I was just using 3TK safety modules from Siemens or other brands and I had no problem with them. I always recommended my customers to use them for their emergency guarding. I might have some limited understanding of their durability, reliability or speed but I thought there might be professionals here who are kind enough to supply me with more information
One customer asked me why he is paying more for a job that can be done with one or two relays or cheap contractors? He wants to know about the exact benefits he is getting from his extra 200$
Your profile says "United nations" as your location, which is too ambiguous to know where you actually are. But we can assume based on the fact that you even ASK this question that you are somewhere in North America. The reason is that basically everywhere else in the developed world, there are stringent machinery safety directives that REQUIRE the use of "Safety Relays" or something like them in what are called "evaluated" machine safety
systems. So nowhere else in the world would this question even come up, it's compulsory.
But here in North America (USA, Canada and Mexico), these rules are not yet adopted in the same way so here, people CAN in fact get away with a "bedsheet and strings" approach to a "safety" system. There is nothing PREVENTING you from using Safety Relays, but there is also nothing COMPELLING you to either. At least NOT YET. Here in the USA, OSHA is our safety governing body (for the most part) and although they have rules, those rules are nowhere near as specific as they are elsewhere. Here, we say that employers must provide safe working conditions and give some basic guidelines, mostl about guarding, but unless there is an accident, OSHA is pretty much "Laissez-faire" (let them be).
So putting that regulatory (or lack thereof) issue aside, what you "gain" in using Safety Relays from a functional standpoint is this: CONSISTENCY in how something is evaluated for safety, how it reacts to inputs, how it is reset and what level of redundancy is involved in the process. There are BOOKS written about all of this, it's way way too much to list here, but here are a few highlights of issues that are covered.
- What happens if a contact welds on a relay or limit switch?
- What do you want to happen if a sensor fails?
- What happens if a contact block falls off of a push button?
- What if a wire breaks?
- What needs to happen if someone trips and falls into a dangerous part of a machine operation?
- Who should have access to dangerous parts of a machine while it is operating and how would they gain access?
- What if someone pulls out an E-Stop or hits a reset button while a machine guarding device is still removed?
- Which parts of a machine must ALSO be shut down if someone enters a different area?
- Etc. etc. etc.
Machine safety relays are used as part of a SYSTEM that is intended to cover as many possible scenarios as would put people in jeopardy AND make sure that everything takes place in the proper sequence of events.
It should be noted that MANY MANY large corporations, especially those that have operations outside of North America, have adopted the IEC safety standards here so that all production systems are consistent wherever you go. So if you build equipment, you should familiarize yourself with the standards and how they apply, because sooner or later it's going to come up.
I'll have to remember that.
