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Large RCD Uplift

aligarjon

Hi Guys

I installed a sub-main today to a domestic garage. Earthing is TT. Ra is 137ohms. Existing house CU has a front end S type RCD. I have run the sub-main from the S type side of the split load board. I have an r1 + r2 of 0.36 ohms to the garage. When I tested Zdb at the new garage board I have just fitted I am getting a reading of 318 ohms. (garage is attached to house for what it's worth). After a bit of investigation It looks like I am getting this massive uplift through the time delayed RCD. Is there likely to be an issue with the time delay RCD or is this normal. Ordinarily 318ohms would not be acceptable as it could indicate the spike being unstable but that is not the case here. I obviously have 30m/a protection for the outgoing circuits at the garage.

Gary

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  • 0

    When you tested “Ze” you likely did so with a high current. Testing Zdb would have been with a low current otherwise the RCD would have jumped out. I assume you made more than the one test to be confident about the meter reading.

  • 0 in reply to lyledunn

    Yes I did test several times.Very consistent readings as well.  I also tested the house sockets which go through the split load 30m/a rcd  and had a similar reading as the time delay trip was also the main switch as was typical with this setup. 

    As you said the Ra/Ze at the main CU was a high current test.  I was just shocked at the uplift

    Gary

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  • 0 in reply to lyledunn

    Yes I did test several times.Very consistent readings as well.  I also tested the house sockets which go through the split load 30m/a rcd  and had a similar reading as the time delay trip was also the main switch as was typical with this setup. 

    As you said the Ra/Ze at the main CU was a high current test.  I was just shocked at the uplift

    Gary

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  • 0 in reply to aligarjon

    The r1,2 is presumably performed with a low value DC current, where the coils  in the RCD look like a straight wire. I suspect the no-trip 'loop' test is done by keying on and off a small test load very fast (so the RCD does not see it), repeatedly in a pseudo random way, and correlating the change in voltage droop with the change in load. The inductance of the RCD will be higher to L- E current than it is to L-N as there is no cancellation current in the second winding.

    The exact inductance will be a bit indeterminate but over 100j ohm  for a one turn winding seems a bit unlikely even allowing for a 'fast pulse ' test frequency that may be equivalent to something more like a few kHz.  (100j at 10kHz, would require 2mH and it will be slower than that, and a core with that sort of single turn impedance in any normal transformer alloy is bigger than any sensible RCD. )

    If there is any doubt, I think I'd consider a wander lead  style test to verify the earthing to the garage, back to the electrode, or a combined Zs and wander lead sort of thing, much like the 137 ohms result, from incomer live to the garage CPC.

    It is certainly not unknown for 'no-trip' tests to behave oddly through RCD coils and over read by a slack handful of ohms,  but this sounds remarkably high. 

    I presume you dont have a tester of another make to compare notes with ?

    Mike

  • 0 in reply to mapj1

    I wonder what  "Lo" current test would have given for "Ze"/

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