Safety Monitor Reset

[intradave]

Anyone know if a PLC output can be used to reset a safety monitor?

I want to use an OIT, in addition to a local pushbutton, to turn a PLC output (relay) on to reset a Guardmaster safety monitor (Cat 3 wiring). Don't see anything in the rules about that.

[Doug_Adam]

I do it all the time.

The particular rules I have to work to require that a safety reset is initiated by an operator, and is not done automatically, so be careful that your code will not allow automatic triggering.
You also need to do a risk analysis for this, which basically shows what will happen if you have a failure and the reset switches on permenently. Best way over this is to ensure your safety relay is set for pulse reset and will not reset if the output is on permenently. All other normal safety requirements should be met, including checking the state of all contactors and relays before reset.

[OkiePC]

Don't forget to consider "stuck button syndrome" when you do something like this. Consider what will happen when the comms cable is yanked out after the reset button is pressed on the HMI. Program it with a timer timing bit or oneshot type of instruction so that a transition from false to true is required to activate the reset.

[Mtech]

We have seen operators use wire ties to hold reset buttons in.

[intradave]

I think the safety monitor won't reset unless it sees a false-to-true transition, so tying a button down or a stuck PLC output wouldn't be an issue. AND all of the E-Stops, Doors Switches, etc... would need to be in the proper positions. Most times E-Stops are kept pressed in until the time to reset....
The only thing that COULD be an issue is a PLC output that is "flickering" at a low rate. But I would think that would be easily detected and corrected.

[Eric Nelson]

Originally Posted by Mtech

We have seen operators use wire ties to hold reset buttons in.

I have seen an operator use a stack of nickels and tape to hold down a flush reset pushbutton. A one-shot solved that issue, and I kept the nickels...

Back to the OP's question. I don't see any issue with this, keeping Doug and OkiePC's advice in mind.

Similarly, I often tie the normally-open contacts of a relay into the safety circuit. This way, in addition to the standard e-stops and guard switches, the PLC also has a say in the matter. At a minimum, this relay is energized when the PLC is in the RUN state. This also give me the ability to trigger an e-stop from within the program to avoid mechanical damage to the equipment.



-Eric

[OkiePC]

Originally Posted by Eric Nelson

...I often tie the normally-open contacts of a relay into the safety circuit. This way, in addition to the standard e-stops and guard switches, the PLC also has a say in the matter. At a minimum, this relay is energized when the PLC is in the RUN state. This also give me the ability to trigger an e-stop from within the program to avoid mechanical damage to the equipment.



-Eric

I did that once to prevent short-cycling of an e-stop circuit that was breaking the input to a $3000 servo drive as well as "breaking" the drive. The PLC would make the e-stop stay "dropped out" for a minimum amount of time to let the DC bus bleed down to a safe level before being re-energized.

It also had the wanted side-effect of teaching the operators not to use the e-stop for normal stop situations since they didn't have the patience to wait 30 extra seconds to start back up again.

[intradave]

Yeah, one of the benefits of a PLC controlled reset is the ability to add a minimum off time before reset, to allow VFDs to dissipate energy....