Trapped key interlocking

Trapped key interlocking utilizes locks and keys for sequential control of equipment and machinery to ensure safe operation. Trapped key interlocks are widely used to ensure safe access to potentially live or dangerous plant or equipment in an industrial setting.

A safe sequence of operations is enabled through transfer of keys that are either trapped or released in a predetermined order. For example, a key is used to isolate a power source (circuit breaker or supply valve), this key is then released and can then be used to gain access through a gate or door to a high risk area by inserting it into an access lock. The key will then remain trapped until the gate or door is closed. A personnel or safety key can be released from the access lock, this ensures that the gate or door can not be closed and the initial key released until this personnel or safety key is returned. This provides increased operator safety.

Trapped key interlocks were used in the French railway system in the 1890s to control track switching operations. Both the Englishman James Harry Castell (1880-1953) and the American R. L. Kirk developed trapped key interlocking systems, and such systems are often referred to as "Castell" or "Kirk" key interlock systems. Both worked in the power generation and distribution industries in the early part of the 20th Century, and both pioneered the use of trapped key interlock for switchgear control. Trapped key interlocks can be found in many industrial settings including electrical utilities, railway, petroleum, and chemical plants as a response to occupational safety and health legislation.

A typical trapped-key interlock device consists of a lock cylinder which operates a sliding bolt through a cam. The assembly is contained in a housing, which is made in different styles to accommodate different applications. The sliding bolt, when extended, mechanically prevents operation of a switch, valve, gate, or other device. Many variations exist, with different shapes of interlock bolt and multiple lock cylinders on an interlock. A significant feature of the interlock is that the key is held or trapped in one position of the lock. Releasing the key indicates that the interlocked device has been made safe; the interlocked device cannot be re-energized until the key has been returned and operated to retract the bolt.

Some complex sequences use key exchange blocks or boxes, that allow alternative sequences of operation. A key exchange block may only hold and release keys and may not have a bolt to interlock process equipment, or may be part of the interlock of a particular machine or device.

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