Customers have no control over incoming supply fault levels and utility suppliers have no obligation or policy to keep customers informed of changing fault levels.  Fault levels may change due to switching arrangements or changing of cables and equipment for the purpose of the increasing capacity.  For this reason it is safer to base calculations on transformer(s) let-through levels.  While the actual fault levels may be lower the values listed below are the maximum fault levels which these transformers can deliver.

Proper selection of earthing and short-circuiting equipment is based on steady state fault current, peak current, tripping times, electromagnetic and thermal stresses.  Transformer impedance and power factor values have a significant import to fault current calculations.  Worse case conditions should be addressed if current limiting devices are not installed.  The following tables may be helpful in selecting effective earthing and short-circuiting equipment.  Impedance values of 4, 6, 8 and 10.8% and a power factor of 0.85 give the following approximate figures for non-meshed series installations.  The contribution to peak current of load and alternative supplies are not included.

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