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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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    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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  • 0

    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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