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Connecting two earthing systems - PME on both - any implications of this?

Former Community Member
Former Community Member
Question out of theorotical interest, can you connect the earthing systems of two PME supplies to achieve equipotential bonding between the two electrical supoplies or devices they power?


What are the pro's and con's of this? I understand the implications of PEN faults which might make the earth have touch voltage but if there is equipotential bonding between two earth systems would the earth fault current (earth which now carries live voltage from neutral) not flow through to the earth of the other PME supply?


Appreciate your thoughts.
  • 0
    May be a naïve response, but isn't this like two neighbours who share gas and water supplies through metal pipes?
  • Former Community Member
    0 Former Community Member
    Possibly and if the neighbourse have PME and have used earth clamps on their radiators and gas meter. Though the reality is both gas and water meters have plastic MDPE pipes under the soil (incoming).
  • 0
    rather depends if they are on the same substation, and if the fact you are adding a parallel link to the street cable neutral matters.

    The correct advice is 'not unless the DNO say it is OK'. They should know the likely diverted current.  Generally they do not like 2 PME supplies in the same buildingg without a lot of deliberation.
  • 0
    Usually requires the DNO's consent.


    Do you have enough information to permit compliance with Regulation 542.1.3.3? How would you propose to size the csa of the cable(s) used to connect the two?


    What happens if one installation at some time in the future becomes out of the control of the person responsible for the other one, and a supply is upgraded?


    Why is this different to bonding of extraneous-conductive-parts? BS 7671 has this covered with the sizing requirements of the main bonding, and the fact that those extraneous-conductive-parts are in contact with the ground, i.e. they are in themselves earth electrodes which help limit the effect of diverted neutral currents. An insulated cable between the two installations would not achieve the same thing.
  • Former Community Member
    0 Former Community Member
    You also need to be aware of the relative physical disposition of two or more PME supplies and their relative cable sizes - if one supply is a smaller cable, and sits in the "wrong" physical location relative to the other supply and source transformer, you can easily overload the PEN conductor on the smaller supply


    It's why DNO's are very wary of multiple PME services serving typical light industrial "sheds" all within the same structure, unless they all enter at the same physical position, rather than into each individual demise


    Regards


    OMS


  • Former Community Member
    0 Former Community Member
    What if a multi core SWA (all cores of a 10 mm) was used to link the earth bars of the two PME systems? Would that be equipotential bonding?

    Are there any risks due to PEN fault between either of the supplies as long as the SWA used for equipotential bonding was able to carry the load?


    If it exceeds 100A on the phase the mains incoming or service fuse should blow anyway hence preventing any risk to people?


    Just interesting to see if this will be a good idea to protect a backup earth fault protection at either premise and form a better path for fault currents (E.g. PEN faults) than the individual.


    Does RCBO provide protection at fault time against Open PEN fault? I believe not but I was told that you would be protected.....
  • 0
    If it exceeds 100A on the phase the mains incoming or service fuse should blow anyway hence preventing any risk to people?

    A fuse won't provide any protection against a broken PEN situation as the fuse will only be in the line condutor, never the PEN. The problem is that the PEN could end up carrying the aggregate N current of several installations, even though no single installation is overloaded (and that's the case whether the PEN is broken or not).


    Likewise an RCBO won't offer any protection at all from a broken PEN situation.

      - Andy.
  • 0
    And just to add my 2P.......544.1.1 is for the bonding...........Which I think the OP is asking about - as he says, "to achieve equi potential bonding"


    Where an installation has more than 1 source of supply to which PME conditions apply, a main protective bonding conductor shall be selected acording to the largest PEN conductor of the supply. 


    The same would apply if a TNS and a TNCS supply were in the same building I believe ..........


    I've had this before and the a new gas and water supply ended up common to a much larger building and needed to be bonded for the larger PME supply between the smaller supply and the PME supply........ LOOOONG runs of 35mm earth in my case.


    There was an interesting article written in wiring matters I believe where the article addressed how long those bonding conductors could be.....
  • Former Community Member
    0 Former Community Member
    tattyinengland:

    And just to add my 2P.......544.1.1 is for the bonding...........Which I think the OP is asking about - as he says, "to achieve equi potential bonding"


    Where an installation has more than 1 source of supply to which PME conditions apply, a main protective bonding conductor shall be selected acording to the largest PEN conductor of the supply. 


    The same would apply if a TNS and a TNCS supply were in the same building I believe ..........


    I've had this before and the a new gas and water supply ended up common to a much larger building and needed to be bonded for the larger PME supply between the smaller supply and the PME supply........ LOOOONG runs of 35mm earth in my case.


    There was an interesting article written in wiring matters I believe where the article addressed how long those bonding conductors could be.....


    So if we had a few cores of 10mm2 or 25mm2 cable then we could use that to link the earth bars of the two PME supplies in the CU / DB?


    My sparky seems to think connecting two earth supplies is very risky and does not even want to have that discussion with the DNO. We got 1x connection through an ICP and the other was historically there through DNO and everything was signed off on both installations.


    The ICP provided supply is 3 phase 100A and the existing supply in the building is single phase 80A. If two circuits from the 3 phase (on different phases) to enter a sole room supplied with only the new power supply how would that work with regards to earth bonding?


  • 0
    Well - every one of the guys who have posted on this topic are way more knowledgeable than I so please do take on board what advice has already been given. They are streets ahead of I in knowledge. 


    But I'd put this up for consideration and comment:


    Considering that the metal pipe work, structural steel, gas supply etc etc is probably common to both incoming electrical services it all needs bonding somehow.............


    See Table 54.8 page 203 in the BS7671 - I think that a 100Amp three phase supply would have probably have a copper equivalent cross sectional area of the PEN conductor of less than 35mm - so you'd need to interconnect the two METS with a minimum of 10mm bonding cable..........


    BUT - Like I said - there was a very interesting article - I think it was in wiring  matters that addressed the problem of the length allowed for this bonding cable - I don't know how to post that article here. 


    And I don't like the idea of using the cores of a SWA as a bonding conductor, (I assume you have a spare SWA already in place) I'd rather a dedicated green/yellow earth beteen the two METS - but if it is  labelled correctly and clearly I guess there's nothing prohibiting it's use. 


    Please don't accept my answer on its own, because its the answer you want to hear..........


    Kind Regards

    Tatty

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