NEC/NFPA 79 and separation of high/low voltages in industrial machinery

[barnesja2]

The knowledge contained in this forum is priceless so I wanted to get your opinions on a code question.

We always separate our 120 and above AC components from our 24 VDC components by putting them in separate panels. The AC side is always interlocked, in compliance with NFPA79 6.2.3.1. The 24 VDC panel is not interlocked, which is great for PLC/network/controls access.

Many of our machines use PCs. We'd like to keep them in a panel to protect them from environment but still have them accessible. The problem is that the PCs run at 120VAC of course, and would require a 120 receptacle in the 24VDC panel or power to run over from the AC panel.

My reading says that any live parts above 50VAC or 60VDC has to be interlocked per NFPA 79 and that adding a receptacle or even running power over to the PCs internal power supply qualifies as "live parts" above 50VAC and therefore is not doable, even if common sense says a 120 receptacle poses very little danger.

Recently it has been suggested that it would be OK to put the PCs in there because we would have a "disconnecting means" (receptacle) kosher with NFPA 79 and there would be no exposed conductors meeting NFPA 70E Electrically safe work conditions requirements.

This is one of those weird spots where common sense and code are at odds. What do you think? Thanks for reading.

 

[Jraef]

I agree with your concept of keeping them separate, it's easier now with all the emphasis on Arc Flash safety too.
Can't you just put the 24VDC power supplies in the other panel and run only 24VDC to the PLC panel? That's what I've done.

 

[barnesja2]

Yeah that's what we do with the PLC. The issue is the desktop PC that we'd like to sit in the 24v cabinet.

 

[GoldDigger]

Yeah that's what we do with the PLC. The issue is the desktop PC that we'd like to sit in the 24v cabinet.

Put the brick in the AC cabinet and run the standard LV cable to the PC? If it is not long enough you may or may not be allowed to extend it, so arrange things accordingly.

Sent from my XT1585 using Tapatalk

 

[petersonra]

The knowledge contained in this forum is priceless so I wanted to get your opinions on a code question.

We always separate our 120 and above AC components from our 24 VDC components by putting them in separate panels. The AC side is always interlocked, in compliance with NFPA79 6.2.3.1. The 24 VDC panel is not interlocked, which is great for PLC/network/controls access.


Take a look at 5.3.5 and 6.2.3.1 exception 2

Many of our machines use PCs. We'd like to keep them in a panel to protect them from environment but still have them accessible. The problem is that the PCs run at 120VAC of course, and would require a 120 receptacle in the 24VDC panel or power to run over from the AC panel.

many PCs these days are available that can run on 24VDC OR have an external power supply like a laptop does. Presumably you could put the power supply in the AC side and just route the Dc side of the power supply cable into the Dc side if you wanted to.

My reading says that any live parts above 50VAC or 60VDC has to be interlocked per NFPA 79 and that adding a receptacle or even running power over to the PCs internal power supply qualifies as "live parts" above 50VAC and therefore is not doable, even if common sense says a 120 receptacle poses very little danger.

Recently it has been suggested that it would be OK to put the PCs in there because we would have a "disconnecting means" (receptacle) kosher with NFPA 79 and there would be no exposed conductors meeting NFPA 70E Electrically safe work conditions requirements.

This is one of those weird spots where common sense and code are at odds. What do you think? Thanks for reading.

Personally, i think you are too worried about NFPA79. it is not a legally mandated standard like the NEC is.

 

[rbalex]

Personally, i think you are too worried about NFPA79. it is not a legally mandated standard like the NEC is.

That's not quite accurate, any ANSI sanctioned Standard is OSHA enforceable.

 

[petersonra]

That's not quite accurate, any ANSI sanctioned Standard is OSHA enforceable.

It can be if you chose to not select your own std of equivilant safety. A lot of places have rules about following such standards with a few minor tweaks to handle cases like this. In any case there a ways to readily avoid the issue the op mentioned.

 

[LarrywdH]

Why can't you run a cable from the high voltage cabinet over to the 24v cabinet and feed a 120v receptacle for your computer and keep everything enclosed, so you have no exposed live parts?
Treat the inside of your 24v cabinet like you would if the receptacle was being added to your work bench.

 

[petersonra]

Why can't you run a cable from the high voltage cabinet over to the 24v cabinet and feed a 120v receptacle for your computer and keep everything enclosed, so you have no exposed live parts?
Treat the inside of your 24v cabinet like you would if the receptacle was being added to your work bench.

I think most of us would do that and sleep well at night but the requirements of the standard can be read as not allowing one to do that. They can also be read as allowing it. personally, I would be inclined to "tweak" things by writing a policy that essentially says the company has interpreted the standard as allowing it. That way you cannot be accused of just ignoring the standard. You show you made a good faith effort to abide by it, but at worst have a slightly different interpretation of the standards requirements than another person does.

It is also possible the equipment is not even an industrial machine as defined by the standard.