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Metal - clad buildings and the confused forum member.

UKPN
One of the members has queried ESQCR bonding/earthing PME requirements for the above. So its worth spending a little time putting across the companies requirements. Where metal-clad buildings incorporate a steel-frame that utilises steel vertical beams that are within the foundations, the steel frame will provide a good connection with the earth which will effectively limit the earth potential rise.

A PME service may be provided to a metal-clad building provided the following criteria are satisfied:


1. The metal cladding is bonded to the steel-frame.

2. The supply is either three-phase with less than 40% unbalance or the supply is single - phase and the frame to earth impedance is les than 20ohms.

Regards, UKPN?

  • 0
    3? The building has only ONE service connected, if more than one service is required due to the building being split into individual units (terrace like), those subsequent services can only be TT?


    True or false?


    That is a trick question?


    If they are each an individual " premises "  and  "terrace like" and all the individual services were provided at the same time, a supply and metering in each individual unit, then a TN-C-S would be provided in each. There are thousands of business parks like that. The DNO provides that on presumption that they stay as separate units, usually the requirement is the fixed installation in each unit is compliant with BS7671.


    That is fine, until someone knocks through the wall of two adjacent units to make a bigger unit, but keeps both services. Best laid plans and all that.


    I have experience of some quite large metal framed buildings that are an individual premises and are detached, that have an original TN-C-S [PME]  single phase supply and later had an additional 3 phase supply added due to a vast increase in demand. Perhaps for economy whoever decided to implement this wanted to keep the original single phase supply also. For the additional 3 phase supply only a TT was offered. So you now have this steel building with a mixture of earthing systems. BS7671 can deal with this, sort of, but it highlights a quirk of the cliff edge created in standards between the supply regs and BS7671. 


    In answer to the true or false, it must be false because the DNO may agree to do it in very specific circumstances, but in reality are likely to only offer TT.
  • 0
    Sparkingchip:



    I stand looking at installations and end up up with so many questions in my head.

    e4bffd996ab1139305dc0f900a75c938-original-20200517_133131.jpg


    A block of five units presumably with five electrical intakes, now walls have been knocked through and five have become two.


    Steel frames, steel cladding, steel roller shutter and steel personnel doors mounted into the other steel work, air conditioning units mounted externally on walls and floor standing on some slabs as

     in the photo.


    The EV charger has been installed as TT, so presumably the main installation is PME.


    Given the location of the rod immediately adjacent to the building and everything else that is going on in the photo was it worth doing so?


    Andy B.


  • 0
    Alan Capon:
    gkenyon:


    That's very interesting ... how is the TT installation immediately adjacent to the PME supplied installation separated from the PME earthing system?


    For us, the issue would be two or more PME services into different areas of a steel building. It would be possible for the steelwork of the building to carry diverted neutral currents, for which it almost certainly would not be rated. 


    Regards,


    Alan. 




    That I understand.


    However, connecting extraneous-conductive-parts to the MET (or inadequate separation of the TT earthing system) effectively connects the installation to PME, This may cause a problem for either diverted Neutral currents in the LV system as before (the TT earth electrode resistance may be very small, and if not properly separated ... it's pretty much connected?). Also, the issue I pointed out previously.


  • 0
    If the answer is "it doesn't matter" (from the DNO perspective), and the TT earthing arrangement is not properly separated, then we are in a tricky situation in cases where the PME earthing system is not connected to the HV earthing system at the supply transformer, as the statement Uf = 0 in Table 44.1 of BS 7671 is no longer always true !

    Do you mean when the PME system is connected to the HV earth? (the situation then becoming similar to the first of the TN rows of 44.1 & Uf might be as high as RE x IE during a HV fault). Even so is there a practical difficulty? We generally construct TT installation using the same materials as a TN system and if a TN system can withstand RE x IE why should we expect a  problem with a TT system?

        - Andy.
  • 0
    AJJewsbury:
    If the answer is "it doesn't matter" (from the DNO perspective), and the TT earthing arrangement is not properly separated, then we are in a tricky situation in cases where the PME earthing system is not connected to the HV earthing system at the supply transformer, as the statement Uf = 0 in Table 44.1 of BS 7671 is no longer always true !

    Do you mean when the PME system is connected to the HV earth? (the situation then becoming similar to the first of the TN rows of 44.1 & Uf might be as high as RE x IE during a HV fault). Even so is there a practical difficulty? We generally construct TT installation using the same materials as a TN system and if a TN system can withstand RE x IE why should we expect a  problem with a TT system?

        - Andy.


    Yes, I did - apologies.


    The importance of this relates to Regulation 442.2.1 (in this case, if you are one of the TT consumers, you are having to rely on an additional earth electrode in someone else's installation - so how can the designer ensure BS 7671 is complied with), the effectiveness of the chosen method of connection of surge protection, and potentially the insulation coordination of  switchgear etc


  • 0
    The importance of this relates to Regulation 442.2.1 (in this case, if you are one of the TT consumers, you are having to rely on an additional earth electrode in someone else's installation - so how can the designer ensure BS 7671 is complied with)

    As I see it, if it was a pure TT system there wouldn't be an issue (Uf would be zero). The problem comes from the influence of the PME earthing system supplying another unit - but that PME earth itself presumably satisfies the requirements of 442.2.1 (presumably by some approximation to a 'global earthing system') - so I'm thinking that if the PME earth in the other unit is deemed safe, then it's influence in our TT unit can't be any more dangerous.

     
    the effectiveness of the chosen method of connection of surge protection

    Being our unit is TT, SPD would either be connected CT2 before the first tier of RCDs (CT1 not permitted in that position for TT) or CT1 or CT2 afterwards. As I understand it both CT1 and CT2 are acceptable on TN installations in both positions - so if our TT installation behaves more like a TN one, I don't see a problem. (Although I can see a potential problem the other way around - if we had CT1 connections pre-RCD and the installation somehow behaved more like TT one.)

     
    potentially the insulation coordination of switchgear etc

    Again the worst case would seem to be for the TT installation (voltage between live parts and exposed-conductive-parts reaching RE x IE + U0 as the local rod holds exposed-conductive-parts close to true Earth while N and line conductors are raised by the HV earth fault) so switchgear designed for a TT installation should if anything have an easier life if the installation behaves more like a TN one.


       - Andy.
  • 0
    If all the units are being built together, then care is taken to balance the loads and a copper earthing tape that is also sized to handle stray current is run buried as a foundation earth between the main earth terminals in each of the separate units, then the risks from lost neutrals and diverted neutral currents can be reduced can’t they?


    Or is that an over simplification?
  • 0
    Sparkingchip:

    If all the units are being built together, then care is taken to balance the loads and a copper earthing tape that is also sized to handle stray current is run buried as a foundation earth between the main earth terminals in each of the separate units, then the risks from lost neutrals and diverted neutral currents can be reduced can’t they?


    Or is that an over simplification?


    If you had N-PE links in each unit then you'd still have diverted N currents flowing through the structure - possibly reduced by having the earthing tape in parallel, but possibly still quite substantial. Years ago I worked in an office in one unit of a multiple block with conventional individual PME supplies - in one corner images on CRT monitors would always 'wobble' - it was only years later I realised that they were between our intake position and next door's and diverted N currents running through the structure were likely to blame. It's hard to balance a supply when different users feel free to turns thing on and off to please themselves.


    Current DNO policy of supplying each unit using split-con from a single point outside where N and PE are split is probably an easier and more thorough solution - especially now other services are likely to be non-metallic.


       - Andy.


  • 0
    I have had conversations in the past separately with a couple of DNO managers and an assessor who visited a large brick and stone building I was working on where commercial office space was being converted to flats, there are two three phase supplies serving the flats and an estate agents office with shared metallic pipework, but the gas meters are in another part of the building with another five commercial units each with a single phase intake.


    They laughed because I made a redundant three phase SWA cable into a main earth conductor using all four 16 mm conductors and the armour in addition to using two more 25 mm conductors and the armour of a SWA cable reutilised to supply a flat, then bonded the shared pipework to both of them. There was one point in an old gents toilet on a landing with metallic pipework running through it that seemed to be the point of most concern, it didn’t look much, but if you figure out the runs of the pipes and the location of the intakes cutting the pipework could have been really dodgy until a substantial section of it was replaced with plastic.


    UK plumbers, gas fitters and others rarely bother to use temporary bonding kits when cutting sections of metal pipework out, you often see the kits hanging up in their vans, but that’s where they stay. They are also not averse to ripping off the earthing and bonding, as it “doesn’t really do anything”.


    Yet I have seen sparks from disconnected pipes, bonding and earthing which don’t half make you stop and think.


    Edit- I could never ascertain that all the intakes within that building were from the same substation and transformer.
  • 0
    A bit of light reading:


    7. EARTHING

    7.1 Separate Services Originating outside the Building

    7.1.1 Where services are to be installed in accordance with subsection 5.2 in low-rise domestic and commercial (retail or offices) premises of conventional construction (brick, stone, concrete), interconnection of the neutral cores of the separate services, either within the building or through the fabric of the building, is unlikely. PME connections, via CNE service cables, shall normally be used.

    7.1.2 In steel-framed or steel-clad buildings (eg industrial units), interconnection of the neutral cores of separate service cables, through the fabric of the building, shall be avoided. Where it is not practicable to establish a single Intake Position, with an incoming PME connection and internal SNE sub-services provided by the Landlord, BNO or Customers, the service connections to the separate units shall be SNE. These may be derived directly from the LV Network or via a feeder pillar, with an incoming CNE connection.

    7.1.3 Where Premises in a steel-framed or metal-clad building with an existing CNE connection are to be divided into separate Premises and where it is not practicable to establish a single Intake Position, the existing service shall be disconnected and removed and each of the newly divided Premises connected externally, using SNE cables. Any other existing CNE connections in the same building, not affected by the division of Premises, may remain as they are, unless problems caused by the diversion of neutral currents are reported.

    https://www.enwl.co.uk/globalassets/get-connected/cic/icpsidnos/g81-library/1---design--planning/5-design--planning/es287---connections-to-multi-occupancy-buildings.pdf


    That is the recommendation, however implementation may vary.


    Andy Betteridge

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