Installation test results

Discussion in 'Electrics UK' started by Licklieder, 2 Jun 2021.

  1. Licklieder

    Licklieder

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    So, as discussed in an earlier thread i've been doing some installation testing before swapping out my MCB's for RCBO's. In general, everything is testing out ok - though i did find one overheating crimp joint in one of my 16A heating radials that was put in by the spark who replaced my consumer unit way back when. Worth doing the testing just to catch that. dropped from 0.57 to 0.11 ohm when i remade that joint.

    I have a question regarding my 16A hot water radial wired in singles.

    IR testing is >200MOhm @ 250 and >500MOhm @500V on all cables - so all good there.

    Continuity test results were:

    L-N : 0.11
    N-E : 1.7
    L-E : 1.7

    which would seem to indicate that the CPC has a high resistance. I need to double check the wire size, but from memory line conductors are 1.5mm; so i guess the CPC is 1mm. So far as i've been able to see there are no junctions in the CPC cable (unless there's one behind the MCB's / consumer unit mounting board). Readings were taken between the cables in the consumer unit and the end point of use.

    Should i be concerned about the high CPC continuity reading ? it seems high to me. My estimate of the cable run is 5-6m. by way of comparison the end to end continuity of my final ring is 1.65Ohm and that's for ~ 20m

    All tests were done using my Fluke 1653B, which is strictly speaking out of calibration, but i did buy a calibration reference block and continuity / IR readings were what they should be.
     
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  3. AndyPRK

    AndyPRK

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    As you say the cpc is smaller, so will increase resistance when testing with it.

    YouTube sparks show off (or are good) by multiplying by a factor before measuring end to end continuity of a ring circuit. To get a value they expect to see for the cpc

    x 1.76 for 2.5mm


    Edit. You say singles. I thought a consistent size was use for singles. Maybe not ?
     
    Last edited: 2 Jun 2021
  4. Licklieder

    Licklieder

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    I'll have to measure when i next open up the CU but I seem to recall that in this particular 1969 / 1970 install the cpc is smaller than the live conductors
     
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  5. Licklieder

    Licklieder

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    So, I make that approx 1.5 if going from 1.5mm to 1mm. Doesn't make much difference as my CPC continuity seem to be > x10.
     
  6. JohnW2

    JohnW2

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    Those figures are certainly odd. Do I take it that, for example, "N-E" means neutral+CPC in series?

    The L+N figure of 0.11Ω sounds low for 1.5mm - 6m of that should be about 0.17Ω. Are you sure that it's not 2.5mm² - for which 6m should be about 0.108Ω ?

    As Andy has said, if it's singles, then it would be normal/usual for the CPC to be the same size as the L &N but, even if it is 1.0mm² (and the L&N 1.5mm²), L+E or N+E should only be about 0.22Ω for 6m - so your measurements are way OTT (and if it were 2.5mm² L & N and 1.5mm² CPC, it should be about 0.14Ω).

    Do you have a 'long-ish lead' which would enable to measure the end-to-end resistance of the CPC directly?
    Again odd. You don't say whether you're talking about L, N or CPC, or some combination thereof, but 20m of 2.5mm² should be about 0.36Ω and 20m of 1.5mm² about 0.58Ω - so, even if you were talking about L, N and CPC (2.5/2.5/1.5 mm²) all in series (and each 20m), the total should only be about 1.3Ω.

    Kind Regards, John
     
  7. JohnW2

    JohnW2

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    Arrrggghhh ...
    upload_2021-6-2_15-40-25.png
     
  8. AndyPRK

    AndyPRK

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    .11 x 1.76 = 0.19 Humm I see what you mean

    yeah probably a fault on the cpc.
     
    Last edited: 2 Jun 2021
  9. JohnW2

    JohnW2

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    Apologies,I forgot to divide all those figures by 2 (for single conductors). It should have read...
    ... which, of course, makes your 1.65Ω figure even more surprising.

    Kind Regards, John
     
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  11. JohnW2

    JohnW2

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    Arrrrgggghh again ...
    upload_2021-6-2_16-39-45.png
     
  12. JohnW2

    JohnW2

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    I'm please to hewar that youy so, but I'm not sure I understand :)
    The CPC resistances certainly appewra to be far to high but, as I said (is the the point you were making above?) it sounds as if his L&N may be 2.5mm² - which presumably would make the CPC either 1.5mm² or 2.5mm² - which would make his measurements 'even worse'.

    Kind Regards, John
     
  13. AndyPRK

    AndyPRK

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    Even if it were t&e, it’s still bad as half the conductor length is the thicker line conductor. And the calculator doesn’t consider that
     
  14. JohnW2

    JohnW2

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    What calculator? Mine is in my brain :)

    If one measures the total resistance of a 1.5mm² L and 1.0mm² CPC (measured in series) in a length of T&E, then that total resistance is 36.5 mΩ per meter of the (T+E) - (29mΩ/2 + 44mΩ/2). Hence, as I said, 6 metres should be about 0.22Ω total for L+CPC.

    Kind Regards, John
     
    Last edited: 2 Jun 2021
  15. EFLImpudence

    EFLImpudence

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    Just for accuracy; it's 1.67 (not 1.76) i.e. 2.5/1.5.
     
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  16. JohnW2

    JohnW2

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    Ah, thanks - if that's what he meant, I think I now understand!

    Kind Regards, John
     
  17. Licklieder

    Licklieder

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    Thanks all. good to confirm that what i measured was way out of whack and needs investigating further.

    When i wrote L-E & N-E those were the series measurements of L&N with CPC. I linked them out using a Wago221 in the backbox of the immersion isolator & measured at the ends in the CU. Even tried cleaning the ends with 200 grit paper to remove any oxidation.

    @JohnW2 - the end to end measurement of 1.65 Ohms i mentioned by way of reference on my final ring is the CPC vs ~ 0.45 Ohms for the live conductors. Again, i need to verify cable diameters, but if 2.5mm for line, and 1mm for CPC that's not too far out. I'm part way through investigating why my L-L readings are almost 0.1Ohm higher than my N-N readings and will be replacing some sockets so may yet find a duff connection somewhere that explains that high reading. I've just ordered myself a R2 wander lead (they're expensive! had to resort to ebay to find one at a reasonable price) so i'll try your suggestion of measuring the end to end continuity of the cpc when that arrives.

    I'll have to dig out the completion certificate i have from when my CU was replaced and see what was written on that - the CPC continuity may have always been that high.

    I still need to do my Ze / Zs measurements, but from memory, I think Ze is about 0.35 Ohm - which would give a Zs of ~ 2 Ohmm and a PFC of ~ 115A. By my reckoning, a 16A MCB / RCBO with Type B curve should still trip instantly (Zs @ 2 Ohm < Max permissible Zs @ (230*.95)/(5*16) = 2.43 Ohm). It's not ideal and I'd still like to know why the reading is so high. One for the next time i power down.
     
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