Trapped Key Interlocking
A safety system that uses physical locks and keys to enforce a fixed sequence of operations on industrial equipment, ensuring each step is completed before the next can begin.
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Origin
Rooted in 1890s French railway engineering, where Paul Boure designed engagement locks for track switching. In 1922, English engineer James Harry Castell (1880–1953) created the first trapped-key interlock to protect workers during the electrification of London. American engineer R.L. Kirk independently pioneered key interlocking for circuit breakers and switches in the early 1930s. Their respective companies — Castell and Kirk Key — remain industry leaders today.
Updated February 22, 2026