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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 ?
when the fault comes on, the voltage drop will be 100% - there will be no L-N voltage at all at the fault point during a worst case fault.
If the supply is a stiff (non collapsing source), but a long resistive distribution, then we can assume that the open circuit voltage divided by the loop resistance will set the current that flows. At the point of LN fault itself, the voltage to earth will be about 50% of the open circuit, as we expect equal voltage drop in L and N as , as we assume that all the volt drop is in the wires and they are equal sizes, so the voltage in the live will rise towards the origin, and the voltage in the neutral will fall towards earth.
Of course, this is not good for the wire, but not dangerous for an earthed person, because as soon as they touch the faulty part some current is diverted, L and N current is not equal, and the RCD kicks in.
when the fault comes on, the voltage drop will be 100% - there will be no L-N voltage at all at the fault point during a worst case fault.
If the supply is a stiff (non collapsing source), but a long resistive distribution, then we can assume that the open circuit voltage divided by the loop resistance will set the current that flows. At the point of LN fault itself, the voltage to earth will be about 50% of the open circuit, as we expect equal voltage drop in L and N as , as we assume that all the volt drop is in the wires and they are equal sizes, so the voltage in the live will rise towards the origin, and the voltage in the neutral will fall towards earth.
Of course, this is not good for the wire, but not dangerous for an earthed person, because as soon as they touch the faulty part some current is diverted, L and N current is not equal, and the RCD kicks in.
