Abstract: A safety interlock apparatus which prevents a machine from operating whenever any of the interlocking conditions are not satisfied. The apparatus provides two levels of protection, where both levels must be satisfied before permitting power to be supplied to the device. The first level is provided by a novel fiber optic switch which cannot be defeated or bypassed in normal operating situations. The switch includes a fiber optic "lock" that must be mated with an infrared "key". The second level of protection involves the monitoring of the AC voltage at the load. For interlock system to be satisfied no voltage can exist without the first level of protection also being satisfied.