Application of BS7671 Table 54.7

Can anyone offer experience on the application of Table 54.7 for Main Earth size on non PME supplies for supplies that have parallel incoming cables.

For example, 2 x 185mm2 PCV/SWA installed in separate ducts.

Thanks

  • Have a look at Table 54.7 and see what it says for live conductors of 2 x 185? You may then wish to calculate the size of your earthing conductor using the adiabatic equation.

  • I am looking at table 54.7 it does not appear to make any recommendations with respect to the application of parallel incoming cables, this is why I am asking experience in the application of this table with respect to parallel incoming cables.

    I appreciate at the design stage these may be sized by calculation by the designer.

  • It will tend to err even more on the side of caution than it already does with a PME system. (usually a calculation shows you do not need as much metal as table 54.7 suggests, but it is safe to assume you do.)

    By how much the main earth  may be reduced rather depends on the earthing system you do have, as this determines the potential fault current. which is the figure you insert into 543,1,3 to calculate a more accurate value of minimum CSA.

    The parallel armoured cables thing is interesting as you should still earth both armours, but maybe only one of them at both ends, depending how this is laid out. In really large sizes it is often more effective to use singe armoured cables trefoiled or quadrifoiled, but then one has to think hard about the currents that circulate in the armour.  And a 185mm SWA will be a right so-and so to thread into anywhere.

    Mike,

  • We are not bringing in the incoming supply, but we need to allow for an indicative design that will be designed and installed by others. Even although the SWA will be earthed we are allowing a direct connection from the Main Earth Bar to the DNO Earthing Point. I know adiabatic calculation will be the way forward at the design stage, but for the indicative design I was hoping to use tabulated figures.

  • I believe the answer to this question is:

    The use of Table 54.7 for parallel cables is not applicable and the adiabatic must be used when faced with parallel cables.

    With 2 cables I think you can get away with it as you can install a cable equal to the main phase and it will be fine. When 3+ parallel cables are used it really is not the way to go.

  • The use of Table 54.7 for parallel cables is not applicable and the adiabatic must be used when faced with parallel cables.

    Humm. You certainly can't apply 54.7 where the line conductor has itself been sized using the adiabatic (i.e. if the line conductor is only just going to survive the fault current, it doesn't then make sense to use half the size for the c.p.c.). But if the overall c.s.a. of each of the line conductors doesn't require consideration of fault currents (e.g. all parallel paths together give Iz≥In) I would have thought you could apply table 54.7 to that. E.g. if each of your line conductors is 2x185mm² then plug in 370mm² which then gives you 185mm² (or 185mm² * k1/k2 if of differing metals) and and you should be safe. If you have parallel protective conductors, it's difficult to control how the fault current will divide (appendix 10 gives some background) - but use the selected/calculated value for each of the protective conductors and you should be safe regardless (but that's probably the same issue whether you select or calculate). As ever, the adiabatic often gives more economicaly attractive results.

        - Andy.

  • Well it depends if the twin 185mms are needed to meet voltage drop or thermal considerations - i.e. how long they are.

    I see the problem, because 'indicative design' really means you do not really have the access to all the numbers that would allow you to downsize the conductors and be sure that  it is safe to do so.

    It should be possible to put some safe upper bounds on it though, especially if you define the 'death or glory' fuse to be fitted.

    The numbers below are not right - and should be taken with a large spoon of salt, but they illustrate one line of thought.

    The worst case is probably when the 185s are short, and needed because the steady sate load current is about 450Amps per core, so 900A per phase total. That suggests a substation of 700kVA to 1MVA . So a PSSC of 25-35 kA is credible, and perhaps a phase fuse size of 1000-1200A.

    Let-through of a fuse of that rating is then maybe 10^7 Amps squared seconds, and use that to size the CPC...

    M.

  • I really don't see much point in this discussion. 2X185 phase conductors and a 185 Earth should be satisfactory on all counts, you need to realise that a short from phase to Earth will heat ALL the conductors involved, but the Earth is quite safe getting considerably hotter under worst-case conditions. Phase Earth faults at this point in the supply should be extremely unlikely and they will never be worse than a phase-phase fault at some point in one of the cables. Such faults may well cause cable damage, but the worst outcome is probably raising everything Earthed to half mains potential while the fault clears, a perfectly normal condition on the whole distribution system (DNO). For a short distance to cost saving of not using 2x185 Earth cables is not worthwhile. For a long distance, a single one will be good to go as the PSSC will be seriously reduced, and an Earth fault to that conductor cannot occur. A fault to the SWA armour is a different kettle of fish, it will almost certainly wreck the cable and probably (maybe) will not clear at the fuse.

    Your typing is going Mike you mean PSSC of 25-35 kA, probably about right.

  • sorry correcting....  part of the problem with this sort of query is that without more site specific info to provide an 'indicative' design has to be either quite conservative, or will for some situation still be inadequate. Equally the technically correct answer 'use the site specific figures and design it properly' is probably not helpful either.

    The chonky substation style fuse is helpful though as it puts an upper bound on the damage any fault downstream of it could  do, almost regardless of supply arrangement.

    Good point on self clearing faults to the SWA armour, it may well burn back from a nail or anything of that size without troubling any ADS . To me SWA seems an odd choice.

    M.

  • I really don't see much point in this discussion.

    I am not too sure I follow your sentiment.

    I am seeking advice in the application of Table 54.7 for Main Earth size on non PME supplies for supplies that have parallel incoming cables.

    I am seeking advice on whether this is a credible/application in the use of Table 54.7. It really is as simple as that. The cable size, type and number are all hypnotical and have no real bearing on the question.

    I am not seeking advice on whether this is the most economical method as we all know the adiabatic calculation will provide the most economic results.