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Sizing CPC between SWA armour gland earth tag to distribution board earth bar, should the half size rule apply?

TaZ10101

Sizing CPC between SWA armour gland earth tag to distribution board earth bar.
I’ve come across this scenario many times, Trimble calcs for a 4 core parallel feed include a single separate CPC, in this case 2x150mm 4core + 50mm CPC.

The issue being engineers quoting Table 54.7, i.e. the calculation is compliant for the parallel feed + the separate CPC but where the glands are terminated the cable size should be half size between the earth tag and the earth bar, in this case that would need 2 separate pieces of 95mm CPC - one from each earth tag to the earth bar to equal 190mm2 of copper, thus satisfying the half size rule. 
The reasoning being that the cable calculation does not account for this final connection from the gland to the bar, therefore in the absence of a calculation to verify we must default to the half size rule, this often causes issues on site with installers opting to use whatever cable they have to hand to bond the glands like 16mm or daisy chain links from one gland to another and without clear design information to follow they don’t recognise there is an issue. 
I’m aware there is Reg 543.2.2 which references requirements for the use of the metal frame of an enclosure as a protective conductor, it seems obvious that under fault conditions multiple cables glanded into a switch board regardless of the bonding internally that some current will aways flow through the enclosure itself. Part (ii) of 543.2.2 states the metal CSA shall be equal to 543.1 or verified by test IAW BS EN 61439, can anyone explain what this means? 
Also from Table 54.7 there is another option k1/k2 x S/2, since steel wire + separate copper CPC is two different materials would this provide a smaller size than simply applying the half size rule?

I would be very interested in options on this subject because for many years I’ve never really found a definitive answer, although I’m sure I do recall on large LV panels that none of the internal bonding cables exceeded 25mm2, and being told at the time the manufacturer data for the panel confirmed it acceptable, but not sure why, maybe it was PME conditions. 

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  • +1

    To my mind, the entire length of the c.p,c, need to meet minimum requirements - there's no exceptions for short lengths whereever they are. That said, c.p.c.s can be shared between circuits and only have to meet the requirements of the most demanding one, so if you had several cables glanded into a plate an in effect several lengths of G/Y in parallel to the earth bar, you might only need to verify that the total c.p.c. was OK for the largest circuit rather than each one having to meet the entire requirement for its own circuit on its own.

    The gland plate situation has always been a bit odd - if steel conduit is used instead of SWA, it's common not to have any extra earth tails at all, even where the conduit i used as the c.pc. Go figure.

       - Andy.

Reply
  • +1

    To my mind, the entire length of the c.p,c, need to meet minimum requirements - there's no exceptions for short lengths whereever they are. That said, c.p.c.s can be shared between circuits and only have to meet the requirements of the most demanding one, so if you had several cables glanded into a plate an in effect several lengths of G/Y in parallel to the earth bar, you might only need to verify that the total c.p.c. was OK for the largest circuit rather than each one having to meet the entire requirement for its own circuit on its own.

    The gland plate situation has always been a bit odd - if steel conduit is used instead of SWA, it's common not to have any extra earth tails at all, even where the conduit i used as the c.pc. Go figure.

       - Andy.

Children
  • +1 in reply to AJJewsbury

    It very much depends what is acting as the CPC - in the case of a metal box and a decently "graunched" joint, it may be that nothing extra in the way of CPC is needed. Now It may be the sort of folk that do SWA vs the sort that do conduit, or perhaps more charitably,  the fact that conduit can't come in at a slight angle and twist after installation, but there seems to be much more  SWA working loose than there is loose conduit,

    Personally I'm a great fan of the "Piranha" style nuts (or one of the variations on the theme) as opposed to banjo washers if there is anything requiring a positive CPC to armour- and plastic boxes / painted boxes etc are often in this category,

    In such a case I'd expect the 'tail' of CPC to match the SWA core diameter, but if pushed I could be convinced that the 'half' rule could apply, but some justification would be needed for using a lot less.

    That said if the armoured cable was a given size for reasons of length and voltage drop, but the currents were limited by MCB or whatever to a lower value, then a section of  thinner CPC or indeed a section of thinner phases and neutral cables for a short hop, may be justified, or even needed to fit the size of terminals or to route easily.

    Mike

  • 0 in reply to AJJewsbury

    Actually there are 3 sets of 2x150 4c each with a separate 50mm CPC, following that logic if the banjos were doubled up and one piece of 50mm earth was used to bond each set to the earth bar internally within the switch board all three together would equal 150mm, therefore satisfying the half size rule. 
    But at the load end within the distribution board one piece of 50mm, or even 2, one from each banjo would fall short and would not meet the half size rule. 
    I did wonder about the cumulative effect of the CPC’s but from an engineering/ design standpoint people only tend to look at one element at a time. 
    I also looked up BS EN 61439 and found some useful BEAMA guidance, essentially I think it demonstrates the metal enclosure withstand, presumably up to the same rating as the busbars which is 50kA. Going by the manufacturer data this would imply that once glanded there would effectively be no real requirement to bond the gland itself, however I know this approach has been frowned upon for a long time now, presumably there are regulations in BS7671 that stipulate the glands should be bonded. 
    The issue is as I see it  is designers never bother to add the manufacturer details or CofC to the design with the cable calcs preferring to base assumptions on equipment sheets which are vague and lack detail, all round I think this issue is caused by a lack of understanding on many fronts. 

  • 0 in reply to mapj1

    I agree, in fact the first thing I checked was the glands into the bottom of the SAIF feeder pillar and they were indeed on a slight tilt, disappointing but given the size of the cables it’s likely the ground was not dug out deep enough to give the joiners the space they needed. 
    I’d have to admit I never considered the piranha option, do they go up to large size glands? I guess this is down to the spec, same size would cut to the chase but in the case of 2x150 this would be 300mm : and this identifies the crux of the issue, in a SAIF feeder pillar the gland plate at the base is bolted directly onto the earth bar at the bottom, sandwiched between the steel enclosure itself. The cable joiners complained that it would be extremely difficult to even get 2 pieces of 95mm from the banjos to the bar in order to meet the half size rule. 
    so if it is physically impossible to install there must be a gap somewhere between what’s on paper in the regs and what can actually be achieved in reality. 

  • 0 in reply to TaZ10101
    TaZ10101 said:
    I’d have to admit I never considered the piranha option, do they go up to large size glands?

    At least up to 63mm apparently: https://www.swaonline.co.uk/cable-glands-accessories/cable-gland-accessories/brass-earthing-nut-locknut-earthingnut

       - Andy.

  • 0 in reply to TaZ10101
    TaZ10101 said:
    The cable joiners complained that it would be extremely difficult to even get 2 pieces of 95mm from the banjos to the bar in order to meet the half size rule. 
    so if it is physically impossible to install there must be a gap somewhere between what’s on paper in the regs and what can actually be achieved in reality. 

    Or the feeder pillar hasn't been designed (or correctly selected) to suit reality. I've seen similar things many times in the past ... no room to get in the required cables, or the installation design doesn't permit adequate space for a sufficient bending radius.

    'The regs' in this case is basic physics.

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