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.

In 1893, French inventor Paul Bouré created engagement lock devices to ensures train traffic safety.

[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.

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.

This is a trapped-key interlock on the door of an electrical switchgear cabinet. It is attached with one-way security screws to discourage casual removal, which would defeat the interlock scheme.
This is a transfer block, part of a trapped-key interlock system. It does not have a bolt to stop operation of a device, but holds and releases other keys.