Calculation of Short‐Circuit Currents

This chapter provides discussion on the short‐circuit currents and sets up the equations required to determine these currents. For the dimensioning of electrical systems, it is necessary to consider three‐phase short circuits in order to guarantee the mechanical and thermal stabilities of the systems and the rated making and breaking capabilities of the overcurrent protection equipment. Two‐phase short‐circuit currents without contact to ground can be larger with powerful asynchronous motors than for three‐phase short circuits. The single‐phase short‐circuit current occurs frequently in electrical networks. The symmetrical breaking current is the effective value of the short‐circuit current I'k (t), which flows through the switch at the time of the first contact separation and is used for near‐to‐generator short‐circuit feeder. The steady‐state short‐circuit current is the effective value of the short‐circuit current remaining after the decay of all transient processes. It depends strongly on the excitation current, excitation system, and saturation of the synchronous machine.