A system that uses one or more key operated interlocks, installed external to the hazard area of the production system, to permit operation only in a prearranged sequence
Pinnacle Systems, Inc. (Safety Sensors and Controls for Industrial Machine Guarding Systems), Safety Glossary, Pittsburgh, Pennsylvania, USA 2014. http://www.pinnaclesystems.com/Glossary-B-I.html, (Ergo - Amer) {759}