This was a question at a planning meeting for a new project.
"" If all circuits have RCD protection that trip at 35 milliamps why does the CPC have to have an impedance less than 1000 ohms ( yes one thousand ohms ) ?
Reasoning for the proposal was as follows.....
A Live to CPC short with a 1000 ohm CPC will create a fault current of 230 milliamps which will un-balance and trip the RCD. If the CPC impedence is about 1 ohm then the MCB will trip on the the 100 amp plus fault over current. This trip may happen before the RCD trips on the 100 amp unbalance. ( possibly the RCD core may be instantly saturated at that fault current and not trip )
It is 100 amps plus and not 230 amps because I fault = 230 / (Rcpc + Rlive)
A Live to Neutral short will not create an un-balance in the RCD and it will not trip but hopefully the MCB will trip on the over current.
So if all circuits are RCD protected at 35 milliamp fault current then CPCs and earth rods need not be low impedance.
As long as all Live to CPC faults, either direct or via a body or other item, result in an un-balance adequate to ensure fast tripping of the RCD the CPC is effective.
The first reaction was " That is dangerous rubbish ""
The proposer then asked. "" If you were getting an electric shock from a live wire would you expect the RCB to operate and save you ? ""
The answer was " Yes of course. that is what RCDs are intended for. "
The proposer then asked " Then you ensure your body's impedance is always low enough that it complies with the impedance of a CPC for that installation where you got the electric shock ? "
"" If all circuits have RCD protection that trip at 35 milliamps why does the CPC have to have an impedance less than 1000 ohms ( yes one thousand ohms ) ?
Reasoning for the proposal was as follows.....
A Live to CPC short with a 1000 ohm CPC will create a fault current of 230 milliamps which will un-balance and trip the RCD. If the CPC impedence is about 1 ohm then the MCB will trip on the the 100 amp plus fault over current. This trip may happen before the RCD trips on the 100 amp unbalance. ( possibly the RCD core may be instantly saturated at that fault current and not trip )
It is 100 amps plus and not 230 amps because I fault = 230 / (Rcpc + Rlive)
A Live to Neutral short will not create an un-balance in the RCD and it will not trip but hopefully the MCB will trip on the over current.
So if all circuits are RCD protected at 35 milliamp fault current then CPCs and earth rods need not be low impedance.
As long as all Live to CPC faults, either direct or via a body or other item, result in an un-balance adequate to ensure fast tripping of the RCD the CPC is effective.
The first reaction was " That is dangerous rubbish ""
The proposer then asked. "" If you were getting an electric shock from a live wire would you expect the RCB to operate and save you ? ""
The answer was " Yes of course. that is what RCDs are intended for. "
The proposer then asked " Then you ensure your body's impedance is always low enough that it complies with the impedance of a CPC for that installation where you got the electric shock ? "