Safety PLC Query

Trying to find an answer to the following;

When referring to machinery safety. When do you need to have a Safety PLC separate from the Non-safety PLC?

You can buy a safety PLC which can combine the two functions. However I have read in process related industries that these should be kept separate. I have seen machinery where the safety and non safety related logic are installed in the the same physical PLC (albeit different programming areas (Siemens F CPU's)). So not sure when you need to transition to a separate PLC.

Thanks in advance.

It's not very well defined.

Machinery, yes the safety PLC can also contain the IO and code for standard functions.

Process control however, both need to be separate. So in effect, a safety PLC and then a standard one for the normal operation.

The difficulty being, when does a machine become a process...

Rob... said:

It's not very well defined.

Machinery, yes the safety PLC can also contain the IO and code for standard functions.

Process control however, both need to be separate. So in effect, a safety PLC and then a standard one for the normal operation.

The difficulty being, when does a machine become a process...

Click to expand...

Never heard of safety PLC's until I saw this thread.
Sounds to me if you can't/shouldn't have both codes/programs in a safety PLC then why use one. Just stick with one PLC and continue using safety relays.

I think in process industries it is less that they NEED to be separate, and more that they need to be redundant. I've seen process systems where there was a pair of redundant CPUs that each did both standard and safety (also S7 CPUs). I've also seen systems where they had 1 standard controller, and then a triple redundant safety controller.

If you read the modern safety standards, it is mostly about reducing risk to an acceptable level, and less about "you have to do things this specific way".

Modern automation gives us many many different ways of solving the same problem. Some people look at how they did things 30 years ago, and say they don't trust newer methods. Some people look at newer methods and say they don't trust the older ones. Oftentimes, both groups selected a valid solution.

So is the safety PLC redundancy/separation deduced by the PLr/SILr?

On a machine I work on, S7 type PLC's, there is the 'main PLC', and the 'redundant PLC'.
All the safety gates, doors, etc. are wired to both PLC's. For example, there is a 'door'
switch on a feed hopper, and there are NC and NO contacts on the switch. The NC go to
the redundant PLC and the NO go to the main PLC. If the door is open, and the machine
is set up to not run with the door open, it won't run. And if one PLC thinks the door is
open, and the other PLC thinks the door is closed the machine displays a 'Mis-Match'
error and the machine won't run.

Poet.

forqnc said:

Never heard of safety PLC's until I saw this thread.
Sounds to me if you can't/shouldn't have both codes/programs in a safety PLC then why use one. Just stick with one PLC and continue using safety relays.

Click to expand...

Safety relays are a fine solution for a simple machine, especially if the only safety function is an E-Stop to kill power.

The machinery I work with usually has a much more complicated safety architecture (wireless systems, AGVs, many robots, distributed light curtains, many safety zones), where safety relays are impractical and safety PLCs are a must.

In the process world, safety relays become even less practical because of the analog nature of the system.

lanester said:

So is the safety PLC redundancy/separation deduced by the PLr/SILr?

Click to expand...

You definitely need to design a system to support your desired PL/SIL, but there are definitely systems with safety and standard in the same PLC that support PLe/SIL3, the highest ratings I've seen. See PCS7 from Siemens as an example. I don't think processor redundancy is required for those ratings, but IO redundancy may be. I have less experience on that side of the fence.

I've never seen a SIL 4 system, so I'm not sure what products would be used there.

Thanks all!