Shielding inside multiple-voltage enclosure

[FireMedic447]

Rebuilding a PLC enclosure for a customer. So far I haven't found anything in the NFPA 70e or OSHA that addresses this, hence the posting:

120vac coming into bottom of the enclosure, goes through DIN rail mounted connectors to a 24vdc transformer, which then runs the PLC and everything else in the enclosure.
Shop guy asked if we need to put a nonconductive (or a grounded metal) plate over the 120vac wiring section of the box, and if that would circumvent the need for subsequent electrical guys having to wear gloves, face shield, et.al. when working on the PLC and it's I/O.
Thanks,
Dave Lomax
Walter Louis Fluid Technologies
Quincy IL

 

[Jraef]

Rebuilding a PLC enclosure for a customer. So far I haven't found anything in the NFPA 70e or OSHA that addresses this, hence the posting:

120vac coming into bottom of the enclosure, goes through DIN rail mounted connectors to a 24vdc transformer, which then runs the PLC and everything else in the enclosure.
Shop guy asked if we need to put a nonconductive (or a grounded metal) plate over the 120vac wiring section of the box, and if that would circumvent the need for subsequent electrical guys having to wear gloves, face shield, et.al. when working on the PLC and it's I/O.
Thanks,
Dave Lomax
Walter Louis Fluid Technologies
Quincy IL

No.
If this were not on the inside of a control panel, the issue with separating the lower voltage circuits would still only apply to what are called "Class 2" circuits, meaning they have limited energy no matter what happens. A Class 2 power supply is like a "wall wart" device, and the TOTAL energy in the circuit cannot exceed 100W. That wouldn't even power up the CPU PLC most likely.

But it's a moot point anyway, the inside of the enclosure is the inside of the enclosure. The only time you need barriers is if you are using what are called "intrinsically safe" circuit devices, like for level sensing in explosive areas, or if you are building a UL listed control panel and planning on using "unevaluated" components by feeding them from a Class 2 power supply, in which case the barriers are intended to keep the OTHER stuff away from the limited energy stuff.

So who is requiring gloves, face shields etc. to work on 120V circuits? That's not a requirement of NFPA70E. Safety glasses, yes, WHENEVER there is electricity inside. Gloves, only when you get up into higher arc incident energy situations, which for 120V controls in a PLC cabinet would have to mean one HELLUVA gigantic PLC!