A relocker is a safe component that permanently locks the safe when a condition is met. The primary purpose of relockers is to increase the time necessary to penetrate a safe, though they may also offer evidence of non-standard use. Relockers are typically triggered by vibration, heat, temperature, or the breaking of a component or barrier, such as a pane of glass.
The glass relocker is commonly a sheet of tempered glass that sits inside the safe-side of the safe door that holds out spring loaded bolts, if broken (by forced attack such as drilling or explosives) the glass breaks, allowing the bolts to spring into the locked position preventing the safe from being opened even with the correct combination.
Care must be taken when moving safes with a glass relocker, as even the slightest bump could cause the relocker to lock.
Some safes are fitted with a thermal relocker to lock the safe should an attacker attempt to open it with a thermal lance or torch, if the thermal relocker reaches a set temperature, it will allow the bolts to slide into the locked position.
Generally these use a "Fusible Link" - two piece of metal soldered together with a metal of a set melting point, causing them to separate at a set temperature.