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The paper describes a computer-aided protection evaluation equipment which has been developed primarily to investigate protective relay performance during the period immediately following fault inception. Modern mathematical techniques allow the transient behaviour of a power system to be determined and the equipment described permits this calculated information to be used to evaluate relay performance. The equipment comprises two main elementsÂ¿a simulation executed on a computer and a means of using the simulation to test protective relays. Novel techniques are described for removing sampling effects from the analogue signals, for small signal modelling of transducers and for the evaluation of unit protection schemes by the simultaneous application of currents and voltages from two points on a power system. Both the hardware and operating software are detailed and information is given concerning the computer simulation of the power system.