False High Zs Readings

by Editor on December 24, 2016

Happy Christmas from Electricians Blog!

Have you been puzzled by higher than expected Zs test readings lately?

High Zs Impedance Test Reading

High Zs At RCDs

I have recently replaced several consumer units. Test results from the Hagar units were fine but the last couple, an MCG And an MK both gave me Zs final circuit impedance readings of almost 1 Ohm higher than expected.

The odd thing was that the Ze readings were as expected and continuity tests on the leads were also good. Thinking that I was missing something, going mad, or that the test leads to my DiLog Multifunction tester were faulty, I replaced them. But the new test leads gave the same results.

It took me a while to figure out the problem but eventually found that the false high Zs readings were being caused by the RCD devices.
Earth loop testing on the input side of each RCD would give similar to the Ze ie. 0.20 Ohms, but the output Zs was 0.97 Ohms. The RCD itself was adding 0.77 .

This problem was confirmed by my ELECSA assessor last week. Apparently the higher impedance readings can be caused by a combination of the RCD and tester’s “Non-Trip” circuitry. The older “D-Lock” Testers don’t seem to create these false Zs readings but unfortunately will likely trip BS-EN61009 RCDs.

The true Zs result can be calculated by subtracting the reading at the RCD input from the output (in this case 0.97 – 0.20 = 0.77) and then subtracting that result from your sub-circuit Zs.
This is definately someting worth being aware of.

More About D-Lock
Robin’s patented D-Lock system uses a DC voltage high current test pulse (20Amps) to “non trip” RCDs for Zs testing. This method worked ok with older BS4293 RCDs but the later BS-EN 61009 types are much more sensitive and the D-Lock test often causes RCD tripping. Hence the introduction of newer lower current and multi pulse “non-trip” circuitry in later Earth Loop Impedance Testers.


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