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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 ?
balderdash and piffle. The limit for L_N loop is set by other things.
There are two restrictions on L-N loop - firstly voltage drop at full load must be acceptable - and if VD is say a stonking 10% , then you will only just fire a C type in the instant trip part of the curve on an LN fault.
It is quite hard to design a circuit that works where the disconnection time is more than 5 seconds. You may not care unless the case below also applies.
.
Second case is for the adiabatic as above - this only applies if the rating of the cable is thinner than the In of breaker that protects is, i.e. for cases where overload is unlikely or there is overload protection at the load end.
So the danger case is a 32A breaker protecting a 100m run of 1mm2 with a single socket at the end of it.
balderdash and piffle. The limit for L_N loop is set by other things.
There are two restrictions on L-N loop - firstly voltage drop at full load must be acceptable - and if VD is say a stonking 10% , then you will only just fire a C type in the instant trip part of the curve on an LN fault.
It is quite hard to design a circuit that works where the disconnection time is more than 5 seconds. You may not care unless the case below also applies.
.
Second case is for the adiabatic as above - this only applies if the rating of the cable is thinner than the In of breaker that protects is, i.e. for cases where overload is unlikely or there is overload protection at the load end.
So the danger case is a 32A breaker protecting a 100m run of 1mm2 with a single socket at the end of it.
