plc interlocking

Does any body know why plc interlocking is not reliable? Why is needed to use external interlocking like contacts interlocking when using plc interlocking? Thanks.
Jose.

You can't trust a computer for safety. If the program stops, then the safety circuits won't work.
Transistors and IC's can fail.
Safety is trusted only to good old relays. They can be wired in a safe condition, and it's easy for someone to understand and test the circuit.

Thank you Keithkyll, I just started learning and studing about plc's. right now I'm working on a project at school about interlocking and thats one of the questions I have. Thank you again and have a good one.
Jose.

When it comes to safety, you need to be as reliable as possible. Basic physics of a hardwired circiut are extremely repeatable, as compared with the alternatives.

Let me answer your question with some questions:

What happens if you are using a sold state output on your PLC and the output fails? Solid state outputs often fail in the ON state.

What happens if your PLC uses a relay output and the relay contacts weld closed in an overcurrent condition?

How can a PLC know if its own output has failed?

If you tie the output back to an input, how can your PLC be sure it is the output that has failed and not the input?

There are specially constructed PLCs for safety applications that do have the necessary hardware to detect and deal with IO failures with redundant backups on the output. Some even have two different processors, each a different brand and with a different OS to make sure that the possibility of an indigenous processor design of OS fault is minimized. But the average PLC does not have the capability.

josesaucedo said:

Does any body know why plc interlocking is not reliable? Why is needed to use external interlocking like contacts interlocking when using plc interlocking? Thanks.
Jose.

Click to expand...

-PLC interlocking is used all over the world and is very reliable.

-PLC interlocking is not used for human safety. A human life is just worth to much.

-PLC interlocking is always used with machine in mind. When they crash, they can just be repaired.

Example:

A month ago I was in a concrete paver plant, just days after they had a death.

The operators used to clean the machines without locking out power to them.

They just stop a plate carrying a layer of pavers inches from its intended transfer position switch. So the operator started to throw pavers to the floor while the tranfers grab was hangin over him. He unfortunatly threw a paver on the switch. The grab lowered itself on him and he is dead.

Now they have a pretty serious locking procedure. This guy was 22. He will never be 23.

I saw people losing fingers and eyes because they did ot lock there system with proper devices.

Nothing is 100% failsafe.

When it comes to the life of a fellow human the safer is a well trainned man using an hardwire safety approuved device.

The word of todays times is "redundancy". In a safety point of view, its a good thing......I'll speak for myself here when I say that when I have calls to machines because when the operator presses a stop button, nothing happens...99% of the time, a N/C contact has fallen off of a contact block. Imagine putting your life or limb(s) in the hands of a plastic clip!


In order to minimize the danger to anyone, "hard" and "soft" stops were developed. "Hard" meaning the actual series loop to a contactor. "Soft" meaning a series loop to a PLC.

Jose,

I think you should specify whether you are talking about control interlocking or interlocking for safety functions.
For simple control interlocking, PLCs are OK. Occasionally, people will put in an extra relay to protect expensive equipment, but that in no longer common.
For safety, to protect people, ordinary PLC interlocking is not good enough. This is due to several reasons. One main one is the "Fail Safe" concept. Can the interlock fail in such a way as to allow the plant to continue running? Will the relay weld shut? Will the transistor short out? The second concept is redundancy, if the relay does weld, how do you switch off the power? Answer, use a second relay. There is a lot more to learn about this, and I am glad you are looking at this while you are a student.

Now, for another point of view, some PLCs are now safety rated. With these, you can use PLC safety interlocking. The first to do this was the Pilz company. Next was Siemens, and now Allen Bradley and others are releasing safety rated PLCs (Although only a small number of PLCs from these manufacturers are safety rated).
Technology can change your answers.

I did not touch on what is needed to make software safety rated.