# PLC mounting near high voltage AC lines ...



## sealyons (Jan 15, 2015)

Obviously, it makes most sense to place PLCs into cabinets with little to no high voltage AC (230V & 460V) signals present to help reduce possible noise issues. However, we have an application where a customer really wants PLCs placed into these cabinets and I'm trying to determine how to proceed. We've recommended adding an additional small panel for the PLC and associated I/O but space and cost are an issue for the customer. These panels have transformers & Softstart motor starters present driving various HP-ratings at both 230V & 460V (individual panels run at either 230V or 460V, those are not mixed within a given panel). Has anyone had specific problems mixing high voltage AC with 24VDC PLCs and associated I/O signals? To what level would you push-back on the customer to keep them from mixing these signals? 

When placing a PLC (in our case a small Omron PLC, self-contained, 14 I/O) in a 230V or 460V panel (as listed above), what is the best mounting approach? I've heard that a 200mm (approx. 8 in) space should be left free of high voltage wires all around the mounted PLC. Does this make sense? Are there any other guidelines that should be followed? We will minimize 24VDC lines from running in ducts with high voltage lines and keep wire crosses at 90 degrees wherever possible.

Appreciate any comments.


----------



## AK_sparky (Aug 13, 2013)

Never had an issue with it, assuming you follow your own advice: keep as much separation as you can, cross at 90's, don't run in the same duct.


----------



## sealyons (Jan 15, 2015)

Thanks. I really haven't had too many issues with it either but since noise issues are so difficult to pinpoint I'm always leery. And it's a more problematic issue when building for a customer then having something internally. 

Do you freely mix PLCs in with AC signal sources or are there some situations where you wouldn't do it? What type of spacing have you used in the past? Do you place the PLC on a totally separate DIN rail in the box without any adjacent AC components?


----------



## oliquir (Jan 13, 2011)

I do that in almost every small panel i made (almost all of the time for small machines), there is always motor starters and plc in same small panel. ive seen a lot of bad design ones where 600v is mixed with analog io and do work ok.


----------



## sealyons (Jan 15, 2015)

Here's one of the panels that we're looking at possibly adding a PLC to. 










As you can see, there's not much space in here. We can adjust component locations on the new builds to free up a little more space but it's a small 24x30" panel to start with. The top & mid areas all 460VAC (transformer, disconnect & breakers, and Softstarters) and the existing small DIN rail at the bottom has primarily 120VAC components and has just enough space to mount the PLC. But this will put the PLC in close proximity to the 460VAC and it will have to live with 120VAC neighbors. Would you be comfortable dropping a PLC in here?


----------



## Michigan Master (Feb 25, 2013)

AK_sparky said:


> Never had an issue with it, assuming you follow your own advice: keep as much separation as you can, cross at 90's, don't run in the same duct.


Good advice. ^^^

Another advantage of a separate panel you may want to bring to the customer’s attention is personnel safety. A low voltage panel will not require any AR clothing for arc flash protection or rubber gloves for shock protection.


----------



## Batfink (Jul 9, 2014)

We have installed countless PLC's in cabinets with 480 or 600vac in them with no issues. Most important thing to do is do a layout ahead of time, know where you want components and how every wire will run. Adjust to keep separation and all is good. 

With photo shown I would try everything I could to adjust the components to swap the left and right lower. Get the control dinrail (where PLC would go) under the transformer add a second dinrail if needed since there would be room. The wires from the disconnect to fuse holders already don't look that good so may as well bring the two components closer together. You don't need that much room under the soft starts so they can scoot down as well after you move them to the right. I always prefer my high voltage to run from top of cabinet to bottom so there is no high voltage running across the cabinet like that.


----------



## Jlarson (Jun 28, 2009)

480 isn't high voltage.

With a cabinet that size I'd use an external transformer to open up some room.


----------



## JRaef (Mar 23, 2009)

Jlarson said:


> 480 isn't high voltage.
> 
> With a cabinet that size I'd use an external transformer to open up some room.


I agree. And in fact, why is that transformer so big? I don't see anything in that box that needs that much power, but I see fuses and a MMS that appear to go to things outside of the panel with maybe 120V. Since most of it is going out of the panel anyway, I'd just get a potted GP transformer and hang it on the outside wall (or back, depending on how this panel is mounted). That will free up more than enough space to do it right.

As I said in another forum you posted in, clearance of 2" on all sides of the PLC is what you will find in 99% of the micro PLC installation manuals.


----------



## sealyons (Jan 15, 2015)

I appreciate the info. Our customer has not requested (nor are they paying for) an engineering review of their design and a re-design, they just want us to do the existing panel builds. They've been selling this product for many years and it supposedly meets their needs. We haven't been given full schematics, access to their engineering team, or any info regarding their design choices at this point. So, it's difficult to comment on their specific component choices or exactly why they chose an internal vs. external transformer.

Certainly agree that moving the trans. outside the box would help greatly with space but that will place more 230/480V wires external to the box which may be a safety issue for them and we'd have to run the 120V back into the box as well. So, as usual, solving one problem really only means you open up a series of other possible problems that have to be addressed.

However, I do appreciate everyone's input on this.


----------



## Jlarson (Jun 28, 2009)

You don't even need to have any wire/conduit exposed. Mount the xformer right to the side and chase nipple through the back.


----------



## JRaef (Mar 23, 2009)

Jlarson said:


> You don't even need to have any wire/conduit exposed. Mount the xformer right to the side and chase nipple through the back.


Yep, that's the way I've done it.









(Not mine, just the only example I could find)

Still, with no access to the design criteria, that's tough, although you might propose it as a "Value Engineering" option.

Is the PLC going to replace that thing that looks like a multi-function timer? You might have room as is if that's the case.


----------



## Jlarson (Jun 28, 2009)

The relay with the blue pots on the front looks like a PLR. That would explain the there wires tapped into the first fuse holders line side terms.


----------



## JRaef (Mar 23, 2009)

Jlarson said:


> The relay with the blue pots on the front looks like a PLR. That would explain the there wires tapped into the first fuse holders line side terms.


Good eyes.

I think the layout leaves something to be desired. I don't like running power conductors on top of wire duct like that. I would have at least stood off the duct from the pan and run those conductors under them, not over them, and strapped them down. If there is ever a fault, those wires will whip around in there. I would have rather stood the big fuse holders off of the back pan and loop up and back down to them so as to be able to put the soft starters on the right side right below their fuses. That way the conductors would not have to cross the ducts at all and I think there would be more room for the other components on the left side.


----------



## GuessLogical (Jan 17, 2015)

Never had a problem at these voltages. I have PLC's in many panels that are mixed with every voltage from 600 on down. Follow the PLC manufacturers guidelines for spacing, and yes if they say keep the PLC 4 inches clear all around that means no wire duct encroaching within that 4 inches.

Regarding signal conductors (analog signals), AK_Sparky hit the nail on the head. If you cannot isolate your signal conductors make certain you cross at 90 degrees to prevent any induction from a power conductor.

Use good shielded cable (foil shields should be outlawed in my book) ground at one end only unless you want to make an antenna, I ground in the panel close to the controller, never had a problem.

Anything under 600V is not really that high voltage.

I never run signal wires in the same raceway as power conductors. 

GL


----------



## sealyons (Jan 15, 2015)

Quite a few points to discuss. Really appreciate everyone's feedback.

First, yes the xformer could be directly mounted to the cabinet (as shown in above image), to minimize external wires but don't know if the customer has any issues with doing that. Have asked, no response yet.

The PLC is currently in another cabinet, it's not something new being added, they just have a desire to reduce panels if possible. Don't believe there is a timer here, the white component (with blue pots) is a Phase Monitor which will stay.

Currently, I believe this entire panel is AC. Bringing the PLC (and HMI (sorry, didn't mention that earlier) into this panel will mean it will have to now carry both AC & DC. Will certainly have to make ducting changes (more on that later). I agree the layout could use some work (but again, customer is not requesting this). I also agree with issues with running the PCS feeds from the fuses on top of the duct (that would certainly be a last resort). Not sure why they didn't rearrange things to get the PCS units to the right under the fuses and put all DIN components on the left where the PCS's are now. I have to believe they tried but who knows why it remained as is. I believe the disconnect at the top could be moved up a bit, the fuses moved up, and the PCS's mounted basically where the DIN is now (need to get more info on PCS's used and their mounting clearances). Hard to tell without more info but I'm guessing the clearance dimensions are why they had to mount the PCS's where they did. I'm also not a big fan of placing the xformer main breaker (CB1) so low in the panel. That is feeding the 230/460VAC right through all the duct work (back & forth). At least that could have been placed on the fuse DIN rail. Also don't like the stick-on cable tie mounts running some of the wires outside ducts. How would others address this? Obviously, if the panel layout could be adjusted (and maybe it can, I've asked and it's up to the customer at this point), things would definitely be done differently.

Regarding ducts, I fully understand the need to keep DC and AC signals in separate ducts if at all possible, mostly to minimize haphazard crosses (I don't believe ducting is going to reduce EM unless there is sufficient separation space). Of course, space is always at a premium and we've had to run some DC control wires in AC ducts at times. Are others staying hard & fast to the separate duct rule or do you find that at times it has to be done? And when running separate ducts, do you follow any specific spacing guidelines regarding the separation distance? What about using spacers within a single duct to create separate channels, any comments on that? Even if the customer agrees to move the PCS's (and it's technically possible) so we can move the DIN rails to the left, I'm not sure we could find room to add another set of DC ducts to the panel. It might be possible. There are not many wires here, so maybe a slightly larger duct with an internal spacer would be an option. Appreciate any comments.

Thanks for everyone's guidance, really appreciate it.


----------



## JRaef (Mar 23, 2009)

PCS? You mean the soft starters? Keep in mind that for those, and the main disconnect, you need the proper wire bending space between the field terminals and enclosure walls. The space they have looks like it MIGHT be a little overly conservative, but I don't know the details. Are you a UL508A shop? If so, those details are in your procedure. That's why I mentioned standing the fuse blocks off of the panel, to allow more of the space for the footprint of those soft starters.

If they are wanting to eliminate a complete separate box where the PLC is located now, can't they live with a slightly larger box for this? They still gain.

PS: Ah, I see. The S+S soft starters are called their "PCS Series", I get it now.


----------

