Earthing Conductor Sizing - Adiabatic

Discussion in 'Electricians' Talk' started by Dale76uk, Feb 14, 2012.

  1. Dale76uk

    Dale76uk Guest

    Hey guys,

    I have an interesting question on the sizing of Earthing Conductors.


    1 Phase - 230v PME (Ze 0.22) Protective device is an 80amp BS88. 16mm Tails, 6mm EC and 10mm MPBC's (Gas/Water)

    is it possible to retain the 6mm EC or will I have to upsize it to a 16mm???

    I was looking at using the adiabatic equation from P160 to calculate the sizing. 6mm seems to be enough.

    I have contacted the NICEIC help desk and they're telling me to use table 54.8???? this is for MPBC's and not EC's. He then told me that it had something to do with ESCQR 2002 as it was a PME. blah blah blah.

    Furthermore, reg 543 says i can calculate it or choose it from 54.7. your thoughts please.

  2. spinlondon

    spinlondon Well-Known Member

    Already told you Dale, the advice given by the NICEIC is correct.
    Where PME conditions apply, earthing conductors are treated the same as main bonding conductors.
    Minimum CSA is 10mm².
  3. Lokkars Daisy

    Lokkars Daisy New Member

    For existing 6mm on a PME installation, presumably the EC was sized correctly when the installation was first installed , and assuming it was a pme supply then, and assuming there have been no major changes to the supply characteristics, and providing the cable isn't showing any signs of overheating , then leave it as it is, you are not required to upgrade it.

    The advice given by the NICEIC is often at odds with itself, I can assure you the 6mm is more than likely quite OK
  4. Dale76uk

    Dale76uk Guest

    thanks, but i'm looking for something more specific.  perhaps a reg number?

  5. Dale76uk

    Dale76uk Guest

    don't forget 54.8 refers to MPBC's and not EC's
  6. spinlondon

    spinlondon Well-Known Member

    I've given you the Reg. Nos. on the other forum.
    542.3 Earthing conductors
    542.3.1: "Every earthing conductor shall comply with Section 543, and where PME conditions apply, shall meet the requirements of Regulation 544.1.1 for the cross-sectional area of a main protective bonding conductor."

    Regulation 544.1.1 refers you to Table 54.8.
  7. Dale76uk

    Dale76uk Guest

    If we look at 543 in more detail, Cross Sectional Areas, 543.1.1 - The cross sectional area  of every protective conductor, other than a protective bonding conductor shall be:-

         i) calculated in accordance with reg 543.1.3, or {calculated}
         ii) selected in accordance with reg 543.1.4 {use table 54.7 when not calculated)

    So, it appears that the Earthing Conductor can be obtained by either of the following above.  As the system is a PME it is also necessary to take into account of 544.1.1 for selecting the MPBC.
  8. spinlondon

    spinlondon Well-Known Member

    Not quite.
    Table 54.7 is used where the supply is TT or TN-S, but with TN-C-S, Table 54.8 is used.
    With TT and TN-S supplies, the minimum CSA for an earthing conductor is 2.5mm².
    With TN-C-S the minimum CSA is 10mm².
    In any event whichever system is in use, the adiabatic equation should also be satisfied.
  9. Lokkars Daisy

    Lokkars Daisy New Member

    just looking at the NICEIC tech manual

    it gives an example as follows ;

    the designer of a proposed small alteration or addition to an existing installation may have found that the main bonding conductors of that installation have a cross sectional area less than required by BS7671

    having carefully considered all the circumstances, the designer may conclude that the deficiency dose not pose a risk to the users and consequently decide that it is acceptable, in his/her opinion, to rely on the existing bonding as part of the protective measures for that installation
    the designer is required to take responsibility for his/her decisions by been prepared to justify that the departures results in a degree of safety no less than would be afforded by compliance with BS7671 and to note the departures

    where protective multiple Earthing conditions apply, careful consideration should be given to any decision to rely on existing main bonding conductors with a cross sectional area less than that required by table 54h or BS 7671.
    There is always the possibility that main bonding conductors may have to carry network circulating currents continuously for long periods the PME supply was made available after 1 October 1988, or in certain cases has been worked upon after 1 October 1988 by the electricity distributor, the cross sectional area of the main bonding conductors has to be in accordance with table 54h in order to comply with the electricity supply regulations
  10. sparky Si-Fi

    sparky Si-Fi Well-Known Member

    1.045KA  *0.1 discon. time for BS 88  (squared) /115 value of K for copper

    0.0028 csa

    Ample me thinks?
  11. spinlondon

    spinlondon Well-Known Member

    Dale's query, is not whether the 6mm² is adequate as a CPC, but whether it meets the requirements of BS7671, and how one should determine that.
    As PME conditions apply, it is required to be sized as a bonding conductor as per Table 54.8 (54H in old money).
    As for your adiabatic equation, what can be said?
    Misplaced the decimal point by 3 places, appeared to have forgotten about the square root, used both measured and tabulated values, and an incorrect value for k?
    Good job you weren't sitting an exam.
  12. J.P.

    J.P. New Member

    Don't know about all the technical gumpfff - which is more relevant to industrial/commercial installs I would think but I do as follows on re-wires - cant say owt about TT as have never done one : -

    TN-C-S ; - 16mm from MET to DB, and then all main bonds in 10mm

    TN-S : - 16mm from MET to DB and then all main bonds in 10mm

    100 Amp Cutout

    Aye there both the same, what the heck its simple and requires a small amount of 16mm but more 10mm : - not cheap at today's copper prices.
  13. spinlondon

    spinlondon Well-Known Member

    With cartridge fuses up to 100A with a value for k of 143, 10mm² is sufficient for earth conductors.
    May require a larger CSA if the value of k is 115, although that will depend on Ze.
    With TN-S and TT 6mm² will be sufficient for bonding.
  14. Lokkars Daisy

    Lokkars Daisy New Member

    Dale asks is it possible to retain a 6mm EC on a pme installation, and the short answer is probably yes, and yes again in many circumstances it will meet the requirements of BS 7671.
  15. spinlondon

    spinlondon Well-Known Member

    Yes he does, and I consider the existing conductor to be adequate, although whether it complied at the time of installation is not clear.
    However he is also querying how the earthing conductor should be sized, and the fact that the NICEIC have advised him to use Table 54.8.
    Because Table 54.8 refers to bonding conductors, whereas Table 54.7 and the adiabatic equation are used for earthing conductors, he has issues with the advice.
    I on both this and another forum have attempted to point him in the direction of the Regulation that requires earthing conductors to be sized as per bonding conductors where PME conditions apply.
    As it stands, it does not meet the requirements of BS7671, as it is not 10mm².
    As to whether it can be retained, or requires upgrading, depends on the individuals opinion.
    If the 6mm² conductor complied at the time of installation, then yes it to my mind it would not require upgrading.
    If however it did not comply at the time of installation, then to my mind it would require upgrading.
  16. harmonic

    harmonic Member

    With ref to regs on earthing conductors and bonding conductors does any one know dates for upgradeing of those cables ie were  bonding conductors ever 2.5m/m or old 7.029

    Also when did they start bonding gas and water 
  17. J.P.

    J.P. New Member

    I don't know what EC means LD other then earth conductor - so I presume we are talking about conductor from MET to DB? If this is so then within PME parameters surely the min size is 16mm - I don't think the Dale was asking about gas/water bonds etc just this. So on an old PME installation with 6mm to DB from MET then this surely this would be coded - perhaps 4 and a recommendation put forth bringing it up to current standards which I believe is 16mm under BS 7671.

    When exactly did TN-C-S start to manifest within domestic electrical deploys LD?
  18. Lokkars Daisy

    Lokkars Daisy New Member

    Yes earth conductor JP, i worked on a housing scheme , 200 units maybe. that was in the early eighties and the supply was pme and the EC was 6mm , they still are afaik, looking at my post above it seems that the minimum of 10mm would have arrived around 1988
    JP the current size for the PME  EC can be 16mm from tables or perhaps less if you calculate it , however with pme it can not be less than 10mm.
    Although if the installation contained no connections to extraneous parts then you could in theory size it less than 10mm
    The thing is that with pme there is a chance of heavy circulating currents , diverted neutral currents travelling through the ground via the main bonding conductors, the powers that be reckon a 10mm conductor will handle these currents, as these currents are also likely run through the EC then this must also be a minimum of 10mm.
    However I don't think the regs are clear on the size of the EC re pme , or maybe it's the way I read it
  19. spinlondon

    spinlondon Well-Known Member

    There is no requirement for a minimum CSA of 16mm² for earthing conductors in BS7671.
    I can only assume that the belief has come about due to the remmomendations in the OSG, or perhaps because of the requirement that bonding condutors shall be half the CSA of the installation main earthing conductor.
    As far as the requirements of BS7671 are concerned:
    The minimum CSA for an earthing conductor (unless it is incorporated in a multicore cable) is 2.5mm².
    The minimum CSA for a main protective bonding conductor is 6mm².
    Where PME conditions apply (i.e. TN-C-S), the minimum CSA for both main earthing and main protective bonding conductors is 10mm².
    There may be other requirements imposed by network suppliers, which would take precedence over BS7671.
    TN-C-S was being introduced in the early 80s I was installing it in 82.
  20. J.P.

    J.P. New Member

    Thanks LD - thats cleared up a few things for me. I always thought that PME demanded a min of 16mm EC but can see that I am wrong. So I would think that the adiabatic equation would more then likely in most cases say a lesser CSA but as you say this would be brought up to 10mm min whatever happens - very important because the price of copper is no joke nowadays and perhaps relying on tabulated values (which is the sort of thing I would do) could add on an appreciable amount to the materials bill especially as cable like 16mm is normally bought on a 50 meter drum - so the cost differential is quite large between 10mm and 16mm.

    Anyway apart from my usual waffling - appreciated LD.

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