process interlock and safety interlock?? the difference?


Thread Starter


I want to understand the difference between process interlock and safety interlock.

as i came to know that process interlock is configured in dcs, while the safety interlock at Plant shuddown system... what is the core difference?

and how are they different from each other and under which kind of requirement do we opt for any of each?
In simple terms ....

Process Interlock : Something that prevents incorrect operation or possible damage to the process or equipment.

Safety Interlock : Something that prevents possible damage or death to people operating that process or equipment.

DCS and many PLC systems are not considered reliable enough for you to risk your life on. Hence safety PLC systems require a greater level of reliability. In addition there are usually legal considerations involved.

In most countries and in most cases you require specialist training and certification to work on safety systems. Unless you are properly trained and certified then you should leave any design, modifications or changes to safety systems to others who are.

normally Safety interlock are SIL certified Loops.

DCS can also perform interlocks, hence non-SIL certified interlocks can be performed at DCS too.

let me know if not clear

Thanks for the reply,

What do you mean by SIL certified loop?

During process design stage, the process engineer give us the requirement as per process study. how instrument/ process technologist decide whether it will be hooked in DCS or Safety PLC? under what requirements/parameters in mind?
Can you elaborate any concrete parameters?

I come across article subjected to layers of protection, a process design inhibit, but still struggling with the same thing in mind.


SIL is an internationally recognized standard for controlling loops that have been identified as Safety critical.

Typically, a risk analysis of your plant should identify plant, units, loops and controls that require a level of SIL control. The controllers for these loops then need to be certified to the appropriate SIL level.

As you can probably see, the more questions you ask, the deeper the rabbit hole gets.


You are welcome.

SIL (SAFETY INTEGRITY LEVEL) is functional safety standard defined by IEC 61508 for the process loops and the evaluation

is done during HAZOP (Hazard and Operability Analysis) involved by Process dept. and end user.

(Check it in WIKI )

It is Process dept. responsibility to tell what are SIL loops and show them on P&ID.

But let me tell u, the process interlock can be performed at DCS OR Non-Safty part of PLC, it depends upon what is the requirement.

If your customer did not specify anything, u can simply take all SIL loops in SAFETY PLC and rest everything in DCS.

Let me know if any further info required.

good luck

i would really appreciate if i can understand the logic behind below statement...we r performing feep study, and i am a trainee engineer, i m eager and desperate to dig out many questions that evolve in my head....would u please elaborate of any example of any of project u come across??

In this feep study, a product is initially stored in a new tank and then pumped & unloaded through loading arm to truck..the low low level of interlock is with pump tripping, pump is also interlocked with totalizer setpoint and control valve to loading arm,once achieved will trip the beside all this, i m still struggling and not very much cleared that what is difference between process and safety interlock ? under what parameters process/ production guys decide the loop to be hooked in dcs or safety plc, as u said its determine in hazop..they are using YOKOGAWA CENTUM 3000, how will they configure?? as they recomended the interlocking to be in dcs...

appreciate if u can mail me some stuff that is presentabl;e and demonstrative..

desperate learner


I know when you start your career you have lot of questions. but my suggestion get answers for each present task and you will learn many things step by step as your career progress.

ok, coming to point, as I told you SAFETY INTERLOCK to be defined by Process guys. are you in instrumentation?? then it is not your job. leave it to process guys. he has to tell you.
if you are process guy then I am not right person to answer how to to do HAZOP.
As I read your interlock description the safety hazard depends upon what type of liquid your filling in the tank? and what plant it is?

YOKOGAWA suggest you to do in DCS? their answer depends upon what was your question? did you specify to them anything about SAFTEY INTERLOCK?

Process guys tell you this interlock is SAFETY then take in SIS/ESD.
If tells you normal interlock do it in DCS.
[email protected] contact me at then I will send some PID for your reference.

Safety interlocks are implemented in your ESD system which can be relay logic, PLC or safety PLC.

Process interlocks are implemented in your DCS.

As an example say you have a process vessel receiving process liquid continuously and you need to keep the vessel from either overflowing or emptying completely. There is a pump which pumps out the process liquid to stop the vessel from overfilling. You could install a high level switch which starts the pump on high level and a low level switch which stops the pump on low level. These are process interlocks and the logic for this would be implemented in the DCS.

But what if either of these switches were to fail? Then either you would overfill the vessel which would result in liquid flowing into the process where you don't want it or the pump does not turn off on low level resulting in the pump running dry and becoming damaged or if the vessel contains gas then this gas could pass through the pump causing possible hazardous condition such as explosion in the process. Therefore you install additional level switches for high high level and low low level which will shut off the pump on low low level and possibly shut off an ESD valve on high high level which stops more process liquid entering the vessel. These are safety interlocks and should be implemented in the ESD system.
I do agree with following definition.
May i get the example of safety interlock in plant.


> Safety Interlock : Something that prevents possible damage or death to people operating that process or equipment.
just remember safety is everybodies job...

if you have a concern, raise the question. It make no difference if you are a construction worker, CAD operator, designer, engineer, instrument or otherwise, it is your responsibility,

you do not want to be on any job where someone gets injured or worse because of a "design" oversight...that was someone elses responsibility