The Source Impedance consists of three inductors, one for each phase of a three-phase AC power network, installed in a half-height EMS module. An additional resistor, of small resistance value, is connected in series with each inductor to keep the reactance-resistance (X/R) ratio of the source impedance close to that of the synchronous generator. The Source Impedance module is designed to limit the short-circuit current drawn from the ac power source when response of protective relays is being tested.