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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
    aligarjon said:
    Ra is 137ohms

    Is that the actual 'RA' of the final circuit (i.e. to the furthest exposed-conductive-part from the earth electrode in the garage), or the RA for the CU ... might we call that RA(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 RA (and/or Ze) for the main DB, so if you have verified (R2 to furthest points of garage, PLUS R2 of garage sub-main, PLUS RA(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 ZDB for the garage CU, BUT you can always derive that from the (R1+R2) of the sub-main

  • 0 in reply to gkenyon

    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

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  • 0 in reply to gkenyon

    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

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  • 0 in reply to aligarjon

    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.

  • 0 in reply to aligarjon
    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?

  • 0 in reply to Chris Pearson

    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.

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