A light weight, self-contained, portable, intrinsically-safe warning device for providing warning to personnel of an unsafe condition is provided. This warning device is especially adapted for attachment to the roof of a mine to indicate unsupported roof conditions or other unsafe conditions. This intrinsically-safe...http://www.google.ca/patents/US6469619?utm_source=gb-gplus-sharePatent US6469619 - Intrinsically-safe roof hazard alert module
