interlocking diagrams - best practices wanted!

[ipupkin]

Hi, All! I think, most of PLC engineers have to deal with equipment interlocking diagrams.

Can anyone point me to best practices of drawing these diagrams?

For example, if motor A stops, then motor B stops and will require manual start. This would be an arrow from A to B.

What if B will not require manual start and will run after A runs again? AGAIN ARROW FROM A TO B ??? This is what WE are drawing currently.

These types of interlock are very common and repeated on hunderds of plants around the world. I suspect there should be a common good tradition of how to represent typical interlocks between equipment. Can anyone point me to best practices of drawing these diagrams?

Sorry for long post

 

[dbaird]

Most industrial interlocking diagrams I am familiar with exist in schematic form only and as such require the user to be technically adapt at reading/interrupting them. However, I have seen simple block diagrams and/or flow diagram illustrating sequence of events. Sometimes the concept is in text form only with very detailed description of what is supposed to happen. Again, it all leads back to initial requirements of end user and their expectations/requirements of the design engineering team for the documentation. This should be discussed in detail and agreed to in the early stages of the project.
David Baird
mrbaird@hotmail.com

Sr Controls Engineer

EET degree.

Journeyman Electrician.

 

[dpc]

It used to be common to prepare logic diagrams prior to doing electrical schematics back in the days of hard-wired relay logic.

There was a standard presentation used, and I believed it was based on NEMA ICS 1-103. OR gates used a circle and AND gates used a square. I suspect this was an ISA standard of some kind.

These were done for each piece of equipment. So there would be diagram for Pump A that shows all interlocking for it, and separate diagram for Pump B that shows Pump B logic, with appropriate signal lines going between the diagrams. Virtually any type of interlocking could be depicted using this system.

These work pretty well as a communication tool, but in the end, the actual wiring (and schematic) and the PLC logic will determine how it actually works.

The logic diagrams are rarely kept up to date after intitial construction, so they are often are at odds with reality.

Hope that helps.

 

[ipupkin]

Thanks for replies!

In our case we've got running sunflower seed crushing plant, with ready (maybe not always up-to-last-minute) interlocking diagrams. Process is very continuous, the most "uncontinuous" thing is drive that is stopped when certain HL is detected and then restarts automatically wheh HL goes off.

That's why we are able to draw very compact interlocking diagrams - to 99% it consists from logical connections like this: "when A is in undesired state, stop B so that manual start of B will be needed later".

May be I shouldn't try to repeat plc program on interlocking diagram since this diagram is compact representation of process logic and thus couldn't be explicit. And maybe noone expects it to be explicit.

Anyhow, thanks for input!