Have you come across the need for a safety PLC? Have you ever wondered what the difference was between a safety PLC and a conventional type of PLC? Let's take a few minutes to get a better understanding then. There is an international standard that covers what's needed for programmable electronic systems when they are used in the safety part of a control system. That standard is IEC 61508. We'll see the details of that standard later... If we go back a few decades (for those of us in industry that long...) we'll see that there were no safety type PLCs. It was recommended not to use a PLC for a safety type application. Still, even today hard wiring a system with electro-mechanical parts is often the best way from a safety view. However, these days more and more safety PLCs have begun replacing some hard wired circuits. Some folks are even using dual redundant PLCs. Is it ok to do that? Let's investigate. A redundant PLC is used as a duplicate of the original PLC to prevent failure to the whole control system if the original PLC should fail. So, we use a pair of regular PLCs in the hopes that if one of the PLCs should fail the other will takeover and bring the control system to a safe shutdown. It is important to note however, that neither PLC is a safety system by itself. They are simply a pair of PLCs running in tandem and configured so that if one should fail then the other will bring upon a controlled shutdown. Another key to utilizing this approach is to include the inputs and outputs of the PLCs to check each other. Usually the safety outputs from the control system are wired to each PLCs inputs. Then, the PLC inputs can monitor the status of the PLC outputs. Also, the PLC outputs are often wired to the PLC inputs. These outputs are used as clock pulses or heartbeats. They tell the system that it is still alive, so to say... The most important part of the system (debatable, for sure) is the software. Most often a programmer needs to write something to monitor and control the safety of the controlled system. This is of course critical to the safety of the controlled system. If we don't know there's a problem we can't react. And if we do know there's a problem but react wrongly it could lead to an even bigger problem. So, in general a dual redundant PLC system is not always the best way to go for a safety system. An out-of-the-box system seems better for most applications. Next issue, we'll go through such a PLC and see how it differs functionally from a standard PLC... and also how it can be considered a safety PLC.
