Constant 120v Source in powered down panel (480v>120v control)

[RGiere]

Hello,

I work in a manufacturing environment and I am trained as a Journeyman Wireman in Nevada, US. In this manufacturing facility we have 480v, 277v, and 120v sub panels throughout the building. Each piece of manufacturing equipment (more often than not) uses a 3 pole breaker from the 480v to supply the equipment panel wit hsaid 3 phase power. The equipment panel then normally has a step down transformer to generate it's own control signal, which is also normally 120v. All the smaller devices within the panel are powered from this 120v, and obviously is able to be locked out completely by the disconnect at the feeder.

Recently our plant manager has wanted to keep the ethernet switches in said panels powered on constantly even with these disconnects locked out. Which means a separate 120v line from a 120v dedicated sub panel is ran to it. Reasons for which is a whole other drawn out explanation, and no, back up batteries were apparently not good enough. My question is that within the NEC or NFPA 70E, is there a section that forbids this activity? Do they allow for a separate race that is labeled and kept separate within the panel? I cannot find a clear answer myself. My concern is that many individuals in our engineering department have access to the inside of these equipment panels and they won't necessarily be aware from the get go that there is a constant hot source of 120v inside the panel, whether they lock it out or not. Seems to be an obvious personnel safety and fire safety hazard to me.

Can anyone point me in the proper direction for an answer?

Thanks!

 

[infinity]

IMO if you can comply with (3) you should be OK with multiple supplies.

409.110 Marking. An industrial control panel shall be marked with the following information that is plainly visible after installation:
(1) Manufacturer's name, trademark, or other descriptive marking by which the organization responsible for the product can be identified.
(2) Supply voltage, number of phases, frequency, and full load current for each incoming supply circuit.
(3) Industrial control panels supplied by more than one electrical source where more than one disconnecting means is required to disconnect all circuits 50-volts or more within the control panel shall be marked to indicate that more than one disconnecting means is required to de-energize the equipment. The location of the means necessary to disconnect all circuits 50-volts or more shall be documented and available.

 

[RGiere]

IMO if you can comply with (3) you should be OK with multiple supplies.

409.110 Marking. An industrial control panel shall be marked with the following information that is plainly visible after installation:
(1) Manufacturer's name, trademark, or other descriptive marking by which the organization responsible for the product can be identified.
(2) Supply voltage, number of phases, frequency, and full load current for each incoming supply circuit.
(3) Industrial control panels supplied by more than one electrical source where more than one disconnecting means is required to disconnect all circuits 50-volts or more within the control panel shall be marked to indicate that more than one disconnecting means is required to de-energize the equipment. The location of the means necessary to disconnect all circuits 50-volts or more shall be documented and available.

Is this also true when the current carrying conductor of the constant hot is ran in the same race, or in these cases, panduit with the other wires that are able to be disconnected at the panel?

 

[tom baker]

There is a similar rule in Art 430 on Motors, Controllers and Control Circuits. As far as the 120 volt circuit, the NEC does not address that and the assumption is if you are in the control panel you are qualified. this would be addressed by by training or SOPs
You might consider a DC UPS - where I worked the PLC control panels, with ethernet switches, sensors, were all 24 VDC power with battery that would run them for several days, and the battery is supposed to be good for ten years. A DC UPS is far more efficient than a AC UPS as it avoids the power conversions inside a UPS and then the power conversion to 24 VDC in the PLC. And 24VDC is much safer to work on.

 

[Cow]

What is an "always on" ethernet switch going to do for you if all the other components(I assume a PLC and other miscellaneous IO), is off?

If I had to, I might be inclined to just run 24vdc from a central remote power supply to each cabinet, since an ethernet switch can't draw much.

 

[hillbilly1]

Would using Poe to power the switches be viable?

 

[jap]

Hello,

I work in a manufacturing environment and I am trained as a Journeyman Wireman in Nevada, US. In this manufacturing facility we have 480v, 277v, and 120v sub panels throughout the building. Each piece of manufacturing equipment (more often than not) uses a 3 pole breaker from the 480v to supply the equipment panel wit hsaid 3 phase power. The equipment panel then normally has a step down transformer to generate it's own control signal, which is also normally 120v. All the smaller devices within the panel are powered from this 120v, and obviously is able to be locked out completely by the disconnect at the feeder.

Recently our plant manager has wanted to keep the ethernet switches in said panels powered on constantly even with these disconnects locked out. Which means a separate 120v line from a 120v dedicated sub panel is ran to it. Reasons for which is a whole other drawn out explanation, and no, back up batteries were apparently not good enough. My question is that within the NEC or NFPA 70E, is there a section that forbids this activity? Do they allow for a separate race that is labeled and kept separate within the panel? I cannot find a clear answer myself. My concern is that many individuals in our engineering department have access to the inside of these equipment panels and they won't necessarily be aware from the get go that there is a constant hot source of 120v inside the panel, whether they lock it out or not. Seems to be an obvious personnel safety and fire safety hazard to me.

Can anyone point me in the proper direction for an answer?

Thanks!

And that's a valid concern.

It seems odd to me that since they ran a separate 120v line into the control cabinets that they didn't isolate that run inside the cabinet with a separate raceway to a backplate mounted receptacle for the Ethernet switch to plug into.

That way the person working inside the cabinet would have to go somewhat out of their way to get surprised by the remote 120v power coming into the cabinet.

JAP>

 

[Jraef]

There is nothing in the NEC forbidding it, so long as the panel is labeled (as per above) and if your facility adheres to NFPA 79 for industrial machinery controls, you would use yellow or orange wire for those circuits.

14.2.4.1* Ungrounded circuit conductors that remain energized
when the supply disconnecting means is in the off position shall be
consistently applied as either ORANGE or YELLOW.

 

[winnie]

What is an "always on" ethernet switch going to do for you if all the other components(I assume a PLC and other miscellaneous IO), is off?

If I had to, I might be inclined to just run 24vdc from a central remote power supply to each cabinet, since an ethernet switch can't draw much.

This! External to the panel provide low voltage, power limited source to keep the necessary electronics up while everything else is off. The voltage and power can be low enough to be considered no hazardous to people and not a fire hazard (though of course both are approximate; there is a story of electrocution with a 9V battery and you can certainly start a fire with 12V 1A).

-Jon

 

[hillbilly1]

This! External to the panel provide low voltage, power limited source to keep the necessary electronics up while everything else is off. The voltage and power can be low enough to be considered no hazardous to people and not a fire hazard (though of course both are approximate; there is a story of electrocution with a 9V battery and you can certainly start a fire with 12V 1A).

-Jon

I think it would be a 70e issue. As long as you kept it below 50 volts, you would be good

 

[jap]

Also, if you happen to have trouble with your remote low voltage power source you run the chance of affecting several machines instead of the individual units.


JAP>

 

[tom baker]

The ethernet switches I are used are made by Allen Bradley/Rockwell, but under the hood they are Cisco. The IT folks are familiar with Cisco and like them for that. The Rockwell version are industrial rated, and DIN rail mounted, UL listed and are intended for control panels. Some have provisions for dual 24 vdc power supplies. The DC UPS and power supply are made by Allen Bradley, again DIN mounted. The accompanying 12 VDC battery is a "Eurobattery" supposed be good for ten years. The key is in the DC UPS, which seems expensive ($500) but monitors the battery charge/discharge, and has a replace battery alarm. the battery is UL 509 listed for control panels, while you can just plunk a battery in the bottom of a control panel, this battery comes with a bracket.

Are your ethernet switches listed for control panels? How are they mounted?
DC Power is much more efficient and having the control wiring be all 24 VDC, helps to isolate the ethernet switches, PLC etc from power issues
And is safer to work on.