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A relocking device (RLD) (a.k.a. "external relocker") 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. Glass relockers are one of the most common types of relockers used in today's safes.
Relockers are typically designed for one-time activation, meaning that once they are triggered the device is locked "permanently" and can only be opened by brute force.
Designed as a defense against torch attacks, these are simply relocking devices equipped with a fusible link 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.
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.