Permissives

[honche725]

what is a permissive in the cause and effect chart? is it the condition necessary to run the equipment/system back to normal again? does it only address the initiator of the shutdown condition?

thanks.

 

[JLSeagull]

The terms permissive and interlock are often used as a shutdown initiator. I regard a permissive as a condition required to reset a shutdown interlock. For some situations it could be the shutdown initiator. In other situations it could be a time delay, a reset button or other process situation required to reset the trip.

 

[jwysmooth]

The term "permissive" can mean many things that only a cause and effect can totally define. If you are unsure, you need to clarify the effect losing a particular permissive should have.

However, generally the permissive is a "cause" that has the "effect" of shutting down or turning off a portion or all the equipment it is protecting. For instance, an Emergency Shutdown button (ESD) would normally be a permissive for operating a compressor package. If the ESD button is depressed, the compression, if running, is shutdown. If the compressor is already off when the ESD is pushed, it cannot be restarted until the ESD has been released.

Normally, in controls engineer vernacular, if a permissive is required only to start the equipment, but then after the equipment has been started, that permissive is no longer important, that would be called a "start permissive". But, if you are unsure, make sure you clarify the effects a specific permissive should have when lost.

jwysmooth

 

[BigInch]

A permissive is a condition that must be true in order to proceed with the next event in a given sequence.

Such as,

"Pump Bypass Valve, States = OPEN",

could be a permissive to proceed with either a pump start or a pump stop.

That's about it.

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