door interlocked disconnect on control panel

Is a door interlocked disconnect always mandatory?

My customer is requesting a control panel that aligns with UL508 guidelines but they do not want a door interlocked disconnect like I specified.

The panel will have 480V 3PH coming in. Total amperage is going to be around 10A. The control panel will only include a 24VDC power supply (to supply power to a remote HMI), some estop circuitry (including a contactor to drop out the 3PH), and overcurrent protection for a remote mounted VFD & Motor.

The control panel is going on a carousel they build. With where they want to mount the panel, they are limited to around 6" in depth because moving parts of the carousel come past the control panel.

So they would like me to figure out some other way other than having a disconnect handle stick out from the front and possibly get knocked off.

1. Do you have any recommendations on other disconnect/door interlock options that would not require a handle mounted on the front. Lever style disconnect enclosures, in my experience, are much larger than what we are looking for here.

2. They would like me to say we can mount the disconnect on the side of the enclosure, but, according to my understanding of the UL508A rules, that would be a violation. Is that correct?

I look forward to your input.

It must have a door interlocking disconnect handle if it is to be UL listed.

UL508A said:

66.1.5 A door of an enclosure that gives access to uninsulated live parts operating at 50 volts or more
shall be interlocked with the disconnecting means such that:
a) The doors are not able to be opened unless power is disconnected;
b) The disconnecting means is not closeable with the enclosure door open, unless an interlock
is operated by deliberate action; and
c) The disconnecting means is not closeable with the door in the initial latch position or until the
door closing hardware is fully latched.

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Aside from that, if this is in the US, OSHA would have a serious issue with not having a way to disconnect power prior to opening a door. OSHA requires that all employers have a program of electrical safety, SUGGESTING that it "look something like" NFPA70E. So the idea of anyone just walking up to and opening an electrical enclosure without having to kill power first is never going to fit within anything that OSHA would not find unacceptable.


You can get side mounted disconnects that still interlock with the enclosure door, there is nothing in UL508A saying WHERE the disconnect is to be mounted, so long as it interlocks with the door. A-B 194R rotary disconnects for example have an accessory (194R-SDK2) that allows them to be side mounted and interlock with the front door. There are probably others, that's just the one I have used.

Thank you jraef. I 100% agree with you. Yesterday I had an engineer tell me that he thought there was some exception. And I am not thoroughly versed with the UL standard yet so I was looking for confirmation.

Thanks for the suggestion of a door interlock with the disconnect mounted on the side. I will check that out

For the CSA panel standard, it needs not interlock with the power so long as there is a tool required to open it. Large screw driver, panel key or something of that manner.

I'm no UL expert, but I believe you can certify just the panel as an open industrial panel (which doesn't require an interlocked disconnect to an enclosure). That could satisfy the customer's UL requirement and allow you to remote the disconnect.

I would still get something in writing from the customer and make sure there's a voltage indicator on the door just to be safe

CYA.

I would not build a panel over 120V that did not have a disconnect interlock on the door.

Aabeck said:

CYA.

I would not build a panel over 120V that did not have a disconnect interlock on the door.

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Why not, its just cheap and reliable population control. Let them open the doors!

Can it be on the side as long as there is an arm that prevents the front door from opening in the on position?
George

When you have such an interlocking disconnect, do you then have a way to bypass it ?
I am thinking on how you would troubleshoot, for example a missing fedback from a contactor. Or when you want to install a clamp ammeter on a motorstarter to see a motors current draw.
edit: Another scenario is making thermograpic photos to pinpoint bad connections and/or underdimensioned cables or busbars.

Just curious.

Almost all brands of disconnects have a screw head or pin hole to open the panel without turning it off. And when the door is open there is usually a trip lever that can be held by hand to turn the power on with the door open.

Also, there are electrical enclosure locks that fail unlocked to serve this same purpose. Shut off power to the enclosure, and the enclosure unlocks. Use a key to bypass. I've used them before. Like this one:

https://www.grainger.com/product/32FF17?gclid=EAIaIQobChMI-9qcna215QIVEpSzCh2YTAIwEAQYASABEgKAQfD_BwE&cm_mmc=PPC:+Google+PLA&ef_id=EAIaIQobChMI-9qcna215QIVEpSzCh2YTAIwEAQYASABEgKAQfD_BwE:G:s&s_kwcid=AL!2966!3!281733020594!!!g!311582192825!

BlueAltezza said:

I'm no UL expert, but I believe you can certify just the panel as an open industrial panel (which doesn't require an interlocked disconnect to an enclosure). That could satisfy the customer's UL requirement and allow you to remote the disconnect.

I would still get something in writing from the customer and make sure there's a voltage indicator on the door just to be safe

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The only way to do such a system is if each and every individual component has its own separate enclosure, with conduit between them. Totally impractical.

Aabeck said:

Almost all brands of disconnects have a screw head or pin hole to open the panel without turning it off. And when the door is open there is usually a trip lever that can be held by hand to turn the power on with the door open.

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The operative point of that is that a QUALIFIED person, with knowledge of the procedure and the correct tool, can open the box if NECESSARY, and by rights SHOULD know the proper protections and procedures to take when doing so. Whether or not that is true is another matter...



NFPA 79, the standard for "Industrial Machinery Controls", dictates that all disconnecting means MUST have that 2nd interior handle to operate the disconnect again with the door open without the need for a tool, not so much to turn it on, but to turn it off again if there is a problem. You will see that most of the rotary through-door disconnect suppliers (of repute), now offer an "NFPA79 handle" option.

jraef said:

Whether or not that is true is another matter...

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As PLC Pie Guy says - cheap and reliable population control!

rupej said:

Also, there are electrical enclosure locks that fail unlocked to serve this same purpose. Shut off power to the enclosure, and the enclosure unlocks. Use a key to bypass. I've used them before. Like this one:

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This is a nice idea. But I am not sure it will pass the UL code?

I am not willing to sell an unsafe panel, so I appreciate your ideas.

My UL508 copy must be a little different than Jraef's. Here is what mine says and I am not sure that a lock like this will fit that standard.

66.1.2 A door of an enclosure that gives access to uninsulated live parts operating at 50 volts rms ac or 60V dc or more shall be interlocked with the disconnecting means such that none of the doors can be opened unless the power is disconnected.
E x c e p t i o n : A d i s c o n n e c t i n g m e a n s f o r m a i n t e n a n c e l i g h t i n g c i r c u i t s o r f o r p o w e r s u p p l y c i r c u i t s t o c o n t r o l d e v i c e s w i t h m e m o r y r e q u i r i n g p o w e r a t a l l t i m e s a r e n o t r e q u i r e d t o b e i n t e r l o c k e d w i t h t h e e n c l o s u r e d o o r s . T h e c a u t i o n a r y m a r k i n g i n 5 5 . 4 s h a l l b e p r o v i d e d .
66.1.3 The interlocking means required by 66.1.2 shall be provided with all the following:
a) Means to defeat the interlock without removing power and which requires the use of a tool to operate;
b) Means to prevent restoring power while the enclosure doors are open unless a defeat mechanism is operated; and
c) Reactivated automatically when all the doors are closed.

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thanks for your input!