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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 ?
So if someone has their fingers in an appliance precisely at the moment there is a dead short and they have a finger in contact with the conductors or a part made live their body introduces an earth fault and should fire the RCD off, assuming there is one installed or if insufficient current flows through them to fire the RCD because they have insulating slippers on and are standing on a sheet of vinyl flooring they will have a voltage on them, which is in theory around half the source voltage, so around 120 volts and stay standing there like a bird on a wire.
It is hard to come up with a scenario where that would happen, too many ifs and buts to make likely.
So I have to stop again and think about the ifs and buts, in amongst these is whether or not the RCBO or RCD has a flying earth lead to reference it to earth a matter to take into consideration? At this site the RCBOs in the shed distribution board all have flying earth leads, so are referenced to earth.
I am back in the shed where the socket I used as an example is, the estimated 17 volt drop for a 13 amp load is very generous as the impedance includes Ze outside of the installation. Before I sat down for a drink and a cheese roll I measured the voltage at 245 volts then whacked a load of around 7.3 kW/ 30 amps on as a load
which pulled the voltage down by 4 volts to 241 volts and I cannot foresee it dipping much below that, there’s just not enough in the shed; and from previous discussions I take it we are only really verifying the voltage drop on fixed appliances as socket circuit voltage drop depends on usage.
So if someone has their fingers in an appliance precisely at the moment there is a dead short and they have a finger in contact with the conductors or a part made live their body introduces an earth fault and should fire the RCD off, assuming there is one installed or if insufficient current flows through them to fire the RCD because they have insulating slippers on and are standing on a sheet of vinyl flooring they will have a voltage on them, which is in theory around half the source voltage, so around 120 volts and stay standing there like a bird on a wire.
It is hard to come up with a scenario where that would happen, too many ifs and buts to make likely.
So I have to stop again and think about the ifs and buts, in amongst these is whether or not the RCBO or RCD has a flying earth lead to reference it to earth a matter to take into consideration? At this site the RCBOs in the shed distribution board all have flying earth leads, so are referenced to earth.
I am back in the shed where the socket I used as an example is, the estimated 17 volt drop for a 13 amp load is very generous as the impedance includes Ze outside of the installation. Before I sat down for a drink and a cheese roll I measured the voltage at 245 volts then whacked a load of around 7.3 kW/ 30 amps on as a load
which pulled the voltage down by 4 volts to 241 volts and I cannot foresee it dipping much below that, there’s just not enough in the shed; and from previous discussions I take it we are only really verifying the voltage drop on fixed appliances as socket circuit voltage drop depends on usage.
