Control power on/off button

Hi,



I've seen this on many machines over the years. Usually on the main control panel there are two buttons, one for control power on and one for control power off.



I have also seen these called "master on/off"


I have seen these perform various functions. Is there any standard on what these are supposed to do? I have seen them drop out certain contactors etc. Just wondering if there is any standard for this.

in the USA, i would look to nfpa 79, standard handbook for industrial machinery. i used to have only a mcr, now i use a mcr safety module (probably not the correct name) with guided contacts and a reset input.
you hit the estop and the unit kills the output. press the reset pb and press power on pb to start the power.
james

The company I used to work for had a control power circuit distinct from the e-stop circuit. The company I work for now doesn't. I don't believe there is a specific requirement for control power as a distinct control. You may want control power on as a secondary action people need to take to be able to do anything after an e-stop is reset. But I don't believe it is a requirement.

Keith

I suppose it depends on what you call control power, if this was the "Single Phase" or "DC supply" for the I/O or other LV circuits then a switch I think is a bad idea, normally, the only LV (be it single phase or DC for control purposes) would only be isolated along with the main power. It is common to isolate output supplies when an e-stop is pressed but leave things like PLC power, sensors etc. on.
I would consider a switch in a panel or on it that isolated the control power only dangerous it may give engineers a false impression the panel is isolated.
I know from experience we had quite a number of machines from the States, many of them killed the "control power" when safety relay was activated, we found this a poor design as an operator pressing the e-stop, with no power to HMI or PLC we had no alarms to tell us which one was activated. We modified them for two reasons, 1 as above, the other was many power downs caused the HMI's or PLC's to fail, we removed the power for the Inputs, PLC power & HMI from the E-stop relay to keep them powered, only control power to things like contactors & like were through the safety, this reduced the power down/up failures of hardware by two thirds.

parky said:

...
We modified them for two reasons, 1 as above, the other was many power downs caused the HMI's or PLC's to fail, we removed the power for the Inputs, PLC power & HMI from the E-stop relay to keep them powered, only control power to things like contactors & like were through the safety, this reduced the power down/up failures of hardware by two thirds.

Click to expand...

Interesting. Does the PLC get an input, to detect that an E-Stop event has occurred, and react accordingly, because otherwise the PLC might re-start the process the moment the E-Stop is reset?

No it will not re-start, every E-stop uses an aux contact fed back to the PLC, also the process is held until safety reset and a re-start is initiated by the operator (standard practice).

PLCDontUQuitOnMe said:

Hi,



I've seen this on many machines over the years. Usually on the main control panel there are two buttons, one for control power on and one for control power off.



I have also seen these called "master on/off"


I have seen these perform various functions. Is there any standard on what these are supposed to do? I have seen them drop out certain contactors etc. Just wondering if there is any standard for this.

Click to expand...

you can add this to the "standard" of anecdotal uses.

But typically the systems we design or make will have a safety circuit that cuts out the common power to outputs along with tripping safety relays.

The safety relays would be tied to all the doors, guards, e-stops, etc. cutting a safety relay would have all things tied to it, including locking guards that won't unlock to allow entry until X time has passed (internal safety relay time delay type). and then any contactors powered by the safety relay to cut off any possible motor movement.

the big one is the control power, so the output power would be cut by the safety relay as well. We leave the power to the PLC and inputs on, so you will still see output lights, and inputs working, and the PLC won't be faulted. you can still get feedback, but nothing can be physically powered by the PLC because the power would run through the safety relays. This was a big deal on a lot of machines that required human interaction to do setups, remove bad parts, etc, from machine that had 20-30 air cylinders that needed to stay retracted, but also shouldn't fire and come down while someone is working. The issue always arose that they could have a faulty valve, or be manually activated, so a HVLP air system was added so that when estops were hit, or doors opened. the guns would have a high enough pressure to hold in place, but too low of a pressure to allow them to transition easily or damage someone if they did get activated somehow.











As far as a standard..... it's always changing, Osha and NFPA are always updating based on new incidents happening. Standards are always changing as new ways to do safety are developed or improved on also. I don't know if there is a dead set standard other than the ones written in the Osha and NFPA guarding/safety standard, just because every process is different and requires a different way to maintaining safety.

I've seen some older and imported machines that would cut control power to everything, including the PLC and HMI, when a key switch was turned off or an e-stop pushed, but I would always try to modify things so that didn't happen. I also try to keep VFDs and servos energized and use their STO inputs whenever possible. If not possible, then I'd use an external control power supply so the PLC can stay connected to the drive.


As for the PLC outputs on new systems, I would typically have one module or one half module (depending on size) of outputs that could stay energized to power things like indicator lights while the safety circuit was de-energized.

parky said:

No it will not re-start, every E-stop uses an aux contact fed back to the PLC, also the process is held until safety reset and a re-start is initiated by the operator (standard practice).

Click to expand...

That makes sense. Thanks!

I use an On/Off control power switch and an E-Stop.
The E-Stop shuts off wire to power PLC outputs.

The reason I use On/Off switch and not "On" and "Off" pushbuttons, is because this is a critical process, so if power fails, when it restores, the system comes back up to continue to monitor the process
(and record critical data) to see if it can resume or take appropriate action.

I never have a specific "Control Power" on/off switch because to me that is a maintenance function more than an operation function.

Just my opinion, but if someone wants to shut off everything, they should just use the main disconnect. For the rare occasion that maintenance needs the main power on but not control power, they can open the enclosure and shut off control power from inside.