well the 10mm copper cores will give you ~ 1.6 milliohms per meter to begin with, so 80 milliohms in the live core. (cold copper - if you are sweating it the 1,6 becomes more like 1,9)

Now are these 3 cores 3phases, or single phase with one core as earth ?

If the latter the armour is in shunt with the copper core, so 80 milliohms parallel the steel wire If 3 phases, the steel wire is on its own

the CSA of the steel on 10mm 3 core is about  40-45mm2 but of course steel is ~ 10 times more resistive than copper, so it is more like 4- 4.5mm2 copper equivalent so if you have a copper core CPC the copper takes almost 2/3 of the current..

The usual solution to a supply with a  high earth loop feeding armoured cables is a 300mA RCD, (or maybe a 100mA type ) either instant or delay type to trap the earth faults, and leave the fuse to do the L-N shorts.

Mike

well the 10mm copper cores will give you ~ 1.6 milliohms per meter to begin with, so 80 milliohms in the live core. (cold copper - if you are sweating it the 1,6 becomes more like 1,9)

Now are these 3 cores 3phases, or single phase with one core as earth ?

If the latter the armour is in shunt with the copper core, so 80 milliohms parallel the steel wire If 3 phases, the steel wire is on its own

the CSA of the steel on 10mm 3 core is about  40-45mm2 but of course steel is ~ 10 times more resistive than copper, so it is more like 4- 4.5mm2 copper equivalent so if you have a copper core CPC the copper takes almost 2/3 of the current..

The usual solution to a supply with a  high earth loop feeding armoured cables is a 300mA RCD, (or maybe a 100mA type ) either instant or delay type to trap the earth faults, and leave the fuse to do the L-N shorts.

Mike