High Zs Reading
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High Zs Reading

by Sparkydelux » Mon Aug 03, 2009 1:58 pm


I fitted some A/C's the other day and did the Testing on them. I ran them through a type C RCBO 20A and when measuring them came across a rather higher value on the furtherest one. The runs are aproximately 17.5m and 13.5m, run in seperate radials and off 6mm T&E.

I wasn't expecting the Zs to be that much higher than the closer A/C as it only exceeds it by about 5m.

I remember something about high Zs values and if exceeded then an RCD must be applied. Can someone confirm this?

What would be the other causes of a high Zs value?

Information is always appreciated.

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by kbrownie » Mon Aug 03, 2009 6:49 pm

Whats do you mean by A/C
What suprizing readings did you compared to the rest of the results, what was R1+R2 and Ze results.
Your Zs reading will be higher at the furthest point on the circuit.

Before testing, the main equipotential bonding conductors are disconnected but not the connection with earth, this is done to prevent parallel earth return paths and to ensure that there is no reliance on the service pipes for gas and water for effective earthing.
[b]Remember to reconnect the main equipotential bonding after this test[/b].
Check no resistors are in circuit, this will also provide a higher reading

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by sparx » Mon Aug 03, 2009 10:25 pm

Hi, Sd,
how is your Ze? 6mm r1+r2 should not vary much between the 2 runs you have, don't have table to hand but should only differ by a few milli Ohms.
if much different would recheck connections first.
Can you get by with a B20A, allows much higher Zs.
RCD will get you 'out of jail' but why are they so different, if it's a bad joint will still give prob. later so better check first!!,
regards Sparx

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