Circuit Breaker and Disconnect in separate enclosure: what about the door ?

[Ken Roach]

Longtime reader, first time poster. I hope I can ask a not-specifically-NEC question here.

I have an industrial machine (I follow NFPA79 and UL 508A) requiring the main 480V/3ph disconnect and circuit breaker to be in a separate enclosure from my control panel. It will be directly adjacent, so I can run rigid conduit between them.

I understand the idea: he wants the machine's main electrical enclosure to have no voltage present at all when the disconnect is open and locked.

But under NFPA79, what we've always done is provide a disconnect handle on the enclosure door that doesn't allow the door to be opened unless the disconnect is open or deliberately defeated.

I can provide this "secondary disconnect", but it gives me a bad feeling to have two disconnects next to one another, one of which will leave the other energized.

How do Forum members handle such a "separate disconnect enclosure" requirement ?

 

[kwired]

Longtime reader, first time poster. I hope I can ask a not-specifically-NEC question here.

I have an industrial machine (I follow NFPA79 and UL 508A) requiring the main 480V/3ph disconnect and circuit breaker to be in a separate enclosure from my control panel. It will be directly adjacent, so I can run rigid conduit between them.

I understand the idea: he wants the machine's main electrical enclosure to have no voltage present at all when the disconnect is open and locked.

But under NFPA79, what we've always done is provide a disconnect handle on the enclosure door that doesn't allow the door to be opened unless the disconnect is open or deliberately defeated.

I can provide this "secondary disconnect", but it gives me a bad feeling to have two disconnects next to one another, one of which will leave the other energized.

How do Forum members handle such a "separate disconnect enclosure" requirement ?

Why the bad feeling?

Why require a disconnect ahead of the equipment and then put a disconnect in the equipment makes no sense either.

Quite often, I do run into installing service rated disconnect for irrigation machines right next to their control panel that already has a disconnect in it, problem is the one in the control panel is not suitable for use as service equipment.

 

[Ken Roach]

Thanks for the reply and example of two disconnects, one service-rated and one not.

The "bad feeling" was just about increasing possible confusion by having two disconnects for the same machine within a couple feet of one another, with different enclosures, handle types, and operating actions.

The usual fused disconnect is a vertical handle on a box with a circuit breaker or fuses inside. The usual door-mounted disconnect is a red rotary pistol-grip.

My probable solution will be to provide an external disconnect as requested, and include an information sticker above the door-mounted disconnect reminding the user that there's another upstream point of disconnection and only one source of power.

 

[GeorgeB]

I have an industrial machine (I follow NFPA79 and UL 508A) requiring the main 480V/3ph disconnect and circuit breaker to be in a separate enclosure from my control panel. It will be directly adjacent, so I can run rigid conduit between them.

I understand the idea: he wants the machine's main electrical enclosure to have no voltage present at all when the disconnect is open and locked.

But under NFPA79, what we've always done is provide a disconnect handle on the enclosure door that doesn't allow the door to be opened unless the disconnect is open or deliberately defeated.

I pretend to be an engineer ...

When I did panel design, our standard was either a through the door or flange mounted disconnect (usually with OCPD). I now see comparable equipment designed to comply with my understanding of 70E, that there may be no energized components in the panel for certain work. The top of the disconnect remained energized. The separate disconnect allows working in a dead panel.

Those I see today are usually nippled to the panel.

My assumptions may be flawed.

 

[hillbilly1]

I pretend to be an engineer ...

When I did panel design, our standard was either a through the door or flange mounted disconnect (usually with OCPD). I now see comparable equipment designed to comply with my understanding of 70E, that there may be no energized components in the panel for certain work. The top of the disconnect remained energized. The separate disconnect allows working in a dead panel.

Those I see today are usually nippled to the panel.

My assumptions may be flawed.

You are correct, it eliminates the arc flash potential in the cabinet by using an external disconnect. With the internal disconnect, voltages are still available inside the enclosure, so proper ppe for the arc flash and shock hazard potential must still be worn making it difficult to service the equipment.

 

[cadpoint]

I am stretching to understand what you’re doing! I frankly think you’re over thinking it. I don’t have to deal now with NFPA 79, ok understand that I’m one of those that have to work off of sealed drawings.

I’m not going to dig into NFPA 79 to try to understand your statements in your second and forth sentences.

What I want to say is that everyday a circuit and an equipment means of disconnect is applicable by NFPA 70, Motors, Disconnecting means, with multiply listing in the Index.

I can read about UL 508A and while it’s a scope of what’s required it seems very versatile through 1.3.3 after that every industrial control panel requirement is redirected to another UL listing!

I’ll assume you’re basically talking about 1.3 specifically?

Don’t mind me I use a lot of posts to gain knowledge of things I’ll never touch much less have to worry about.

 

[Ken Roach]

Thanks for your comments.

I come from a programming and process control background and do not have detailed NEC experience, so I was trying to ask a general question rather than a chapter-and-verse code question.

I've been introduced to an external disconnect enclosure from Hoffman, that they call a "SEQUESTR" enclosure. These have a left-side cutout for an Allen-Bradley 1494V disconnect operator and allow you to interlock the disconnect enclosure and the main enclosure doors with a single operator handle. If I can make it fit up with my main enclosure door latch, it will be ideal for me.