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Dcs Interlock System

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#1 NoobMi


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Posted 29 September 2012 - 07:33 AM

Not sure if this is the correct forum for this question, but since it is in the industrial forum, i guess most would be experienced enough to tell me some basics about it :)

What i now is that there are mainly 2 type of interlock, "soft" and "hard". Can anyone enlighten me on their difference/what do they mean by soft and hard interlock?

#2 Shivshankar


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Posted 29 September 2012 - 12:08 PM


Hope this helps.



Edited by Shivshankar, 29 September 2012 - 12:09 PM.



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Posted 30 September 2012 - 08:14 AM

Hi NoobMi,

Hardware and software interlocks are intended to interrupt a sequence of events that could lead

to an accident. Hardware interlocks are typically electrical or mechanical devices that “fail-safe”

in their operation. They are typically used in critical safety applications, primarily to control

hazardous energy sources. Software interlocks are often used to interrupt a process before activating a hardware interlock. In this

capacity, the hardware interlocks back-up the software interlocks.

Documenting all of the safety interlocks (both hardware and software) helps build the hazard

picture for the analyst and reviewers. Describing the interlocks within a table that lists the

interlock, the action taken, and the protection provided will allow for easy incorporation into

system manuals.

best regards


Edited by azzu, 30 September 2012 - 08:16 AM.

#4 TS1979


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Posted 30 September 2012 - 01:06 PM

First time to see definitions about soft interlock and hard interlock. I am very doubt that if the definition exists.

All interlocks in plants are for safety purpose. Most interlocks are triggered by measurement elements (Flow, pressure, temperature, level, vibration, flame scanner, etc.) which send unnormal signals to PLC through wires. The interlock will trigger a series actions of hardware, such as opening (closing) of valves, starting (stopping) of pumps, etc. Is this called a soft interlock?

Another situation is the emergency S/D button which requires operator interference to take action. However, once the button is pushed, the wire to the push button will send a signal to the PLC and PLC will trigger the series actions of valves, pumps, etc.

There is no difference for the signal flow pass system. All the actions of valves, pumps, etc. are determined through the PLC. I don't think there are direct wires connecting the emergency push button to all the valves, pumps, etc. which will take an action when the emergency button is pushed.

#5 jr31


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Posted 10 October 2012 - 02:12 AM

It would be difficult to define precisely "Hard" or "Soft" interlock terminolgies as these day any control/interlock implementation would need physical (Hard) as well as logical (Soft) connectivity. Implementation by Independent Instrumentation I think is more "apt". E.G. API requires an independent high high level switch on HC tank for overfill warning/protection.

Usually DCS is used for Control purposes. For Critical processes a Safety System (SIS) shall be used for Interlock implementation and shall generally require dedicated instruments to initiate Interlock action. For Instance a level transmitter signal shall be physically wired to SIS system (Hard Wired) but The SIS logic shall implement interlock (which possibly could be cosidered Soft) & finally signal shall be sent say to a pump by phically wiring SIS output to pump control circuit.

Only difference would be to have dedicated instrumentation to implement this and not to use level signal from DCS.

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