I am on a project where the AHU manufacturer has factory wired and mounted a PLC in the same control cabinet as the 460V feed, going to relays for fan operation, compressor start/stop, etc. Normally we would like to see this controller mounted in its own steel cabinet for both safety sake as well as protection from RF and EMF. I think we're asking for trouble if we approve this installation.

I do know that all wiring insulation has to match the highest voltage, which may help, but I am looking for some muscle to quote that will require either mounting the controller in its own enclosure within the control cabinet on the unit, or in a separate enclosure outside of the control compartment.

 

PLCs are pretty immune to AC power noise. I can't tell you how many thousands of them are installed in cabinets with lots of 480 and working just fine. Not generally my practice personally, especially on larger jobs, but the PLCs don't usually seem to mind. I think feed power quality and adequate environmental (temp and humidity) control are bigger issues for most PLCs. I've even seen PLCs right next to SCRs switching 480 for phase controlled heaters operating with no problems.
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Steve Myres, PE
Automation Solutions
(480) 813-1145

 

We have cabinets where the PLC is mounted in the same enclosure as 415V motor control and VSDs, no problems experienced. But if your specification had segregation, or the requirements of a hazardous environment then I would push for the change otherwise the manufacturer can ask for a variation order and then charge for it.

 

There are no codes that I know of that prohibit this type of installation, and in fact it is very common.

There is no reason you cannot specify you want it done differently though.

I prefer to see the power stuff seperated from the control stuff, but in the same cabinet, just because it is easier to work on and test that way, but convenience is not code.

You might be able to convince your management to spring for the extra money to change it by claiming it is a safety issue, but unless it really is, it is best not to go to the safety well too often.

 

there shouldn't be a problem. if you check the TRANE Chillers you will see 480V/600V and the Controllers in the same box.

matt