Abstract: A voltage spike suppressor for polyphase systems including those with alternatively connectable polyphase sources, the suppressor having a diode bridge network connected to couple overvoltage transients exceeding the nominal line voltage by more than twice the forward drop of a diode in the bridge network to a capacitor and resistor network. In four-wire systems, a center-tap in the capacitor and resistor network can be connected to the neutral lead in the power circuit. Metal Oxide Varistors may be used to clamp the center-tap voltage in the capacitor-resistor network. A diode clamp circuit may be used to limit common-mode voltage transient swings in the capacitor-resistor network and in the neutral terminal when present.