PLC cabinet with only 24vdc, does it need disconnect and/or door interlock?

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Bob_S

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Hello,

1st post after lurking & searching around for a bit. I hope this is not too much of a dead horse issue but here goes:

I'm working on the controls layout for a new project. The customer wants the system PLC to be in it's own bay of a modular free-standing control cabinet, with the PLC bay having a physical barrier wall between it and the next cabinet over, which will have a 480v main disconnect, control transformer down to 120vac, a few motor VFDs, etc. The idea behind the PLC in it's own bay with no 480v in it is so a qualified person such as a controls engineer could have live access to the plc with minimal protective gear on (safety glasses, long sleeves).

The PLC they want fed by a battery as part of a UPS, so I am considering is putting the UPS in the adjacent drive cabinet and feeding just 24vdc to the PLC cabinet. If I only have 24vdc in the plc cabinet, does it require a disconnect? Does it require an interlocked door?

If I end up putting the UPS and possible other 24vdc power supplies in the PLC cabinet they would be fed by 120vac so in that case I believe that I would need a disconnect with interlocked door.

Thanks,
Bob
 
NFPA 79 Electrical Standard for Industrial Machinery 2012 Edition

NFPA 79 Electrical Standard for Industrial Machinery 2012 Edition

NFPA79:2012:6.2.3 said:
When required by 5.3.1.4, each disconnecting means mounted within or adjacent to a control enclosure that contains live parts operating at 50 volts ac (rms value) or 60 volts dc or more shall be mechanically or electronically interlocked, or both, with the control enclosure doors so that none of the doors open unless th epower is disconnected. Interlocking shall be reactivated automatically when all the doors are closed.
Click to expand...

NFPA79:2012:13.2.4.1* said:
The color ORANGE shall be used to identify ungrounded conductors that remain energized when the main supply circuit disconnecting means is in the off position. This color identification shall be strictly reserved for this application only.
Click to expand...

A reminder that a UPS does not de-energize.
 
petersonra said:
why would you need PPE for a cabinet with only 24VDC in it?
Click to expand...

The main enclosure has 480Vac in it Bob. They want to isolate the PLC into a separate enclosure with only 24Vdc so they won't need PPE to reprogram the PLC. The question is whether a 24Vdc cabinet requires disconnect interlocking.
 
If the PLC has battery back up then the main switch can be thrown and there will only be 24 volt battery power so no PPE required right?

This is the way the lift stations I did was done.

jpppp021.jpg


jpppp012.jpg
 
hurk27 said:
If the PLC has battery back up then the main switch can be thrown and there will only be 24 volt battery power so no PPE required right?

This is the way the lift stations I did was done.
Click to expand...

I can never see most the images :(

PPE is to be worn according to the hazard of the ENERGIZED enclosure until you have both disconnected the power and VERIFIED that the power is absent. So ... if there's originally FR2 hazard when powered the engineer would have to wear the FR2 until he tested that it'd been removed. Or so I understand it. Where's Zog?
 
only 24vdc

only 24vdc

Thanks all for the replies so far.

What I'm going to propose to the customer (end user) is that we put ONLY the plc system & Ethernet switch in the plc enclosure. We'll put the 24vdc power supply, battery controller, and back up battery in the adjacent drive cabinet. There will be a physical barrier between the enclosure bays, so the plc bay will have the 24vdc power wires coming into it feeding the plc and enet switch.

I would like to leave it at that, but the customer also wants a through the door programming port with 120vac convenience receptacle and enclosure lighting, both of which they will feed from a separate maintenance lighting supply.

Maybe I'm overlooking something but I think that with this arrangement (if they approve it) there would be no need for a disconnect (although they may want to be able to open the 24vdc power feed) and no need to interlock the door.

Bob
 
Bob_S said:
... but the customer also wants a through the door programming port with 120vac convenience receptacle and enclosure lighting, both of which they will feed from a separate maintenance lighting supply. ...
Click to expand...
120Vac is over 50Vac so you get to interlock the enclosure again :) or mount it on the high side of the cabinet.
Bump your 24Vdc supply to feed 24Vdc lighting to the PLC side of the enclosure.

And don't forget the GFCI :)
NFPA70:2011:210.8(B)(8) said:
Garages, service bays, and similar areas where electrical diagnostic equipment, electrical hand tools, or portable lighting equipment are to be used
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At least in our shop we have polished concrete floors where things called "trucks" drive in and out through "garage doors".

Oh and consider 406.9 in case they have fluid exposure. We treat our shop as a damp environment.
 
Originally Posted by NFPA79:2012:13.2.4.1*
The color ORANGE shall be used to identify ungrounded conductors that remain energized when the main supply circuit disconnecting means is in the off position. This color identification shall be strictly reserved for this application only.


I always thought it was Yellow for conductors remaining energized or interlock wiring?

Thanks, learned something
 
Prior to 2002 the color was YELLOW.

2002 permitted either YELLOW or ORANGE as long as you were consistent in use.

2007 the color was ORANGE only.
 
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