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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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  • 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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  • 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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