Pull cords & Conveyors 3

[ReliabilityTech]

General question about the standard or regulations involving how a pull cord e-stop circuit is wired.
Can a pull cord circuit be wired through a PLC for a stop command to be implemented, or does it have to be hard wired back to the contactor of the starter for the conveyor motor?
Are both acceptable?

 

[itsmoked]

You want it to be as fundamental as possible. A PLC could spaz-out and ignore a signal. It could respond correctly but have a hardware output failed ON which maybe is causing the need for an emergency stop, etc., etc.

You would want it going to the motor contactor in series with the starter's coil circuit, any other safety switches, E-Stops, over temp, anything. You can also send it to the PLC so it realizes the motive power has ceased and handles it correctly.

If you have a VFD involved instead of a contactor you'd use new Safe-Torque-Off (STO) inputs found on most now, to disable the VFD output at the most fundamental level, below even the VFD's internal computer.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[FreddyNurk]

There are safety level PLCs that could be used in this application, I wouldn't be using a 'standard' PLC for the same reasons as itsmoked.
There are also lower level safety relays that can likely be used with a pull cord. Some of them actually negate standard 'normally closed' e-stop failure modes that aren't generally expected.

EDMS Australia

 

[ReliabilityTech]

I appreciate the responses.

And yes, I should have mentioned that this conveyor is/will be VFD driven.

I guess I am set in my ways of its predecessor which had a multi-conductor teck running along the length of the conveyor, picking up devices (side travel, horns, pull cords, etc.) which were all wired back to the control wiring in the softstarter's e-stop circuit. I trusted an open circuit to stop a soft start more than I would trust a PLC to stop a drive.

I was thinking that there would be regulations for the wiring method for this scenario, but I couldn't find very much specifying how it MUST be wired. (ISO, CEC, NFPA, Mining Act, even NEC)

Thank you again

 

[stevenal]

OSHA PLC E-Stop Interpretation

 

[itsmoked]

There are special cases where, say, you need the VFD to actually stop the system. Lots of things stop faster with the VFD than without. Those get harder to make safety decisions on. If you have to do that then you want the safety system to be tested constantly, daily. You do that by having the safety system become the way the system is started and stopped normally. This way when it's called upon in an emergency it's known to be functional (because it was tested five minutes ago).


Also we've had a lot of discussions on conveyor E-stopping. Loooooong AC safety circuits can have serious issues that need to be dealt with correctly.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[ReliabilityTech]

itsmoked: It's my understanding that you would want your controlled stop to be just that, a slow, ramping, controlled halt to the conveyor. The e-stop input would have different characteristics in its stopping curve. Likely your normal stop function would have a timer to ensure all material is free of the belt, so one full revolution. It would be convenient if the stop command can be the same input as the e-stop command as there would be a test of that system at least four times in a 24 hour period.

 

[miningman]

This issue caused me a lot of grief about 5 years ago. I forget the details, and I am "electrically challenged" so I would not be the right individual to articulate an opinion. If memory serves, it was the Canadian federal document " The use of electricity in mines" that became the critical document. Again if memory serves , the new edition ( 2005???) had a few revisions that specifically concerned conveyor estops and it gave us a lot of trouble really understanding what the requirements were. Check it out

 

[itsmoked]

RT: I see your questions and they're certainly valid. A normal stop running a full belt emptying cycle while an E-Stop stopping NOW. Not sure of the best solution in that case but I'm sure there's an official document somewhere as miningman is showing. I defer to those sources as that would be the way to stay in compliance.

Keith Cress
kcress -

http://www.flaminsystems.com