Connect - Sizing of Earthing Conductor

My project has an LV supply fed via a sub-station with an outgoing 800A fuse. The supply is 90m and connects to a panel board with an incoming 630A MCCB.

The earth sizing is currently calculated at 2x185mm2 cables in parallel. When I fix this to a single cable of 400mm2 CSA the disconnection time is not achieved and the circuit fails. When connect is left to calculate on a single earth the 630mm2 cable is still not sufficient.  

Having carried out a check adiabatic calc using the values of I2t in the model, and with fuse data, the 400mm2 earth is more than sufficient. And as this exceeds the total CSA of 2x185mm2 cables should be sufficient in the model to clear the fault in under 5 seconds.

Can you please some commentary on the above sizing.

Thanks

Stuart O'Loughlin

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