This investigation pertains to the composition and general condition of the thermally activated trigger mechanism of Pressure Relief Devices PRD's], safety devices used on compressed natural gas cylinders commonly used to store fuel on transit buses. These trigger mechanisms provide the essential function of the PRD, and any problems with the trigger will result in poor device performance. The report analyses the condition of the eutectic material used in the triggers, its composition, condition, and any flaws that may occur. Following the experimental investigation, failure criterion for the triggers were computed based on observed flaws in the trigger material. These computations provide a measure of the reliability of the triggering devices, and thereby of their effectiveness.