Controls,  P


A switch or relay logic control system in which several process conditions must be met before a piece of equipment can start. (Example: A permissive system can be designed to allow a compressor to start only when there is sufficient suction pressure. Each process condition requirement is called a permissive, in which all are wired in series, so if any one of them detects an unsafe condition, the circuit will be opened, causing a trip.)

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