# How to measure Zs

Hi can anyone tell me how to measure Zs from a c/u, or is this measurement taken else where.I know how to do Ze and i know Zs = Ze+(R1+R2) its that i've noticed a requirement for this test to be carried out on the installation certificates. This is my first certificate that i have had to fill in,hopefully i will learn how to fill them in.Its just that i thought that Zs was the impedance supply i.e outside. Any help in the right direction much appreciated
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Ze at CU is Impedance (Z) of [e]xternal phase to earth loop ie from substation to Cu and return path.
as you say R1 + R2 circuit L from CU out to farthest point on circuit + E return back to CU
Zs is those 2 readings added together to give Impedance (Z) of [s]ystem.
EG Zs = Ze [0.01] + R1 [0.1] + R2 [0.15] = 0.26 Ohms.
N.B. Zs is Z of your 'SYSTEM' (circuit) not 'SUPPLY'! which is confusion I think B.W.
SPARX
sparx
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Quick thought! if you measure Ze + [r1+ r2] & note answer, and seperately measure Zs they won't add up exactly the same! don't panic ,
you did nothing wrong! due to the practice (unnecessary by regs) of putting earth pigtails in back boxes it introduces parallel earth return paths so r2 will be lower than expected, via damp & any thing connected to circuit under test,
put what you find in each column of sheets & be ready with this answer when asked at some point by some inspecting body!!!
regards SPARX
sparx
Rank: Project Manager
Posts: 2166
Joined: Wed Mar 21, 2007 8:33 pm
Location: The fifth continent.

Hi Sparx,
thanx for the quick reply, although passed the part p course still have a lot to learn.through experience. I often read through the forum and see you are a member that gives out a lot of good advice (also blakey). Where would us mere immortals be without your sound advice most punters to this forum owe a lot of thanxs.
So on behalf of everyone and myself many thanx and keep up the replies.
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