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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.

Reply
  • 0

    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.

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