A trapped key interlock system is disclosed. A pair of hollow tubular casings located one inside the other and each having a series of elongate slots are used. When the lock is in a locked condition the elongate slots are aligned as inner and outer pairs with engaging members extending through the pairs of slots is preventing rotational movement of the tubular casings relative to each other. The engaging members are biased towards this locked condition. On insertion of the correctly coded key the engaging members are caused to move against the biasing force out of engagement with the slots in the outer tubular casing thereby allowing the inner tubular casing to rotate relative to the outer tubular casing and moving the lock to an unlocked condition.
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