I hope it is OK to resurrect an old thread to ask my question about safety relays.
I have been digging through old threads on this forum and elsewhere online trying to understand how to use them properly and I feel that I am missing something. Please excuse me if my terminology is wrong but I will try to explain my application.
Say I have an e-stop circuit with 1 or more e-stops wired in series that activate a relay. I have heard a relay used in this manner called safety relay and master control relay interchangeably. I understand that a safety relay is not the same as a normal cube relay which should not be used in safety applications.
My understanding is that the point of using a safety relay for this application is because it is "failsafe", that is it monitors the condition of the contacts so that in case of a fault, such as contacts welding themselves shut, the relay will still open when it is supposed to.
*** Correct - it not only monitors the output contacts through an auxiliary contact but also typically dual channels of the safety input device (sometimes with cross short circuit recognition).
So my question is: why would the output of such a relay be wired to energize the coil of another non-safety relay? I have worked on many panels where e.g. there is a safety relay energizing the coil of a motor contactor, but if the safety relay cuts out and the motor contactor has welded itself shut you'd still have power available to your motor. I have worked on panels from several different engineering companies that are built this way, and all the literature and wiring examples I am coming across online seem to indicate that this is acceptable, though I can't understand why.
*** The safety relay / controller output would typically be wired to redundant external safety relays or contactors with feedback to the safety relay. Generally everything in the system needs to be or should be redundant.
If I have a panel controlling say 10 separate conveyor drives and I need to cut power to all of them in the event of an e-stop, should my 3 phase power to each of those drives be running through safety rated relay contacts?
*** If new drive application, I would look into safe-torque-off models. They have safety inputs that can make this simplier. If it's an existing drive, you would many times use an immediate safety relay output (low current) to stop the drive (to help with the motor decel) and then a timed safety output (contactors, etc..) to remove drive power (or sometimes motor power)
I hope someone can enlighten me here. Also thanks to all of the professionals who post here, this forum is a great resource for newbies like myself.
