PC power supply de-energized when PLC power supply is de-energized

I had a question and wanted to know if someone can steer me toward the right answer. I recently started at a company that kept shutting down the PLC because the programming software on a PC would not reset due to a fault with motor drive. Every time the PLC was shut down, line power to the PC was also de-energized. I was asked to supply a separate power source to PC so that the computer based software controlling the PLC, along with other PC's, would not loose power when the PLC is de-energized. My gut feeling tells me to leave as is. Any suggestions why or why not I can supply a separate power source (120V) to the PC?

Is this a windows based PC? Are you shutting it down gracefully, then removing power, or just cutting the 120? I don't think I understand the whole process. Am I correct that you are cutting power to the PLC in order to reset a VFD that is faulted, which also cuts power to the PC?

Is your question about having multiple power sources to the cabinet, or about the wisdom of not removing power from a PC without shutting it down first ?

Let's go ahead and agree that providing a way to prevent uncontrolled PC shutdowns will tend to increase the reliability of a PC and its filesystem.

The fact that you have a "programming software" package that can't gracefully handle a drive fault is a separate issue; that's something to solve later.

Multiple AC power sources for a cabinet are relatively common, so they are fairly straightforward to perform.

NFPA 79 has some sections that will help guide you. I'm going from the 2002 edition I have handy so the section numbers might have changed.

I'm not sure which section it is that requires you to label the enclosure noting that there are multiple sources of power. You'll find it, I'm sure it's there.

Section 5.3.1.1.2 Requires you to label each source disconnect with the location of the other source disconnects.

Section 14.2.4.1 Requires yellow color code wire for wire that will remain energized when the main disconnect is in the OFF position.

Those citations I took from MikeHolt.com, a great resource for NEC and NFPA type code questions.

Yes, it is a windows based PC. Before I got to the company, they were shutting down the PLC because they did not know the fault. After I investigated the issue, the VFD had malfunctioned and needed replaced. After replacing it, the fault was able to be cleared. However, the PC itself is fed out of the PLC control cabinet. So, when the PLC disconnect switch is de-energized and power to the PLC was turned off, power to the 4 other PC's also are de-energized....operating software for conveyor systems. Line voltage is fed to the PC's from the PLC control cabinet. The PC is shut down gracefully, but when I tested the receptacle feeding line voltage to the PC after changing out the input card and replacing the VFD after shutting down the power to the PLC control cabinet all together, there was no power to the receptacle as well feeding the PC's....also tracing the circuit to make sure that the 120V circuit going to the receptacle was coming out the control cabinet also. It makes me think that the engineers who wired the PLC wanted to shut down the PC's connected to the PLC also when the PLC was de-energized. My question is...what risk(s) would there be feeding a separate power source to the PC that has control wiring going to the PLC...even when the PLC is de-energized? There must be some reason why it was installed that way, but it doesn't make sense why the PC's would be fed out of the PLC control cabinet as apposed to a 20A / 120V circuit from an electrical panel.

It is very common for PCs to be powered by a DC power supply or an AC transformer located inside the control cabinet where they are located.

This does not arise from any functional desire to shut down the PC whenever the control system is de-energized, but rather from the desire to have no hazardous voltages inside the enclosure when the disconnect is opened, or because the system specification says that the machine "shall have one 480V 3 Phase supply".

Because you mentioned a receptacle, it sounds like the control cabinet has a 120V transformer meant to feed "utility receptacles" related to the machine, and somebody has just decided to use them to power PCs nearby.

I frequently find items in machine specifications that require a utility receptacle to power a laptop, so I use a Graceport or Mencom hardware port to provide that, and provide a transformer and fuse for 5 A.

On a recent machine they were having weird noise problems with their thermocouples starting right after lunchtime. I found a little dorm-sized refrigerator plugged into the laptop receptacle.

Ken Roach thanks for the reply but I am aware of the current article pertaining to the NEC regarding tagging separate power source in the same enclosure. That is article 210.5 (C) (1), (2), & (3) of the 2011 NEC. The second post you made clarified an issue that I was in question of. That was what I thought as well....so additional hazardous voltages would not exist in the cabinet when the control cabinet disconnect is opened. That is why I was skeptical about changing the circuitry format of the existing wiring. That answer makes sense and that was what my gut feeling was telling me. Thanks for that 2nd reply.

Thanks for those citations; I am far from my office and don't have my current paper specifications.

If you provide a plug, somebody is always going to plug something into it no matter how you label it.

I would love to have a little logger program to find out how many cell phones and USB drives have been plugged into the port on this machine labeled "KEYBOARD ONLY !!"

You could run another circuit from a power panel for the PC power, and not have it go into the PLC enclosure. Is the PC used for monitoring/changing the PLC program or does it have an HMI application running as well? I don't see a need to power down the PC when the PLC is powered down.

Killing the power to your PLC is never a good solution, no matter what the problem may be. That this action is also detrimental to your HMI workstation is secondary, in my opinion. I would be more concerned about ending the power killing practice and establishing a more appropriate protocol for dealing with your control system problems.

"Cycle the power" should never be the first response.

Thanks for all the responses. I agree with all of you regarding not opening the disconnect switch to the PLC control cabinet. Other company associates are doing so and I have frowned against it. Killing the power the PLC has been done before I got to the company because they didn't know an answer why the fault was occurring. In this particular PLC cabinet, I corrected the fault by replacing the VFD. Instead of them cutting power, I needed to know what was causing the fault so I could eliminate them from doing that. Unfortunately, they are doing the same to another PLC cabinet in order to stop a conveyor system that continues to operate when the controls are depressed to tell it to stop and it won't. That is another issue that I am current investigating.

As far as what the PC is used for....it is used to monitor, status changes occurring for the conveyor system, and for also changing the program to the PLC. There is not an HMI installed as of yet. That is in the works for a future addition on the PLC network.

The PLC is a PLC-5 family of Allen Bradley using RSLogix 500 software. The program was installed by a separate company that is sub-contracted out to service the PLC's. The company has the RSLogix 500 software but the software runs on XP and not Windows 7. I have tried to download the software in compatibility mode for XP, but the drivers will not install because of the compatibility issue. So either an XP platform will need to be used for their existing software or the company will have to purchase RSLogix 500 software made for the Windows 7 platform. Running the software in compatibility mode does not work with the drivers needed.

An additional problem that will occur when I get the software running is to eliminate others from accessing the program that has NO knowledge of PLC programming. That will be another issue. I am really tempted to supply my own laptop in addition to my own software to access PLC programs in the system to eliminate that issue. Afterwards I can consult with the engineers who wrote the program to get their password so I can have the PLC in Monitor / Run mode to see any additional problem that I am missing at this point. Without access to the software, I am rather blind on the issue regarding internal problems with the system. Until that point it's basically trial and error. That is in addition to the fact that the company doesn't want to pay for the sub-contracted engineers who wrote the program to service the PLC's. The company is basically penny-pinching the issue. That is where I come in.