Mistura Systems has a safety interlock for large machines which acts directly on the main power isolator. Rather than a safety interlock as part of the control circuit or emergency stop, this consists of a trapped key, manufactured from stainless steel to ensure strength and durability, offering control over a large switch which breaks the main power feed to the machine. Safety interlocking of large machines is problematic. It can be expensive and complicated, involving dual circuits, crossmonitoring of contactors and special relays to ensure safety. This new interlock is a very practical method of power interlocking. It is capable of switching 315A, 75kW of power, and means that safety is ensured with very little additional wiring. The power must be turned off before the trapped key can be removed, and power can only be restored once the key is returned to the lock. The key is effectively a personnel safety key that ensures potentially hazardous maintenance can be carried out safely. It also provides protection for the equipment against abuse or incorrect operation as power can only be restored by authorised personnel.