OT: The PLC & HMI -vs- the lock out disconnect

Too often I find that there is a need to lock out all motors, etc on a machine with a single disconnect, yet I don't want to power down the PLC and/or the HMI computer. The fact that they have to logout, shut down the computer, and then reboot and log back in when they are done IMO actually discourages the operators from using proper lockout/tagout procedures when doing maintenance that should require it. I have been known to put two disconnects on an enclosure, one to kill everything and one to kill only whatever is necessary to render the machine safe.

Just curious to hear what you all do.

We have isolators for all motors not directly coupled to a single machine (basically if you have a palletiser the lift motors and internal conveyors don't have an isolator but conveyors feeding the palletiser do have isolators).


On the machine itself, all emergency stops are latching and all gates are Cat 3 so we relly on the operators to push the E/stop before opening a gate to enter.

For sparkies disconnecting motors CB or fuses are in the control cabinet so they can isolate what they need to that way rather than the MS.

I don't like to drop all power as batteries go flat and I am a worry wart!

with ours, they just shut it off via the hmi and then flip the breaker in the control panel and lock it out. They do this for each machine they need to lock out.

We also run 2 estops for everything. One estop goes through the plc to only shutdown certain sections and the other e-stop uses mechanical relays to power down everything.

I build alot of equipment for the BIG 3 auto makers. On all of our enclosures, the main disconnect turns off everything except PLC power and cabinet lighting. For that, we install an external disconnect.

We generally take the PLC and HMI feeds from a separate circuit to the rest of the equipment. This is not isolated when the main board is isolated, and we placard that fact on our cabinets.
Cabinet lighting and GPO power for laptops is also not isolated.
If this stuff has to be isolated, it has to be isolated from the main distribution board.

I try to use a completely seperate supply for the PLC CPU and the HMI through a UPS. I also wire in a control power present input so that the PLC knows what is going on.

Most operators I know won't shut down an HMI before locking out, not a good thing for windows based HMI's.

I like to do the same thing that allscott does but still add a placard like Doug does.

Leon

We have also installed a separate disconnect for the PLC and HMI.

An example is here. We also have warning labels on the cabinet and the wires are yellow.

This is more or less what I've been doing. I was mostly just wondering what others do. The second disconnect is to provide a maintenance disconnect that renders the equipment safe to perform mechanical maintenance work on without shutting down the HMI & PLC. The enclosure door is interlocked with the first disconnect so that everything is still powered off when the cabinet is opened unless a qualified person operates the interlock defeater.




edit: Yes, those are cotnrol loads - thats my story and I'm sticking to it because I'm not going back to fix the damn picture.

I tend to use motor disconnect plugs from meltircs. This is certified as a disconnect and it makes maintenance and repair a hell of lot easier. They can be disconnect under load as well.

controlled said:

I build alot of equipment for the BIG 3 auto makers. On all of our enclosures, the main disconnect turns off everything except PLC power and cabinet lighting. For that, we install an external disconnect.

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ditto here, standard practice is we run the PLC and HMI off the lighting disconnect that Rick Densing pointed out. All of the wiring is yellow (with appropriate colored tracer). The main disconnect is for I/O power and 460VAC stuff.

Disconnect

We keep the PLC and I/O Power on a different MCC. The motor Feeds supply only motors and their VFDs. We havent had a lot of good luck with large disconnects used as lockout. They dont tend to be designed for many operations. We have to almost daily, adjust the linkage. This is the isolated non fused disconnects that Allen Bradley makes. Bolts on the handle tend to break a lot as well. It is a better idea to use one disconnect,in my opinion.

Bruce99 said:

We keep the PLC and I/O Power on a different MCC. The motor Feeds supply only motors and their VFDs. We havent had a lot of good luck with large disconnects used as lockout. They dont tend to be designed for many operations. We have to almost daily, adjust the linkage. This is the isolated non fused disconnects that Allen Bradley makes. Bolts on the handle tend to break a lot as well. It is a better idea to use one disconnect,in my opinion.

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True that. Anything over 600 A does not seem to hold up very well with lots of cycles. I had a 800 A 460 V switch (spring) let go and short phase to phase. Thank goodness for for good AK brakers. That machine was/is locked out at least three times daily for regular adjustments/maintenance. The guy that pulled the switch has a new respect for electicity.

Our control circuits (PLC and other) were always seperate from our motor circuits. PLC output voltage could be locked out on most machines. On newer machines we could lock out both input and output power but still kept the processor hot. We did not use the MCR for lockout. All had labels and disconnects.

Motors could be locked out in several places depending on what work was being done.

Each motor had its own disconnect.

Motor groups, or MCC's had sectional disconnects grouped by machine.

Then we could take out a substation, or the entire medium voltage (13.8 KV) source and shut down the mill.

Our motor numbers were traceable to the source. Motor 2-14-C3 would be substation 2 MCC 14 starter C3. This worked well when you needed to take the lockout to the next level.