Fail-safe switch which renders HID lamp inoperative upon breakage of outer envelope

摘要

A high-intensity discharge lamp which has a lamp current-interrupting device in series circuit with the arc tube which is sensitive to any breach occurring in the protective outer envelope. The current-interrupting constituent of the device initially incorporated into the lamp comprises vanadium sesquioxide. The device has predetermined dimensions and an initial predetermined operating resistance. During normal operation of the lamp the device operates at a predetermined temperature sufficiently low so that said device is stable in the presence of the non-reactive atmosphere enclosed by the outer envelope. The predetermined temperature also being sufficiently high so that upon a breach occurring in the outer envelope permitting oxygen to enter, the vanadium sesquioxide rapidly converts to vanadium pentoxide to interrupt the continuity of the power circuit to the arc tube and prevent the operation of the lamp.