A relocking device (a.k.a. "external relocker

Relocker

A relocker is one of various mechanisms intended to keep a safe or vault locked in a burglary attempt.There are two basic classes of relockers:* Relock trigger , an internal relocker built into the primary lock...

") is an auxiliary locking device intended to be activated during an attempted burglary of a safe or vault. Such a device will keep a safe or vault locked even if the primary lock is defeated. This independent mechanism is designed to maintain the locked state of a safe even if the lock itself is destroyed. This auxiliary locking device usually consists of a spring-loaded bolt of some type, held in check by a bracket or cable that is rigged to release the mechanism in a burglary attempt. The device will either block the main boltwork from retracting or block the door from opening.

Thermal relocking devices

Designed as a defense against torch attacks, these are simply relocking devices equipped with a fusible link

Fusible link

Fusible links include mechanical and electrical devices.A mechanical fusible link is a device consisting of two strips of metal soldered together with a fusible alloy that is designed to melt at a specific temperature, thus allowing the two pieces to separate...

designed to melt and release the relocking device if the temperature inside the door exceeds a certain temperature (usually 65°C), as would happen in a torch attack.

Glass relocking devices

A glass relocking device is a piece of glass, usually tempered, placed where it might be expected to break in a burglary attack. It is attached, usually with wires, to one or more spring-loaded bolts, which are often randomly located. A drill or torch may break the glass, releasing the bolts.