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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 (assuming that no duplicate keys are available). This provides increased operator safety.
In 1893, French inventor Paul Bouré created engagement lock devices to ensures train traffic safety. They were used in the French railway system in the 1890s to control track switching operations and were manufactured by Trayvou, now known by the name "Serv Trayvou Interverrouillage" (STI) and owned by Halma.[1] Later, the Englishman James Harry Castell[2] (1880–1953), Frenchman B. Trayvou and the American R. L. Kirk [3] also developed trapped-key interlocking systems. Therefore, such systems are commonly referred to as Castell, Bourré, Trayvou, or Kirk keys. 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.