A fault clearing time is required for determining permissible touch and step voltage limits.
The fault clearing time should be determined along with the earth fault current.
If the upstream protection settings are not known, the following general protection clearing times may be used (generally applicable for 132 - 11 kV networks) . These clearing times are based on a worst case scenario. Linear interpolation between values may be used (for example, a Fault current of 6000 A would equate to a Primary clearance time of 0.65 s).
 Earthing Design Instruction, Distribution earthing design, Document No: EDI 100 Amendment No: 1, Endeavour Energy.