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Neutral earth links in feeder pillars

Lee Petherick

So, we have a private HV network on site that feeds feeder pillars and LV switchboards as a TN-S system.

I have multiple locations where I think there are neutral earth links that shouldn't be there, but even though it doesn't "feel" right to me, especially when I see 26 amps going down the earth conductor and only 16 amps going down the neutral in one particular place.

Example one - TP+N from TX into new feeder pillar which then feeds onto an old feeder pillar - neutral earth links in both, and they are only 2 metres away from each other.  One of these fuseways then feeds a building that has another neutral earth link in the switchpanel.

Example two - TP+N from TX into LV switchpanel (ACBs and MCCBs) which then feed two separate feeder pillars - both which have neutral earth links in them.  Although I haven't seen it myself, I am guessing that the LV switchpanel has it's own neutral earth link too.

I hope it's not correct, as I just don't see how it can be, but always willing to learn!! Slight smile

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  • 0

    Are both the neutral and earth cables sized for full potential current? You maybe need just a few to be low in capacity to provide additional reasons for 'fixing' any problems. Just a thought.

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  • 0

    Are both the neutral and earth cables sized for full potential current? You maybe need just a few to be low in capacity to provide additional reasons for 'fixing' any problems. Just a thought.

Children
  • 0 in reply to Philip Oakley

    Is this part of your on-site distribution, or consumption side ?

    I.e are any current consuming loads connected to the TNS part between the links ? 
    If not you might be marginally legally OK, even in the UK. Depending how the system was set up, a TNC distro to each building say, but in the building feeding TNS final circuits is permitted - for none consumer circuits only (DNOs do exactly that of course)
    Legality aside, it needs some serious thought to get it right, and then does not work well if the earths of different areas are cross linked,  and what you have sounds like it may be if it is diverting current through the earthing and perhaps the building superstructure which is not a good sign.

    Don't remove anything yet though - if there are high resistance links masked by the down stream ones it may make it vastly worse. If there are separate neutrals and CPCs in the feeds to the pillars/ cabinets or whatever, then the CPC and neutral continuity should both be verified, as should the presence of upstream link,  before removing the load end link and converting to TN-S, (or TN-s depending on your preference, I tend to follow a former forum contributor's convention of the small 's' load side, and the large one distribution side)

    Don't move more than one thing at a time and don't have it energised if/when you do.

    Also, related, where is the CPC earthed to terra-firma - where are whatever is acting as the system LV electrodes ?
    Mike

  • 0 in reply to mapj1

    Morning, sorry, just trying to decipher the first bit.

    Each feeder pillar has TP+N plus CPC supply, so wouldn't be TN-C.  Not sure what is meant by on-site distribution or consumption side though, although it may be asking whether I believe that there is TN-C to each feeder pillar and then that's where the neutral earth link is, which I don't believe it is.

    Would definitely be eyeballing and testing each and every part before taking anything out, when it's isolated of course.

    With regards to the LV electrodes, we have a combined HV/LV earth electrode as we are not a hot site. 

  • 0 in reply to Philip Oakley

    The neutrals are bigger than the earths, not taking into account the armouring of the supply cable.  These systems have been working without issue for the last 17 years, but that doesn't mean that the earth should be carrying load current in its normal working life.

  • 0 in reply to Lee Petherick

    Well distribution cables only supply boxes doing the  ADS and perhaps metering, that in turn  supply final circuits that may have loads directly connected,. If it has a directly connected load at any point  its not "distro".

    I think what you have could perhaps be described as TN-S-C-s, as NE is first both common then separated then connected then separated again, which is not really a recognized combination in the UK unless you are operating your own distribution network ;-)

    I'm assuming you are in the UK. In much of Europe, or in AU/NZ this sort of thing would, or at least if done rightl could, be fine.

    Mike.

  • 0 in reply to mapj1
    mapj1 said:
    I think what you have could perhaps be described as TN-S-C-s, as NE is first both common then separated then connected then separated again, which is not really a recognized combination in the UK unless you are operating your own distribution network ;-)

    'reconnection' of N and PE is not permitted by BS 7671, and I don't think it has been at least as far back as prior to 15th Ed of the Wiring Regulations.

  • 0 in reply to gkenyon

    'reconnection' of N and PE is not permitted 

    quite. But it is very common on the network operator side of the intake fuse, and in what we would now call the "building networks" of some older blocks of flats that used to be maintained by the area boards, neither of which fell/fall under the wiring regs / '7671.

    Quite sensibly, since the rule changes that turned some unsuspecting freeholders of flats into unskilled owners and operators of building networks, the distro in new blocks of flats,  is now designed and tested  to '7671, and normally with SWA or trunked cables with no additional NE bonding. 
    But, there is a lot of older stuff out there and we don't know who put this in, or when, or who they were working for and private networks can be a bit odd, to say the least.

    Mike.

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