An external interlock is a safety circuit provided for customer use. Most interlock circuits consist of two terminals provided on the customer interface connector. A connection must be made between these two points for the power supply to be enabled into the HV ON mode. It is strongly recommended that these interlock connections be made via fail safe electro-mechanical components (switches, contactors, relays) as opposed to semiconductor transistor devices. If the power supply is already in the HV ON mode and the connection is broken between these points, the unit will revert to the HV OFF mode.

This simple circuit allows the customer to connect their own safety interlock switch to the power supply. This switch could be an interlock connection on a HV access panel. In this way, if the panel was inadvertently opened, the high voltage would be turned off, greatly reducing the risk of bodily harm or physical injury. Spellman strongly recommends the use of interlock circuitry whenever possible.