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Maximum ZS exceeded

Newfutile

i am testing in some flats which has cleaners sockets (in communal areas) wired as a ring final.

these have C32 rcbo circuit protection.

i have measured 1.0 ohm ZS on the 4th floor sockets, maximum zs (80% OSG) would be 0.54 ohm.

as its a rcbo this meets ADS for earth fault current via the rcd part of the rcbo.

if we assume the neutral is the same as cpc (its probably not) this will take over 5 seconds for a line to neutral fault as only 230 amps will flow, a C32 needs 320 amps to disconnect on the flat part of the curve.

we own the flats so its up to me to make any remedial repairs i deem required.

should i change the rcbo’s  for lower rated B curve type?

Do we even have line - neutral disconnection times? 

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    Do we even have line - neutral disconnection times? 
    no we don't  its not a safety of life matter, just overheating and or risk of slow insualtion damage- we do have a voltage drop requirement at full load though, which sets some sort of upper bound on the L-N loop .

    So at 32A ,  if you had a 1 ohm L-N loop you may have lost 32V (hahaha - that may be OK on some South American holiday destinations but this is the UK so back to the drawing board )- now it rather depends how much is in the live on the way out or the earth on the way  back. 
    I'd starting with no more remedial than plug the tester in again with NE swapped an see if you really do have that weedy a supply.  If so, a 16a breaker is the cheapest way forward ;-) 
    M.

    Sanity check ,estimate the cable run length to the 4th floor and round the loop. and see if it stacks up. (or post length and cable type here and I can impress you by messing up some simple arithmetic in public.. )

    Or power down and pull the worst  4th floor socket off and ring round with the ohm meter L-L, NN and EE and compare - so easy to test on a ring so impossible on a radial. Maybe its a long spur wired in thin spaghetti.

  • 0
    Newfutile said:
    these have C32 rcbo circuit protection

    Why?

    "Cleaners sockets" implies plugging in a vacuum cleaner, or perhaps floor polisher, for limited periods on an occasional basis. I doubt that there will be a whole army of cleaners operating together. So, the load should be moderate.

    I don't think that 1 ohm need be excessive, and of course it depends somewhat on the Ze/Zdb.

    I'd install a B32 or even B20.

  • 0
    Newfutile said:
    Do we even have line - neutral disconnection times?

    We do where the conductor size is less than would be expected given the rating of the over-current device (i.e. it won't automatically provide overload protection) - a ring final with 20A conductors on a 32A MCB is one example of that. It's not a fixed time, but however long it would take the conductors to reach an unacceptable temperature - the adiabatic equation does that (for up to around 5s at least - longer than that it tends to be unduly pessimistic). MCB manufacturer's energy-let through data I think doesn't work once you're past the "instantaneous" part of the curve, so it's the old-school approach plugging values into t=(k² S²)/I² and comparing that with the disconnection time from the graph for the fault current I.

       - Andy.

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    Newfutile said:
    as its a rcbo this meets ADS for earth fault current via the rcd part of the rcbo.

    Unless the RCBO is functionally dependent on line voltage with a classification of 'E2' or 'E3' according to Table Z1 of BS EN 61009-1:2010+A1:2012+A2:2013. See Table 6.3.4 of 2026 On-Site Guide, page 47, row that starts at left hand column 'characteristic' with 'Method of operation according to dependence on line voltage (Table Z1 of BS EN 61008-1 and Table Z1 of BS EN 61009-1)'

    If it's functionally dependent on line voltage and classified as E1 (or by  inference if there is no marking for line voltage dependence, then it must be line voltage independent ... but if it's a global product to IEC 61009-1 it might not have a marking even if it is line voltage dependent) these products can be used for ADS as well as additional protection.

    See this BEAMA guide: https://www.beama.org.uk/static/dda8f294-e43c-484f-8061e92b1f1e64e0/Technical-Bulletin-Guide-to-Residual-Current-Device-RCD-E-Marking.pdf 

    The guide also notes that some RCBOs on the market are currently marked 'E3' although the device might actually be suitable for both ADS and additional protection ... so probably best to check with the manufacturer.

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