How to calculate Zs

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Shamrez

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My question is:
how to calculate the impedance of all circuits in system
Zs =Ze+(R1+R2) . Do we need to calculate all rings and radial circuits one by one or only one further most (R1+R2).
I red in testing we look at worse Sanario.
All circuits are parallel and adding parallel resistance means sum will be less and at end Zs will be less
I hope you understand my point

Many thanks
 
Welcome to the forum..

I think you may be confusing 'Insulation Resistance Testing' with 'Earth Loop Impedance testing'..?? :unsure:

i.e. A global IR test at the CU will include ALL circuits and numerous possible parallel circuit paths, thus will be lower than testing individual circuits..

Whereas reg 314.4 requires that where more than one circuit exists, they must be independent of all other circuits.. Thus circuit 'Earth Loop Impedance testing' will NOT involve parallel paths.

e.g. you can do a global IR at the CU..
But Zs are on individual circuits not at the CU!
 
In simple terms, the point behind Zs is finding out the impedance back to the supplier transformer, to see how much fault current can flow.
This tells us whether the protective device(s) on the circuit will operate under fault conditions, and how long they will take to do so.
It therefore needs to be considered for every circuit in the installation.
 
I think they mean by every circuit being parallel is that the R1 and R2 for any individual circuit is parallel within that circuit.
 

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