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The paper describes a computer method for analysing substation electrical configuration in order to define protection zones in the substation, and to devise the correct strategy to be adopted in the event of a fault being detected in any of the zones. The protection strategy logic provides maximum discrimination in fault location, takes account of changes in substation switching arrangements, considers remote tripping requirements, is able to handle simultaneous faults, and includes the facility to initiate back-up protection operations when necessary. The scheme ensures fail-safe operation in the event of a wide range of switch status indication errors, and voltage and current measurement errors. The scheme is suited to a substation whose control functions are co-ordinated by an on-line digital computer. The results of a simulated application of the methods to a 132 kV substation are described.