[1] The device operates by shunting excess current when the induced voltage exceeds the avalanche breakdown potential.

It automatically resets when the overvoltage goes away, but absorbs much more of the transient energy internally than a similarly rated crowbar device.

A unidirectional device operates as a rectifier in the forward direction like any other avalanche diode, but is made and tested to handle very large peak currents.

The actual clamping occurs in roughly one picosecond, but in a practical circuit the inductance of the wires leading to the device imposes a higher limit.

These fast over-voltage transients are present on all distribution networks and can be caused by either internal or external events, such as lightning or motor arcing.