Disconnect Required?

[Strahan]

I'm currently going over the proposal for the installation of a new piece of equipment and have a concern that I know you guys can help with.

System is 480V total load 49amps PLC controlled. The proposal submitted has the 480v power circuit components located in a separate box from all the 120v plc controls to reduce risk of arc flash when entering the box to access the plc. My question is the two boxes will be located on opposite ends of the machine the power section box which houses all the 480v "stuff" has a main disconnect located on the door. Would I still need a main disconnect at the panel containing all the 120v controls? Thanks guys for any help you can give.

 

[mcclary's electrical]

Most every PLC conrol cabinet Ive ever seen has a Selector switch inside that turns control power off. No need to be lockable since OSHA won't let you use control power as a Lock/tag/ try procedure, it's just there for maintenance purposes

 

[tom baker]

There are appropriate rules for this application Art 430 Part VI control circuits. But does Art 430 or NFPA 79 Industrial Machinery or UL 408A apply?

 

[John Valdes]

I see no reason why they need to be seperated. What arc flash? One lockable disco that kills everything is all you need. If I misread your question please post back......John

 

[eric7379]

As tom suggested, NFPA 79 may play a role here. It depends on the circumstances of the instalation. Is the control power derived from the 480 panel by a transformer, or is the 120v control power ran as a separate source to the control panel? Take a look at NEC 430.75(A).

In looking in NFPA 79:

5.3.1.2
The supply circuit disconnecting means shall disconnect (isolate) the electrical equipment of the machine, including all control circuits, from the supply circuits when required (e.g., for work on the machine, including the electrical equipment).

But here is another thing to consider:

NFPA 79
9.1.1.4
The source of supply for all control circuits shall be taken from the load side of the supply disconnecting means.
Exception: The power supply circuit to memory elements and their support logic requiring power at all times to maintain the storage of information shall be permitted to be taken from the line side of the supply disconnecting means or other power source.


It really depends on the requirements of the machine. In my particular case, we have a few machines in which the emergency stop button can be pushed and it kills the power to the whole machine, but the computer processors stay powered up. They can be powered down separately, if needed.

Unless it is a case like that, I would say that only one disconnect is needed, unless the 120vac is brought in separately. Then, make sure you tag and label!

 

[buldogg]

Most every PLC conrol cabinet Ive ever seen has a Selector switch inside that turns control power off. No need to be lockable since OSHA won't let you use control power as a Lock/tag/ try procedure, it's just there for maintenance purposes

OSHA does accept certain safety lockout systems such as the PILZ system in which control voltage is locked out.