purpose of a safety rated contactor

[dayman86]

I apologize if this comes across as a dumb question.


what is the purpose of a safety rated contactor, what separates it from standard contactors, when should they be used?

thank you in advance

 

[SceneryDriver]

dayman86,
They are used in situations requiring the guaranteed disconnection of motive power in a machine. Stopping a saw motor during an emergency is a simple example. They differ from standard contactors in that their contacts, including the AUX (signaling) contacts are force-guided. Force-guided contacts are designed such that they MUST ALL move together, open or closed. Failure of a pole to move results in a complete failure of the armature to move. They are typically built to a higher standard of construction and their price reflects it (as will all safety-related components).

The safety controller or safety relay is wired through the AUX contacts of the safety contactor such that is checks to see if all three poles of the contactor are open, and will disallow a reset of the safety circuit if one or more are closed. This is to mitigate the hazard of one or more poles welding shut in a fault condition and remaining undetected.

Depending on the level of hazard the machine represents (determined by a safety study / hazard analysis) a pair of safety contactors may be wired in series, and cross checked by a two-channel safety relay to ensure no single point of failure can go undetected and create a hazardous condition.

jaref can most likely add to this, if he sees the thread.



SceneryDriver

 

[Jraef]

Nope, you did a thorough job of describing it. I had the same question when they first came out, it's something we (meaning North America) kind of took for granted; that when you cut power to a coil, it drops out and opens the circuit. We all KNEW or had experiences of welded contacts, and where the aux contacts went ahead and changed state, even though the circuit was not open. But it was so rare, we didn't give it a lot of thought.

But for the hyper safety conscious Europeans (mainly Germans) who want to cover EVERY possible scenario in a "Machine Safety Evaluation", that possibility, however remote, was unacceptable. So that concept of "force guided" contacts was developed to eliminate (or nearly so) the possibility that the aux contacts could open without all of the power contacts opening. Mostly though it's not just the design, because that part is easy to change, it's the CERTIFICATION that's important, at least to the EU folks. Eventually we will get around to adopting those safety standards, but there is a LOT of resistance to that here. Think "Nanny State" arguments.