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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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    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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  • 0
    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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