Although it is possible to ‘crack’ a combination lock, its rarely a method used by criminals, or even by safe manufacturers called to open a safe whose combination has been lost. Many safes have time locks, so even if the combination is cracked, the safe will not open outside office hours. All post offices have time locks on their safes. Many safes also have a simple microswitch inside the combination lock connected to the alarm system to trigger it whenever the dial is turned. For these reasons, most attempts to break into safes by-pass the lock altogether. One method is to ‘drill’ the door. With engineering drawings of the bolt mechanism, it is possible to find a point on the safe door to drill through. A screwdriver can then be pushed through and manipulated to release the bolt mechanism – by-passing the combination lock altogether.It goes on describing all sorts of techniques and the counters used against them.
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A popular alternative to ‘drilling’ safes is to use explosives. Gunpowder was used with some success in 19th century America, simply pouring it in through the keyhole. The shock burst the front plate of the door open, making the bolt mechanism accessible.