Untitled Document. Permissive and interlock circuits : LADDER LOGIC. A practical application of switch and relay logic is in control systems where several process conditions have to be met before a piece of equipment is allowed to start.
A good example of this is burner control for large combustion furnaces. In order for the burners in a large furnace to be started safely, the control system requests "permission" from several process switches, including high and low fuel pressure, air fan flow check, exhaust stack damper position, access door position, etc.
Each process condition is called a permissive, and each permissive switch contact is wired in series, so that if any one of them detects an unsafe condition, the circuit will be opened: If all permissive conditions are met, CR1 will energize and the green lamp will be lit. Note that the high fuel pressure contact is normally-closed. Table of Contents - Practical Process Control by Control Guru.