Permissive circuits in PLC


A practical application of switching and relay logic is in control systems where several process conditions must be met before a computer is allowed to in this session we are going to discuss about  Permissive circuits in PLC

Permissive circuits in PLC

A good example of this is the control of the burner for large combustion furnaces. For burners in a large furnace to start safely, the control system requests “permission” from various process switches, including high and low fuel pressure, control of fan air flow, position of the exhaust damper , position of the access door, etc. the condition of the process is called permissive, and each permissive switching contact is wired in series, so that if one of them detects an unsafe condition, the circuit will open:

If all permissive conditions are met, CR1 will be energized and the green light will turn on. In real life, something more than a green lamp would be energized: normally a fuel valve solenoid or control relay would be placed on that rung of the circuit that would be energized when all the permissive contacts were “good”, that is, all were closed. If any of the permissive conditions are not met, the contact series of the breaker will be broken, CR2 will de-energize and the red light will turn on

Note that the high fuel pressure contact is normally closed. This is because we want the switch contact to open if the fuel pressure is too high. Since the “normal” condition of any pressure switch is when zero (low) pressure is applied and we want this switch to open with excessive (high) pressure, we must choose a switch that is closed in its normal state

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