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

In combination with a high-intensity discharge lamp comprising a sealed elongated radiation-transmitting arc tube which is enclosed by and supported within a sealed light-transmitting protective outer envelope which is opaque to short wavelength ultraviolet radiations, said arc tube enclosing a discharge-sustaining filling and having electrodes operatively positioned therein proximate the ends thereof, electrical lead-in means extending through said arc tube and electrically connected to said electrodes, electrical adapter means affixed to the outer surface of said protective envelope to facilitate electrical connection of said lamp to a source of electrical power, and electrical conductor means electrically connecting said electrical adapter means to said electrical lead-in means, the environment included between said arc tube and said protective envelope being non-reactive with respect to lamp components exposed thereto, and said lamp having a predetermined normal operating current and a predetermined start-up current, the improvement which comprises:

• a lamp-current-interrupting device included in series-circuit arrangement with said electrical conductor means to conduct lamp current when there occurs a breach of said outer protective envelope to permit oxygen to enter therein, said lamp-current-interrupting device having predetermined dimensions and comprising vanadium sesquioxide and having a predetermined electrical resistance, the predetermined dimensions of said lamp-current-interrupting device coupled with the predetermined electrical resistance thereof and the normal operating environment within said protective outer envelope causing said current-interrupting device, as included in current-conducting series-circuit arrangement with said electrical conductor means, to be at a predetermined temperature sufficiently low that said current-interrupting device is stabe in the presence of said non-reactive atmosphere which is normally enclosed by said protective envelope, and upon a breach of said outer protective envelope, the predetermined temperature of said current-interrupting device as included in current-conducting series-circuit arrangement with said electrical conductor means being sufficiently high that contact with oxygen admitted into said breached outer protective envelope will rapidly convert said vanadium sesquioxide to vanadium pentoxide which in turn will interrupt the continuity of said electrical conductor means to prevent the operation of said lamp.