Lockable Emergency Stops - Thoughts Please 1

[papafrancisco]

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.

 

[ScottyUK]

I would be wary of relying on a control signal for a safety isolation. A few modern drives have a 'secure disable' input or whatever name Marketing have dreamed up for it which disables the gate drive to the output stage or opens the DC link. These are qualified for use in safety systems with fairly onerous performance requirements, up to the old EN 954 Category 4. Personally I would rather see a lockable disconnector with a padlock on the front of it. If the drive doesn't have such an input then I wouldn't be happy relying on a control signal inhibit to protect my life. If that costs $$$ or £££ then that's is my employer's problem and a cost of them doing business safely. In the UK it would likely be a legal requirement, although you're under a different legislation.

 

[DRWeig]

Agree with ScottyUK wholeheartedly. I wouldn't get in that machine without a visually-verifiable disconnect of power (safety switch) that I can then lock out.

Best to you,

Goober Dave

Haven't see the forum policies? Do so now: Forum Policies

 

[papafrancisco]

Thanks all. I agree whserviceoleheartedly with you.

 

[tygerdawg]

howerever this would cost $$$

Less expensive than an injury & lawsuit.

TygerDawg
Blue Technik LLC
Virtuoso Robotics Engineering

http://www.bluetechnik.com

 

[dpc]

You may want to refer to NFPA 70E-2012 and its section on Establishing an Electrically-Safe Working Condition. The language is intentionally vague when it comes to disconnecting means. But I have never been in any facility where locking of an E-Stop would satisfy their Lock-out/Tag-out requirements. You must open a disconnect on the main power connections and lock (or tag) it out.

Dave

 

[stevenal]

http://www.osha.gov/pls/oshaweb/owadisp.show_document?p_table=INTERPRETATIONS&p_id=25947

 

[itsmoked]

It sounds like you want to keep the PC running but can disconnect the "energy source". To wit you could possibly use a lockout to remove the energy source from reaching the motion generators while the PC remains running.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[waross]

Good idea Keith.
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

 

[MikeHalloran]

On the few CNC mills with which I am familiar, power for the embedded PC is derived from the major 3-phase circuit feeding the spindle and translation motors. An isolated ground receptacle is typically provided (and typically not marked or colored as such) for the purpose of powering a local printer. Connecting the PC to a local printer that's plugged into a building line socket will typically burn out both the printer and the PC motherboard. You can probably guess how I know that.

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

 

[waross]

Mike;
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
--------------------
"Why not the best?"
Jimmy Carter

 

[LionelHutz]

I wouldn't accept a E-Stop as a lock-out device but then everyone is already in agreement on that one.

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.

 

[papafrancisco]

First of all I thank ya'll (southern twang) for your feedback. For all practical purposes a lockable e-stop does remove control power however it is not the primary voltage to the motor. My feeble mind keeps saying what if the drive were to malfunction without control voltage feeding the drive. From this my innerself says I have put the operators at risk. Thus from this and your feedback there is only one solution i.e. remove the primary volatage to the drives. Thank you for keeping me on the straight and narrow.

 

[controlsdude]

If its just the PC you want to keep power on just provide a transformer with a disconnect on outside to keep power on the PC, no matter what happens to the power for the prime movers. This would satisfy a safety committee at your plant and would be to osha or whoevere compliant.

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.

 

[itsmoked]

I suggest you have an EE come in who understands controls and have him assess the feasibility of disconnecting the power to the drives that can cause motion in the area the worker has to access.

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