Hello everyone. I'm working on project where the power source (Transformers) supplying the project loads has its neutral point earthed through a resistance of 20 ohm. Considering a 400V, 3ph system (230V, 1ph), this will lead us to a maximum line to ground of fault of 11.5A. This shall enable us to go with the mimimum CSA of protecive earth conductor as per BS7671 (4mm2 for non mechanically protected conductors). However, for the main bonding conductor within the substation, my understanding that it shall be at least half size of the earthing conductor as per 544.1.1.
In the light of the above, is there a minimum CSA for the main earthing condutor or it can also follow the same regulation of the 4mm2 for non mechanically protected conductors. Please also note that the transformers and medium voltage switchgears are already located within the very same room and will be bonded to the same bonding conductor (maximum LG fault at HV side shall be 700A)
Thanks in advance for your help.