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

Question

Sizing CPC between SWA armour gland earth tag to distribution board earth bar. I&rsquo;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&rsquo;t recognise there is an issue. I&rsquo;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&rsquo;ve never really found a definitive answer, although I&rsquo;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.

AJJewsbury · Accepted Answer

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.

mapj1 · Answer

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

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

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