Cross sectional area of a protective conductors

The 11kV side will likely have a Neutral Earthing Resistance of something like 6.4 Ohms so the HV fault current under EF is limited to around 1000A - of course it could also be a 4 ohm unit and they may be in parallel on the HV feeders so 1.7kA seems credible. It may be an earthing transformer that replaces the NER's. In any event, the HV electrode (or the steelwork earth) is simply to limit earth potential rise and operate earth fault protection. The OP notes that this is a fully cabled system with earthed cable screens (ie the reference to "global" earthing)


It is simply not credible to put 100kA into the electrode - expect concrete to be spalling off in all directions if that was true due to rebar heating.


As suggested, draw out the fault loop - that should show that the rebar and bonding conductors are in parallel with a thumping great chunk of copper which is the neutral plus the earthed screens of this conductor and each phase conductor - the relative resistances should be showing you that the division of current will dominate in the neutral not the electrode or the bonding.


Depending on the type of facility, the design might consider the possible fault scenario where the phases are intact, the system neutral is severed and under earth fault, the only route would be via bonding and rebar back to the transformer - but that also assumes a catastrophic loss of system protection etc. I might consider that scenario for certain specific clients where a LOOP event would really be quite dangerous, but if so, we would also be designing deep earthing electrodes under the building, surface earthing electrodes around the building and so many divergent paths of the mesh back to the transformer slabs as to still make multiple 300mm2 conductors unlikely. Keep in mind that even at lightning levels of fault current, most standards still only mandate between 50mm2 and 70mm2 of copper (typical copper tape is 3mm x 25mm to give 75mm2 CSA)


Regards


OMS

The 11kV side will likely have a Neutral Earthing Resistance of something like 6.4 Ohms so the HV fault current under EF is limited to around 1000A - of course it could also be a 4 ohm unit and they may be in parallel on the HV feeders so 1.7kA seems credible. It may be an earthing transformer that replaces the NER's. In any event, the HV electrode (or the steelwork earth) is simply to limit earth potential rise and operate earth fault protection. The OP notes that this is a fully cabled system with earthed cable screens (ie the reference to "global" earthing)


It is simply not credible to put 100kA into the electrode - expect concrete to be spalling off in all directions if that was true due to rebar heating.


As suggested, draw out the fault loop - that should show that the rebar and bonding conductors are in parallel with a thumping great chunk of copper which is the neutral plus the earthed screens of this conductor and each phase conductor - the relative resistances should be showing you that the division of current will dominate in the neutral not the electrode or the bonding.


Depending on the type of facility, the design might consider the possible fault scenario where the phases are intact, the system neutral is severed and under earth fault, the only route would be via bonding and rebar back to the transformer - but that also assumes a catastrophic loss of system protection etc. I might consider that scenario for certain specific clients where a LOOP event would really be quite dangerous, but if so, we would also be designing deep earthing electrodes under the building, surface earthing electrodes around the building and so many divergent paths of the mesh back to the transformer slabs as to still make multiple 300mm2 conductors unlikely. Keep in mind that even at lightning levels of fault current, most standards still only mandate between 50mm2 and 70mm2 of copper (typical copper tape is 3mm x 25mm to give 75mm2 CSA)


Regards


OMS