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I think your observation (problem?) with diverted neutral currents to earth through the cpcs may be as a result of your oil fired boiler and its external tank and pipework between the two. I wonder if the pipework is metal so that the tank and it are acting as another earth electrode which is close the DNO's earth electrode where the TNS has been converted to TNCS (see # below). How the current divides between the DNO electrode and the oil tank set-up depends on their relative earth electrode resistances. It is notable that the biggest change in cpc current is when the cpc to the boiler and manifold is removed so maybe there is something in what I have said?
Now I am not experienced in such a set-up but is it not the norm for the pipe to be plastic and the tank connected to earth by an electrode so that when the tanker comes to fill up there are no touch voltages to terra firma or between tanker and tank?
If the pipe is metal I remember reading (but cannot find a reference quickly) that there should be a plastic insert in the pipe to isolate the tank from the dwelling's earthing system. At the dwelling end the metal oil pipe is bonded to the MET.
Moving on, design effort needs to be put into how the stand-by generator (and its separate fuel tank?) is/are going to integrated with the installation, taking into account ADS, the earthing and bonding systems of mains supply and generator, isolation, switchover, etcetera. But you have probably begun to think of that.....
Just some thoughts. Others can correct or amplify further.
Re# Again I assume - cannot be certain then - that to convert an underground TNS cable to TNCS the cable is located close to the dwelling, N is connected to a buried earth electrode to establish the connection between the local terra firma and the dwelling's internal cable bonded equipotential zone and make a CNE, L and CNE then enter the dwelling and CNE is separated to provide N and E/MET. The supply cable's metal sheathing is continued into the dwelling but insulated to prevent use - but remains earthed via star point at transformer.
Now I am not experienced in such a set-up but is it not the norm for the pipe to be plastic and the tank connected to earth by an electrode so that when the tanker comes to fill up there are no touch voltages to terra firma or between tanker and tank?
If the pipe is metal I remember reading (but cannot find a reference quickly) that there should be a plastic insert in the pipe to isolate the tank from the dwelling's earthing system. At the dwelling end the metal oil pipe is bonded to the MET.
Moving on, design effort needs to be put into how the stand-by generator (and its separate fuel tank?) is/are going to integrated with the installation, taking into account ADS, the earthing and bonding systems of mains supply and generator, isolation, switchover, etcetera. But you have probably begun to think of that.....
Just some thoughts. Others can correct or amplify further.
Re# Again I assume - cannot be certain then - that to convert an underground TNS cable to TNCS the cable is located close to the dwelling, N is connected to a buried earth electrode to establish the connection between the local terra firma and the dwelling's internal cable bonded equipotential zone and make a CNE, L and CNE then enter the dwelling and CNE is separated to provide N and E/MET. The supply cable's metal sheathing is continued into the dwelling but insulated to prevent use - but remains earthed via star point at transformer.
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