An auto-guard was a security device that was armed when no crew was present to guard a facility. It sensed disruptive activity in a corridor that was meant to be still, and discharged powerful electrical rays upon any offending moving objects it detected within its range. Aboard Nerva Beacon, a Wirrn Queen was killed when she was attacked by an auto-guard. She managed to disable the device and plant an egg within Dune before dying.
The Fourth Doctor unintentionally reactivated the auto-guard while trying to turn the oxygen on. By using one of Harry Sullivan's shoes to distract it, he was able to use the cut out and deactivate it. (TV: The Ark in Space)