Lockable Emergency Stops - Thoughts Please
Lockable Emergency Stops - Thoughts Please
(OP)
Good morning all. Let me preface by saying I am old school. Now to my question... We have an existing CNC system that has a disconnect on the outside for all power. There is 3 phase for the motion controllers and of course control voltages i.e. 120vac & 24vdc. On the control voltage side we have a PC based HMI/PLC. When recovering from a LOTO, the PC takes a bit of time to come back on line. For our daily operator maintenance, the operator is required to be in side the machine. We currently have a lockable emergency stop where the operator must remove the key. I have researched the web trying to determine if this is acceptable. I have talked with the motion device manufacturer and their contention is it is up to end user. Can anyone cite from our NEC, OSHA, NFPA's governing bodies anything that says this an acceptable means to LOTO for non-maintenance tasks such as cleaning? I myself prefer to have the motion device source voltage on a disconnect howerever this would cost $$$. Thanks for any response.





RE: Lockable Emergency Stops - Thoughts Please
RE: Lockable Emergency Stops - Thoughts Please
Best to you,
Goober Dave
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RE: Lockable Emergency Stops - Thoughts Please
RE: Lockable Emergency Stops - Thoughts Please
TygerDawg
Blue Technik LLC
Virtuoso Robotics Engineering
www.bluetechnik.com
RE: Lockable Emergency Stops - Thoughts Please
Dave
RE: Lockable Emergency Stops - Thoughts Please
RE: Lockable Emergency Stops - Thoughts Please
Keith Cress
kcress - http://www.flaminsystems.com
RE: Lockable Emergency Stops - Thoughts Please
When I started in the field, lockable stops where in the process of being outlawed because people were dying despite locks on the emergency stop. I strongly resist any suggestion to use an emergency stop as a safety stop. They may be locked to prevent unauthorized operation but I don't like that either. You are setting a mine field for the new guy.
But, if a lockable disconnect removes all power from all devices and from the I/O interfaces it should be safe to keep the PLC energized. Beware those who will propose short cuts. Remember; Most accidents are a result of at least two or three co-incident events. Don't let any one implement a short cut that supplies the first enabling event.
Your suggestion passes my "Old School" standards Keith.
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Lockable Emergency Stops - Thoughts Please
So I always order an isolated cable when someone asks me to hook up a printer to a CNC. Actually, that's my second tactic; my first is to feign ignorance of the subject matter entirely and suggest they call the CNC supplier.
Where I'm going is, there's a significant chance that providing power to the PC while disconnecting the power circuits is likely to involve a major rewire (actually a redesign) of the CNC, and when you're done, and _anything_ happens, who they gonna call?
If they really have a problem with the PC boot time, I'd investigate subbing a faster PC and/or a solid state drive. ... but that may not be easy, because some CNC 'PCs' are customized specifically so a generic PC won't quite fit or work, locking you into buying replacement motherboards from the CNC people. For that reason, extended warranties are a really really really good investment when buying a CNC. Would you believe nine motherboards in the first year for a name-brand CNC machine? (Not including the one I burned; that was on a different machine.)
Mike Halloran
Pembroke Pines, FL, USA
RE: Lockable Emergency Stops - Thoughts Please
That sounds like a wiring fault. It could be one of several problems but likely current circulating on and between the ground connections.
Plugging the PLC into the printer receptacle also may solve both issues.
Bill
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"Why not the best?"
Jimmy Carter
RE: Lockable Emergency Stops - Thoughts Please
Installing a second control power disconnect beside the main disconnect would solve the problem. The main disconnects the power devices and the control disconnects the transformers or power supplies for the control system. You basically move the control power connections to this new disconnect and then feed the new disconnect from the incoming of the main disconnect. The disconnect would be an appropriate fused disconnect or breaker. Put a Control Power Disconnect label beside the handle to properly ID it.
RE: Lockable Emergency Stops - Thoughts Please
RE: Lockable Emergency Stops - Thoughts Please
Some designs in past i have seen have a separate feed for the controls such as the plc, hmi, and pc. Then the main disconnect on the panel would be for the prime movers.
This way your not waiting for the pc plc to reboot on disconnect turn on.
RE: Lockable Emergency Stops - Thoughts Please
There are a lot of little contradictions that could be present. Primarily you want to understand what happens when the things the PC is interfacing to loss power while the PC doesn't. Lots of things shouldn't care - some might. Some that do could be pretty easy to mitigate.
You also want to assess if anything gets confused by this power sequence. For instance does the PC lose track of where positions are because, say, the encoders or the drives that are providing power to them is lost. It may be easy to re-power them from the power left ON during this service or to just recognize you need to re-issue a "home" command before running jobs again after the drives are energized.
You should be successful with this if you have some electronics knowledge available and two or more heads to bounce conciderations between.
Keith Cress
kcress - http://www.flaminsystems.com