and you have to choose one or the other.
Therefore having both a full main earthing system within the installation (as per normal), and an additional earthing conductor back to the substation, would be non-compliant with Part 2.
In particular, it would, as you've noted, result in load current being carried in a protective earthing conductor; contrary to 8.3.8.1 (a) (also, in 2018 edition, 5.5.2.1).
Therefore, if the engineer insists, he will have to produce a certified design for work to Part 1; i.a.w ESRs 58 & 59.
Required documentation for Part 1 work is set out in 1.9.4.
Calling it a "bonding conductor" doesn't make it one. Equipotential bonding is about dealing with voltage difference between exposed conductive parts and extraneous conductive parts.
Touch voltages can be cased by the high fault currents carried by the protective earthing system; resulting in voltage along the length of the PEC.
This effect is mitigated by using one of the 3 methods of "fault protection" permitted by 2.4.1. In this case almost certainly using automatic disconnection of supply, with the mandated time limits for operation mitigating the risk of anyone receiving a shock from simultaneously accessible parts.
The other main case of touch voltage is differences on potential between parts connected to different buts of land. There are many causes, but these are what equipotential bonding is designed to deal with.
Harmonics cause a lot of problems, but not touch noted for producing touch voltage. Since that's not the problem being addressed, calling this proposed conductor 'bonding" is just playing with words.
You don't state what the symptoms are; but since there are large VFDs involved it's likely that there are induced HF currents (as distinct from "harmonics"), that may well be causing HF touch voltages. Clause 5.4.8 refers (though not in any detail).
This is another form of touch voltage, and as for EPR, the answer is ensuring that all simultaneously accessible conductive parts are at similar potential. Fact is that normal earthing, designed for 50 Hz fault currents, is often a high impedance to HF. Therefore to deal with HF induced touch voltage, we need an earth path that has low impedance to HF
And if this is the problem, then again the proposed additional earthing conductor between MSB & TX won't do anything to help.
First step would be to ensure that the advice in the code of practice for VFDs has been followed.
Power Drive Systems (PIB 49); published by Radio Spectrum Management:
https://www.rsm.govt.nz/about/publications/pibs/pib-49/
------------
With regard to the detail of where the MEC can be connected, you can choose either Exception to 5.5.1.1.
Exception 2 allows for connection to a "neutral" bar if so required by the distributor.
However using a combined N&E bar would be non-compliant, as 5.3.5.1 requires an "MEN link".
The Exceptions allow it to be at the sub, but you can't not have one.
Re: 5.3.5.1 MEN location exceptions
Thanks as always Aleck for your insight.
The symptoms is that some consumers on the same feeder are suffering from flickering lights. Couldn't agree with you more that this is not a bonding wire, the term gets misused by almost every electrical worker.I believe an earth wire was temporary connected during some testing which showed a very slight reduction. I cannot go into much detail, nor have I been to site, i have just read a report and now asked to install some data logger equipment around the area, whilst engineers fiddle with the vsd settings. They are also wanting me to reconnect this earth , which got me wondering whether it would be compliant to do so.
The symptoms is that some consumers on the same feeder are suffering from flickering lights. Couldn't agree with you more that this is not a bonding wire, the term gets misused by almost every electrical worker.I believe an earth wire was temporary connected during some testing which showed a very slight reduction. I cannot go into much detail, nor have I been to site, i have just read a report and now asked to install some data logger equipment around the area, whilst engineers fiddle with the vsd settings. They are also wanting me to reconnect this earth , which got me wondering whether it would be compliant to do so.
Re: 5.3.5.1 MEN location exceptions
Agree the terms are widely mis-used (as are many others).
Protective earthing & equipotential bonding are two different concepts; and need to be dealt with separately.
That said, often the same fittings / conductors are performing both functions - basically wherever there's a protective earth, it also performs an equipotential bonding function.
So we can't classify a PEC as being solely about (protective earthing).
But that's its primary purpose, so that's the label it gets.
A key feature if our MEN distribution system is that the neutral is tied to earth a multiple places, to maintain it at close to earth potential.
That's an equipotential bonding function; performed by the (PEN) conductor alongside it's load-carrying (N) and protective earthing (PE) functions.
We mostly just call it a "neutral".
Since there's already a conductor (mains PEN) between distribution NE and installation NE, any significant differences in potential between the two points are already sefectively bonded. Adding another conductor in parallel can't make a significant difference to the bonding; and shouldn't be (mis) labelled as a "bonding conductor".
-------------
The fact that the symptoms are in other installations counts against the cause being earthing system components with high impedance to HF. That problem is typically limited to the equipment supplied by the VFD(s).
However other aspects of that EMC document may be relevant.
As may harmonics - but i've never seen any thing to suggest that harmonics could be dealt with by any form of equipotential bonding.
There is a known issue relating to reduced-size neutrals, selected / installed on the basis that the neutral load current will be significantly less than the load currents in the actives. Add more harmonics-generating equipment (such as VFDs), and the existing mains N may now be under-sized . See 3.5.2 (b)
In which case adding a conductor in parallel might well have some effect - but only as far as CCC goes, it can't reduce the harmonics.
Protective earthing & equipotential bonding are two different concepts; and need to be dealt with separately.
That said, often the same fittings / conductors are performing both functions - basically wherever there's a protective earth, it also performs an equipotential bonding function.
So we can't classify a PEC as being solely about (protective earthing).
But that's its primary purpose, so that's the label it gets.
A key feature if our MEN distribution system is that the neutral is tied to earth a multiple places, to maintain it at close to earth potential.
That's an equipotential bonding function; performed by the (PEN) conductor alongside it's load-carrying (N) and protective earthing (PE) functions.
We mostly just call it a "neutral".
Since there's already a conductor (mains PEN) between distribution NE and installation NE, any significant differences in potential between the two points are already sefectively bonded. Adding another conductor in parallel can't make a significant difference to the bonding; and shouldn't be (mis) labelled as a "bonding conductor".
-------------
The fact that the symptoms are in other installations counts against the cause being earthing system components with high impedance to HF. That problem is typically limited to the equipment supplied by the VFD(s).
However other aspects of that EMC document may be relevant.
As may harmonics - but i've never seen any thing to suggest that harmonics could be dealt with by any form of equipotential bonding.
There is a known issue relating to reduced-size neutrals, selected / installed on the basis that the neutral load current will be significantly less than the load currents in the actives. Add more harmonics-generating equipment (such as VFDs), and the existing mains N may now be under-sized . See 3.5.2 (b)
In which case adding a conductor in parallel might well have some effect - but only as far as CCC goes, it can't reduce the harmonics.