Crowbar resistors are used to protect the outputs of power sources in railway systems against the short-time (transient) and long-time overvoltage failures.

Output voltage rises very fast (in microseconds) when semi-conductors of power source fail (short circuit) or the control circuit collapses. Protection system fully independent from the control system of power source detects the fast increase of the output voltage and switches the crowbar resistor connected to the power output with the semi-conductor switching system. In this case, crowbar resistor create an over current limited to a value that will not damage the power supply but will activate the protection equipment such as fuses and circuit breakers and attenuate the over voltages very quickly.