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Needing to meet mcb zs with ln loop.
Former Community Member
Where an rcd is relied on for fault protection due to high zs it has been suggested to me that the loop between live Conductors needs to be less than the mcb max zs. I can't find anything on this anywhere but he said he had this confirmed by an niceic assessor. Have I got it wrong ?
Socket circuit in a TT installation protected by a B32/30mA RCBO.
Disconnection time required by CPD 0.4 seconds rather than 0.2 seconds (Table 41.1), maximum disconnection time by RCD 40 milliseconds.
Maximum earth fault loop impedance for a B32 MCB to disconnect in 0.4 second 1.37 ohms, maximum earth loop impedance for a 30 mA RCD 1666 ohms.
Time/current characteristic for B32 RCBP overcurrent characteristic current for time 0.1 sec to 5.0 second.
So for the RCBO to trip due to a short circuit, overload or an earth fault it needs to satisfy the requirements above.
So the RCBO relies on its RCD function to protect against earth faults and it's MCB function to protect against shorts and overload and all the requirements to allow it to function correctly seem to have been met.
So why isn't the live neutral loop test satisfactory?
Socket circuit in a TT installation protected by a B32/30mA RCBO.
Disconnection time required by CPD 0.4 seconds rather than 0.2 seconds (Table 41.1), maximum disconnection time by RCD 40 milliseconds.
Maximum earth fault loop impedance for a B32 MCB to disconnect in 0.4 second 1.37 ohms, maximum earth loop impedance for a 30 mA RCD 1666 ohms.
Time/current characteristic for B32 RCBP overcurrent characteristic current for time 0.1 sec to 5.0 second.
So for the RCBO to trip due to a short circuit, overload or an earth fault it needs to satisfy the requirements above.
So the RCBO relies on its RCD function to protect against earth faults and it's MCB function to protect against shorts and overload and all the requirements to allow it to function correctly seem to have been met.
So why isn't the live neutral loop test satisfactory?
