PLC mounting near high voltage AC lines ...

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. 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 (small Omron PLC, self-contained, 14 I/O) in a 230V or 460V panel (with transformers), 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.

sealyons,

The plant I used to work at before it closed and went North was dealing with Arc Flash requirements.
on large enclosures we had the 480 volt section in 1 section,
a 240 volt section (if required), a 120 volt section for the power supplies, mcr, safety relay, interface relays, and then the plc section.

on small panels we would place the 480 / 240 volt power & transformer in its own enclosure and feed the power to the plc enclosure.

regards,
james

When you say "sections", what do you mean? Horizontal, vertical? How much space did you leave between "sections?"

Just to help you avoid embarrassing yourself around power engineers, everything 1000V and below is referred to as "Low Voltage". "High Voltage" is between 69kV and 230kV. So your 480V is not really "high voltage".

In general, PLCs and their I/O are designed to be used in industrial systems where they would indeed be in proximity to voltages that are much higher than they actually work with. When designing them there is a NEMA test criteria (ICS 2-230) called a "showering arc test" which subjects the components to the relatively extreme EMI/RFI fields created in a spark gap that is spewing metal fragments, and more importantly, ionized gasses, all around them. The EM fields around regular 480V conductors are insignificant in comparison. Most modern PLCs now will tell you that 2" of clearance all around is all that is necessary. But this by the way is why you should NOT use "home or office grade" electronics in industrial systems, there are no such testing rules involved, only FCC rules about what they EMIT, and they tend to be made as cheaply as possible.

That said, it is VERY important to protect all electronics from the effects of extreme RFI, such as from VFD output cables. That isn't just about PLCs though, hence the recommendations to use shielded power cables coming from VFDs.

And that leads to another important issue. It's not so much of a big deal as to whether or not higher voltages are present, they almost always are. What's far far more important is to pay close attention to the details of how PLCs and their I/O are wired and grounded. RTFM...

I learned a long time ago something in the fire department that transitions over very well to all aspects of my life. When deciding whether or not to do something that might be risky, ask yourself, “what would the headlines say”? If you don’t like what you’re reading in your head, don’t do it. Yes 480VAC isn’t “high voltage” but it’s still nothing to screw around with. If you’re not comfortable, don’t do it. I don’t know all the “ins and outs” of panel building but you own what you build or add too. The very first thing I’d do is find out what the local regulations are and follow them. If you follow the laws, you are less likely to be held accountable if something goes wrong.

sealyons,

we have used both.
as far as spacing goes, some customers want everything crammed in a small enclosure or allow the designer to space things based on experience and manufacturers guidelines.

we try to give at least 2 in. typically 3 in. between terminal blocks and wire duct. that's just a start.

there is not enough time to explain everything.

regards,
james

We have 400V and PLC in same cabinet all the time. Regular 24V I/O and 400V power lines arent that easily interfered, if you are dealing with analog signals and Frequency converter then you have to be more carefull. Mainly with the EMC cable from the frequency converter. We have never experienced any problem but we don't deal with that sensible equiptment either. And it depends on the load of the 400V aswell.

I mean, if you run a 24V I/O in parallell with a loaded 400V line. Do you get a signal at your input? I have never.

Try not to mix

Had a panel with 4 vfd's and a PLC with a RTD card, enough noise to screw up the readings. Did not use VFD cable and it did not matter that RTD cable was grounded correctly, enough noise from VFD's.

If you don't use screened cable for the VFDs you are bound to get trouble

Had a cabinet that was acting wierd. When I came to the site I noticed that they had pulled the screened cable from the VFD away from the screening. Resulting in a high speed input just counting up, up, up, up the regular 24V signals was no problem.

The more sensitive your equiptment is, the more it matters.

I had a 1hp motor running on a VFD. There were 2 stop buttons near the motor and the 24vdc wiring for them was in the same conduit. If the motor was running on 60hz, everything worked fine. If the motor was slowed down, the stop buttons did not work. There was enough voltage induced on the wires to activate the discrete input. I put in a relay and solved the problem.

T Gibbs said:

I had a 1hp motor running on a VFD. There were 2 stop buttons near the motor and the 24vdc wiring for them was in the same conduit. If the motor was running on 60hz, everything worked fine. If the motor was slowed down, the stop buttons did not work. There was enough voltage induced on the wires to activate the discrete input. I put in a relay and solved the problem.

Click to expand...

And you had screened cable for the motor?

sealyons said:

.....

When placing a PLC (small Omron PLC, self-contained, 14 I/O) in a 230V or 460V panel (with transformers), what is the best mounting approach? ....

Click to expand...

Common sense answer.....

Refer to the installation manual provided by Omron, and follow it precisely !! If there is any doubt, do things "better" than they recommend.

You will get no recourse from the PLC manufacturer if things go wrong, and you have not adhered to their installation guidelines.

rQx said:

And you had screened cable for the motor?

Click to expand...

No, if so it probably would have been ok. It was just thhn in 1/2" emt. I should have run a separate conduit for the stop wires, but we were on a budget and time crunch so I took a shortcut and it bit me in the ***.

The majority of machines I work on have 480VAC and the PLC in the same cabinet, that's why people use PLCs instead of a $35 Raspberry Pi. In fact, most PLC IO is optically isolated good to 600V, so if you end up with 480VAC on a 24VDC input, it won't fry all your stuff.

A good panel will have separate cable trays for AC, DC/feeback, and motor cable, usually keeping the AC lines segregated. This is purely for noise issues on communication and analog feedback lines. Motor cable is the noisiest and, for very high performance servo motion, actually fairly vulnerable to noise itself, we try to keep them separate and get them out of the panel in as short a distance as possible (we usually mount drives along the bottom and shoot the motor cable right out the bottom of the panel).