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What exactly is permissive
What is permissive exactly?


Pump - G0400
HS - HS0001
Level Not Low - LAL0300
Level High High - LAHH0500

Condition: Pump can be started by HS0001 and when LAL0300 is not de-activated (meaning there is no LAL). Pump will trip if the LAL0500 de-activated (meaning there is LAHH).

In this case, what do you call HS0001 and LAL0300?
I come to know that the only permissive is LAL0300, before my knowledge is that both HS0001 and LAL0300 are permissive. Now I don't know how to call the HS0001?

Hi there,

The way I read this is that if you change your sentence slightly it will make more sense, so if you write it like this,
"Pump can be started by HS0001 BUT ONLT IF LAL0300 is not de-activated (meaning there is no LAL)"

a Start permissive is a final contact of a coil at the end of a interlock chain that must be true before you can start your machine.
In this case the LAL0300 must not be in a low level condition so this is one of your permissive interlocks. LAHH0500 is also a interlock but in a trip chain. The final coil's contact in this trip chain will also be in the permissive interlock chain to confirm there is no trips active.

HS0001 is the push button on your panel so that will only become true once you push it to start the machine. You will only get power to the HS0001 if all the permissive interlocks have been met and the final permissive internal coil have become true. This coil's contact is your final permissive and will supply power to HS0001 as well as display on the HMI that the machine is now ready to start.

HS0001 is not a permissive since it's state is not determined by a condition in the field but by a action, so it does not qualify as one of your permissive interlocks. It's a push button, so call it your final control element if you want.

In some machines the contact of this final permissive interlock coil will be send to the CCR and they alone can then send a final permissive contact to the local controller in the field. Only then can the machine be started by the operator in the field.

You need to move away from the word permissive to indicate the interlocks since it can be confusing. We normally just call them start-up interlocks, trips, alarms and permissive contact.

Thanks a lot Sam.


Another question in my mind, what is INHIBITION.
In our C&E diagram, some FAL (from flow transmitter) has an effect of INHIBITION to compressor.

What is it exactly?/

By David Ferguson on 16 June, 2010 - 7:20 am

I agree with Sam but...............there is much confusion about interlocks and permissives. They are two DIFFERENT things and need to be treated as such.

A permissive is needed to get a piece of equipment going, an interlock is needed to keep it going. An interlock can also be a permissive, but a permissive is only a permissive. For instance, if I need the level in the tank to be above a certain point to start a pump, it is a permissive, if I also need that level to be above that same point to keep it running, now it has become an interlock.

If I need the start button to get it running, it is a permissive, if that button can go away after I start (normally), it is just a permissive. The stop button is usually a normally closed button that opens when pressed, so I need it "not pressed" to start and if it goes away I shut down, (interlock and permissive so therefor an interlock) but when it comes back I do not start until the other "permissives" are satisfied. (including the start button).

This can be confusing as maybe I have a pressure switch, it indicates low PSI in a line that I pump liquid from point A to B. If the pressure is low I shut the pump off. Now when I am sitting down, of course I have low PSI. If this "interlock to keep running" was also in the permissive code, the pump would not start ever (low PSI), instead I would start the pump, wait an amount of time and then engage this "interlock" to keep it running, so in this case the interlock is not also a permissive. Now on the other hand, the tank level above is both a permissive (to start) and also needed to keep running, so it is both. The start button is a permissive only.

The programming for permissives and interlocks is different and needs to be treated different, for instance say have a string of many things that I FIFO (first in first out) trap my reasons to see which one shut me down, I would only want to FIFO the interlocks because as soon as you let go of that start button (permissive only), it would set the FIFO trap and say "the reason you shut down was the start button", so they are treated as two different things.

I have this discussion many times and there is much confusion on this topic but they are two distinct different items...................I hope I have not confused you more, hard to explain (for me) on paper............

Dave Ferguson
Control Systems Engineer

Hi there,

Thanks for the input Dave, I have to agree with most of what you said although some items you mentioned might be a matter of opinion or what you are use to and grew up with. You will most probably find every one believe that the way they learned and got to know something is the way it should be all over the world so it is important to keep a open mind. The above includes me as well.

Let's use Utz's question as a example as to what I mean.

On the plant I grew up on we would talked about putting on a "inhibit" when we want to prevent a executive action to take place on a loop.

When I started working internationally I hear the word "MOS". It stands for "maintenance override switch" but it is exactly the same as a inhibit.

I also grew up with the words ESD and DCS Transmitter, out there it is called the trip and control transmitters - again exactly the same thing. I think the above discussion is pretty much the same thing in that I prefer start up interlock and you are more use to the word permissive.

Sorry Utz, back to you.
Again I am unfamiliar with the word INHIBITION since English is not my first language but if I derive it from the word inhibit which means stop or prevent INHIBITION will mean stopping or preventing. So in your case the flow alarm low (FAL) will prevent the start of your compressor or you can say it places a inhibit on the compressor's start or inhibits the compressor from starting up. So it is a start up interlock on the compressor. Dave used a good word, the FAL needs to be satisfied before you will get the final permissive to start. In other words it must not be in a low flow alarm state. Since you say it is from the FT this is just the software alarm contact of the FT from the DCS. These alarm and trip settings can be modified on the faceplate of the FT on the DCS. You most probably will have a FALL trip on there as well.

Once it is satisfied or true the compressor can be started but the FAL will also flag a alarm condition once the flow goes into a low flow alarm condition after start up. If the flow drops down more and goes into a FALL state the compressor will trip. FALL cannot be activated unless FAL is activated first so that is why they used FAL as the start up interlock and not FALL.

Some years ago i worked with some Nuovo Pignone Reciprocating Compressors.

The PERMISSIVES, are some conditions that need to be satisfied before you can start the machine.

Example: you need to compress the gas "A". The machine will start if you have the gas "A" at the inlet of the compressor. So, if you don't have the gas (don't have pressure) the compressor don't will start. That is a permissive.

INHIBITION: Inhibition is a trip condition that you will inhibit during a fixed quantity of time, it is necessary to start a machine.

EXAMPLE: The compressor will trip if the discharge pressure goes very low. So, the trip is inhibited (overrided) during some seconds or minutes. After that time the TRIP condition is established. If a real problem happens and you loose pressure the machine will go to a safety condition (shut-down). The functional logic diagram must show that information. Generally you need an "OR" to simulate the healthy of the system. The PLC/DCS, etc will put a "1", during the start-up procedure, after that time the PLC/DCS/ETC will put a "0". Now the safety will depend of the pressure transmitter / switch...


Hi there,

I agree with Maurick that is most probably what it is since you will have a FAL and FALL before you start the compressor.

If the FT is not part of the compressor package it could be that you have to have a flow in some header line before the compressor before the compressor will be allowed to start, but if the FT is part of the compressor package I think the way Maurick described it is most probably the way it will work. It is standard practice to do it like this and in most cases you don't have to do anything and all this is done in the plc so most people are not even aware of the fact that some of the trips will be inhibited during start up. The C&E will also be the place to indicate this so it all makes sense.

Well done and thanks for the help Maurick.

So what about the stop button which is normally closed. Based on your "action" vs "field condition" logic this one shouldn't be a permissive or a shutdown but normally it is considered a shutdown if it trips the machine. Will you consider it to be a permissive too?