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Supplementary CPC/SWA Resistance Ratio

fiftyhertz
Evening all, as the title above i am just trying to get my head around how a supplementary CPC will help lower the Zs to achieve a sufficent 5s disconnection time. My thinking is that the SWA and CPC are supplementary mechanically connected at both earth bars (supply and load) so the fault current will divide based on the resistance ratio (even if it takes the longer route through the supplementary CPC if there was a fault on the amour to line conductor).

 

I have attached a sketch of this for clarity. 


I also know strapping a cpc on the side of the armour is bad practise, however im just trying to get my head around the principles.
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    The idea is indeed to provide a parallel path and lower the resistance that way, of course if the resistance in the live path is also high, then there is the additional  advantage  of lowering the voltage at the point of fault to less than half of the line voltage. (if the R1 and R2 are equal then the voltage at fault will be exactly half, but generally it isn't, and if you can get it down to less than 50v then there is no need to disconnect at all.)
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    I have attached a sketch of this for clarity.



    Looks about right to me. It's worth playing around with some example numbers - I suspect you'll find that the worst case Zs for a fault is not at the far end, but at a point some way along the cable length.


    In real life it's a bit tricky to calculate accurately - as it's the impedance rather than resistance ratios that matter, and impedance of steel armour (especially when there's another part of the fault current running either in the same direction or in the opposite direction just outside it) isn't always obvious. Some basic numbers with resistance will certainly give you a general idea though.


      - Andy.
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    Zs = Ze + (R1 + R2)


    Therefore, if R2 is reduced, all other things being equal, Zs will be reduced.


    Note that if Ze is high, R1 and R2 don't matter a jot; and if R1 is high, it may not be possible (or desirable) to get R2 low enough to make Zs compliant.


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    Strictly speaking it is not good science to directly add resistances to impedances. It would be better to say Zs = Ze+Z1 +Z2. In addition fault current flowing in the SWA armour may have a choking effect on the current flowing in the external CPC. To be safe and have a margin of comfort have a full size additional CPC to achieve your desired Zs.
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    John,


    Makes sense, im assuming the choking effect is what is discussed in Annex NA (taken from GN8) as the screenshot attached. I cannot find this annex anyway, could you expand on what you mean of the choking effect - Is it that the higher resistance of the steel armouring will initially lower the fault current?
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    mapj1:

    The idea is indeed to provide a parallel path and lower the resistance that way, of course if the resistance in the live path is also high, then there is the additional  advantage  of lowering the voltage at the point of fault to less than half of the line voltage. (if the R1 and R2 are equal then the voltage at fault will be exactly half, but generally it isn't, and if you can get it down to less than 50v then there is no need to disconnect at all.)




    Mike great explanation, so if you get voltage at the fault to 50v via a low resistance cpc, there’s no ‘need’ to disconnect, however by nature of the short circuit between live and earth won’t it will still automatically disconnect as the fault current will trip the breaker (even if the majority of the voltage is dropped in the line conductor)

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    rather depends on the length of the line and the breaker or fuse rating at the origin- I agree if it is designed for a BS7671 complaint voltage drop, then the line loop will be low enough to take out the breaker, but there are odd cases where the fuse may be slower than the 0,4 seconds you may imagine you need, but actually do not need if the bonding is good enough to keep the touch voltage down. An over sized breaker may be a fire hazard, but not a shock hazard.

    It is worth asking for an extreme example how fast the ADS on a 55-0-55 supply needs to be for safety of life, and with almost any earthing at all the answer for safety of life is "never" is good enough. Of course to stop it catching fire in fault , some finite trio time is desirable, but  you see where this is going..

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