Large RCD Uplift

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.

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

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

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

aligarjon said:

Ra is 137ohms

Is that the actual 'R_A' of the final circuit (i.e. to the furthest exposed-conductive-part from the earth electrode in the garage), or the R_A for the CU ... might we call that R_A(DB)?

I know this is a whole new level of a******e pedantry, BUT ...

Anyway ... 

aligarjon said:

When I tested Zdb at the new garage board I have just fitted I am getting a reading of 318 ohms.

Is that test necessary? See Regulation 643.7.1 b).

You have R_A (and/or Z_e) for the main DB, so if you have verified (R2 to furthest points of garage, PLUS R2 of garage sub-main, PLUS R_A(DB)) does not exceed Table 41.5, and tested each RCD is operating correctly, then isn't that all that BS 7671 needs?

(I know forms ask for Z_DB for the garage CU, BUT you can always derive that from the (R₁+R₂) of the sub-main

The Ra reading was taken at the consumer unit at the meter position where the house consumer unit is. This is where my sub-main originates. There is not a spike at the garage. The garage is attached to the house.   The 0.36ohm  r1+r2 reading is the distribution circuit to the garage.  I agree I could have calculated the Zdb at the garage but I did a live test  on the new sockets in the garage that gave the higher than expected reading so tested at the new CU as well, then all the way back to the house CU. The reading changes significantly either side of the 100m/a time delay trip so I know that is where it happens.   I am confident the circuits I have installed are sound.  I have seen plenty of RCD uplifts over the years but not to this level.  I possibly haven't seen them on a TT system to be fair.

Gary

I'm not saying you shouldn't be asking ... it's a very fair question from a practical perspective.

However, a good illustrative point as to why BS 7671 doesn't always require what would, at first, seem the obvious choice.

aligarjon said:

I am confident the circuits I have installed are sound.

I am sure that they are, but what about the MFT?

If the tolerance for each measurement (high current and no trip) is 10%, that would give a maximum EFLI of 151 Ω upstream of the RCD and a minimum of 284 Ω downstream of it.

It seems to me that either the RCD is introducing a high resistance, or the MFT is faulty. Time for a calibration check?

The RCD is introducing  a high impedance, not a resistance at DC or 50Hz, but presumably un-cancelled inductance (only seen on LE path as as there is no reverse current to unmagnetise the core going up the neutral) that is upsetting  whatever weird fast bouncy waveform the tester uses for its 'no trip' thing. 

I agree, it seems like a lot - I'd have expected to see single figure ohms.
Mike.

Could the charging of the capacitor in the delay circuit of the S-type RCD account for the droop(s) in the voltage provided by the tester thus giving higher than expected results?