PLC Enclosure Interlock

[RAJH]

I have a small enclosure housing a PLC control system. The control system is powered by plugging it into a 120VAC outlet. The plug is the disconneting means. If I interpert NFPA 79 (6.2.3, Exception #2) correctly I do not have to interlock the door. Is my understanding correct?

 

[petersonra]

If you are required to abide by this standard, the answer in general appears to be a convoluted yes, provided a tool is required to gain access.

 

[pfalcon]

Woah! Let's take a step back here.

The interlocked disconnecting means on an enclosure is to prevent access to live parts in a control enclosure. What else do you have in that cabinet? How did you bring the 120VAC into the cabinet? What type of I/O are you running? Are there terminal strips?

On the surface, cord and plug equipment requires neither a disconnect nor an enclosure. But before I go out on a limb and say you're okay, why do you have a PLC? What's connected to it? Why did you put it in an enclosure in the first place?

 

[petersonra]

Woah! Let's take a step back here.

The interlocked disconnecting means on an enclosure is to prevent access to live parts in a control enclosure. What else do you have in that cabinet? How did you bring the 120VAC into the cabinet? What type of I/O are you running? Are there terminal strips?

On the surface, cord and plug equipment requires neither a disconnect nor an enclosure. But before I go out on a limb and say you're okay, why do you have a PLC? What's connected to it? Why did you put it in an enclosure in the first place?

I don't think any of those things matter one iota to the original question.

i said the answer is convoluted. First off there is the question of whether he is required to abide by NFPA79 or not.

If he has to abide by it, here is the chain of logic.

6.2.3 When required by 5.3.1.4 you have to have the interlock.

5.3.1.4 If the disconnect is cord and plug, you only have to comply with 6.2.4 and 16.2, so you don't need the interlock if it is plug and cord.

6.2.4 says if it does not have a disconnect it has to require a tool to open the enclosure or the parts inside are separately guarded.

16.2 has to do with signage.

 

[RAJH]

I don't think any of those things matter one iota to the original question.

i said the answer is convoluted. First off there is the question of whether he is required to abide by NFPA79 or not.

If he has to abide by it, here is the chain of logic.

6.2.3 When required by 5.3.1.4 you have to have the interlock.

5.3.1.4 If the disconnect is cord and plug, you only have to comply with 6.2.4 and 16.2, so you don't need the interlock if it is plug and cord.

6.2.4 says if it does not have a disconnect it has to require a tool to open the enclosure or the parts inside are separately guarded.

16.2 has to do with signage.

I will respond to both replys with this reply.

First- Thank you both for replying!

The PLC control system is used to manually operate five, 24VDC control valves.

Power to the small enclosure is provided by inserting a plug, attached to a 6' power cord, into a 120VAC receptacle. The plug is the disconnect. The 120VAC hot leg is wired directly to a 2A, finger safe fuse. Everything inside the enclosure is finger safe. A tool is required to open the enclosure. Safety signs are located on the front cover of the enclosure.

I thought NFPA 79 was something we always had to comply with when desigining control systems. Are there exceptions?

 

[pfalcon]

First off there is the question of whether he is required to abide by NFPA79 or not.

Bob,
First off, when are you going to chill out on NFPA79 requirements. He may not be required to comply but he did in fact ask if his situation would comply under NFPA79 interpretation. And in that vein I answered.

6.2.3 When required by 5.3.1.4 you have to have the interlock.

Second off, there is no interlock requirement that comes out in his question. Plug and cord does not require either an enclosure, nor a disconnect if mounted in an enclosure. Section 6.2 only applies to exposed live parts; which is not the case with plug and cord.

5.3.1.4 If the disconnect is cord and plug, you only have to comply with 6.2.4 and 16.2, so you don't need the interlock if it is plug and cord.

Disconnects are not plug and cord but we are loosely taking this to mean "in substitution for" the disconnecting means. Without live parts there is no requirement for the enclosure, a disconnect, nor an interlock. Section 6.2 does not apply.

6.2.4 says if it does not have a disconnect it has to require a tool to open the enclosure or the parts inside are separately guarded.

Once again there is no statement of live parts therefore there is no requirement for an enclosure and therefore no interlock and therefore no tool required for access.

16.2 has to do with signage.

And therefore no enclosure to paste a sticker on.

Therefore on the face of it he can do whatever he likes with his toaster PLC. Only back to my original post. Why did he get a PLC in the first place? What else is in the enclosure? Based on that reply; he may need an enclosure, which may need a disconnect, which may need a door interlock or a sticker.

 

[petersonra]

I thought NFPA 79 was something we always had to comply with when desigining control systems. Are there exceptions?

Its not a standard that is mandated by law like the NEC is. Its a standard that is adopted voluntarily, by end users, or by equipment manufacturers.

So, more or less, you only have to abide by the standard if your employer says so.

I am not even entirely sure that what you are doing would be covered by the standard, now that I think about it, unless the valves you are controlling are part of a machine. If they are part of an air handling system, for instance, they would not be covered by NFPA79, although the general principles in NFPA79 could still be followed.

Just so no one gets me wrong, NFPA79 is a pretty decent standard. I am not suggesting you should ignore it, especially if your employee says you have to follow it.

 

[pfalcon]

As Bob was saying, to the best of our knowledge, NFPA79 is not required by any legal agency either local, state, or federal. It is simply good practice.

Again, and in support of what Bob said, the standard is primarily for industrial machinery though related devices can benefit.

If we presume you are running a machine then:
NFPA79:2007 6.2 does not apply to the PLC itself any more than it would apply to a vacuum cleaner. Plug and cord is not covered by this section.

NFPA79:2007 6.2 does not apply to the control circuit wiring because the circuit is 60 volts dc or less.

The last thing to consider then is the 120VAC/24VDC transformer. It also should be a sealed unit. Depending on the PLC you purchase then it may be integral to the PLC.

So, if it is an industrial machine, and your company requires NFPA79; You are still below the requirements. You do not need an enclosure except for being a mounting base; You do not need a disconnect as you have a power cord; You do not need an interlock as you are below 60VDC. Have fun.