Electrical sanity check: nonzero voltages to ground with Red Lion G10

Hi all. I am not an electrical engineer (mechanical actually) but would like to think I have an okay understanding of basic industrial electrical.

My current understanding is that you should never see much more than 0.2V from basically anything to ground. If you do, that indicates current is flowing through ground, and you have a short, right?

Okay, so, based on that premise, I just sent a G10 back to Red Lion for repair and got a new one, but am seeing the same behavior with the new one: basically, all I have to do is touch the cabinet's DC common to the (-) negative terminal on the G10 and I see 3.15V between DC + and ground. I have several other things in the cabinet and none of them caused this. I originally thought I had a short somewhere, checked all wiring, thought the PS or UPS could be bad, switched them out, same thing still happening. I used a different PS altogether and hooked up +/-/GND to the G10 directly (separate from cabinet wiring) and observed the same. As stated before, I am seeing the exact same thing on a brand new one.

I know that a lot of times PSes and whatnot will have a varistor/capacitor/whatever between power and ground for surge/filter purposes, so sometimes you can see a decreasing voltage on a multimeter, but this was staying consistent at 3.15V.

So, am I a retard here? Pretty stumped. Only conclusion I can draw right now is that the circuitry in the G10's power supply causes this.

Any help would be appreciated, thanks all.

It is hard to say what is causing your stray voltage without a wiring diagram of the power coming into the panel and the panel itself. I would first of all see if this voltage is also on the ground of the PLC.

https://en.wikipedia.org/wiki/Stray_voltage

Here is some information on stray voltages. Is the G10 currently not operating correctly?

Regards,

Get a real electrician to help you. Put the meter down and step away from the cool tools. What is a G10? What voltage does it require? Are you sure it gets fed with DC? Most things with a power supply get fed AC...

Wait, what's the problem that's causing you to send the G10 back? Is it fried/smoked?

It just sounds like your ground and DC common aren't tied together causing you to see that stray 3.15V when you wire up the G10.. perhaps bad practice, but not the end of the world.

rawdeal73 said:

Put the meter down and step away from the cool tools. What is a G10?

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This guy..lol

AGill said:

This guy..lol

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Yeah, I never pass up the opportunity to give a mechanic grief Sounds like he is chasing his tail. What's wrong with the G10? Should have been a G6? V8? K so I'm kind of a smarta$$.

You might look through an installation manual for you RedLion panel, and see if there is a note for grounding the hmi back to your panel or power supply. I think AGill is correct that separately derived power sources without common bonding can have potential between them.

The wiring arrangement is very straightforward, it's a small box. Basically incoming 120VAC -> 24VDC PS -> 24VDC UPS -> DC +/- buses -> fused disconnects for device +'s. Incoming earth ground is tied to the box, as is the earth ground connection of the PS. I do not have DC common tied to earth ground.

I sent the G10 back because of seeing nonzero voltages to earth ground (and everyone else here agreeing that wasn't normal, including an electrician), and a control valve that was working (everything just sitting here in shop, no one touching it) connected to the same box died about the same time the Red Lion was powered on (which now it's sounding like was just a fluke). I had assumed that there was a short somewhere that damaged it; found out the Red Lion was the cause of this nonzero voltage to ground and assumed there was an internal short. Got the new one today and seeing the same ****.

Red Lion installation manual says that it can operate with DC common tied to earth ground or not tied, but that the ground terminal should be connected to earth ground, which we have done.

G10 was operating correctly before sending it back.

Just a wild *** guess. The G10 has a coin type lithium battery inside.

ps: silly nanny filter.

I do not have DC common tied to earth ground.

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If you dont have the DC common tied to earth, the DC common is floating and can have a voltage differential to earth ground.

So re-reading what I put, just one thing to clarify, "My current understanding is that you should never see much more than 0.2V from basically anything to ground. If you do, that indicates current is flowing through ground, and you have a short, right?" was a brainfart, I meant to say that something is putting a potential to ground, not current through; simply having a different in potential between the two points doesn't mean current is flowing (except a small amount through the multimeter).

Gomez_ said:

If you dont have the DC common tied to earth, the DC common is floating and can have a voltage differential to earth ground.

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So this makes sense to me, but again I've always seen otherwise, i.e. DC common == ground == 0 V roughly. Nothing else I've ever connected in a cabinet has changed the ground potential when connected.

Admittedly I was taught how to make cabinets by one other guy (who quit about a year ago, so it's just me now), and he never tied DC common to ground. I've only heard about doing so on here, and on the internet after searching about it.

Is tying to ground the de-facto standard? Where is it usually done, at the DC common bus?

It depends on what the devices connected to the power supply need (sometimes they need to be floating). Most of the time in our shop we tie the common to earth ground right on the power supply (if they have multiple terminals).

A >0 voltage reference from DC common to ground is totally normal if it is not electrically tied to ground, as explained. There is much debate over if that is required, but the mere fact of having a voltage potential between common and ground will not necessarily cause problems. Some 24VDC components INTERNALLY ground the common without making it obvious (PanelViews, for example). That may be why you don't typically notice it. But do you really measure DC common to ground voltage on all panels you build?

we have floating 24 and 120 V DC systems on all the plants we build, another company grounds the DC negative. I think in Europe they sometimes ground DC positive. I would say there is no standard and it depends on the application.

We like floating because the battery chargers we use have alarms when there is a positive or negative ground fault, so then we can go find out where there is an unintentional ground before a short circuit occurs. The downside is we have to make sure all of the equipment we use doesn't internally ground + or - DC, and if it does we either mount it isolated or get an isolating supply.

interesting EIP library in your sig.

BTW, thanks everyone for your input/insight. Greatly appreciated!

Discussion on tying 24VDC common to gnd from earlier this year:

http://www.plctalk.net/qanda/showthread.php?t=23355&highlight=floating+voltage