Here is a situation that we've never come across. We haven't really looked into it (as it will be a change order and it's part of a national rollout campaign)

We just installed circuits and outlets for Plasma, LCD and other TV's/Home Theatre Equipment. The entire time, my guys would get what seemed like bad static shocks when the touched building steel. Didn't seem like electrical shocks, but static. Didn't get any readings from the steel.

Finished the installed, turned everything on, and there were lines across the TV's. Told by the Project Manager that this has been happening at all the sites, it was usually a ground problem.

Tested the outlets, had 1.0V between ground and neutral at the outlet. Went back to the panel (these all came out of the same sub-panel) did not have any voltage between G-N. Now mind you, these are all homeruns.

Checked the rest of the store, had between 0.5V and 1.5V in the rest of the store.

A few important facts:
3PH 120V circuits.
Shared Neutral (3H, 1N, 1G)
The Ground was shared between 3 sets of wires (all in one conduit)
All boxes were grounded through the EMT and with ground screws.
All circuits 20A run with #10 THHN (no run longer than 120')

When the TV's are plugged in with a two-prong adapter (disconnecting it from the ground) it gets rid of the lines.

Apparently, there are engineers working on this as we speak. But has anyone ever come across this?

BTW- this store sells Home Theatre systems. These TV's and components will be installed in homes with questionable electrical systems. How unhappy will people be when their $3000 TV doesn't work right in their 40 yr old house?

Are the monitors and the signal sources(distribution amps, etc.) feeding them on the same circuits? If you disconnect all cabling except the power cord, then feed one of the monitors from an isolated signal source (like a portable battery-operated DVD player, perhaps), do you still get the noise bars in the picture?

I suspect that there may be a "ground loop", causing a current flow in the shields of the video/antenna cabling between the signal source and the monitors. Possible solutions include getting the monitors and signal sources on the same branch circuits, making sure that both circuits are grounded back to a common reference point (isolated grounds might help here), or installing special isolation transformers in the video feeds to the monitors.

NJ,

We have a total of around 12-14 circuits feeding the outlets.

If it is a ground loop, how could we eliminate it? The circuits are already homeruns. With insulated ground wires.

My first though is to replace all the outlets with IG receptacles and disconnect the ground wire from the boxes. I have a feeling that we are getting the loop through the conduit attached to the building steel. But this is just a theory.

All the TV's are connected back to a main A/V system through their A/V cables. These cables also appear to figure into the problem.

But my main concerns are these:

This same problem has occurred in other stores across the country.

The entire store had an issue with the grounding.

If the TV's/equipment is this sensative, how will it work on the average home electrical system?

Mahlere, just curious if the building steel is grounded?

not sure. Service is grounded to the steel. But not sure if the steel is grounded. Possibly a bad connection on the water main?

Building is about 15 yrs old, service is original.

About 45,000 sq ft. I'm gonna guess that the steel is grounded at some point.

Also, main service is 480/277V. Tranformer is feeding all the 208/120V. Appears that all grounding conductors are installed. Didn't open the transformer to check though.

The 1.0 volt between neutral and ground is likely the result of voltage drop on the neutral. I doubt it is causing your problems.

I do believe NJ is nailing it.

This sounds all to familiar, we did a series of stores for the same purpose. We also had these same problems.

They did not send me out to troubleshoot, the person they did send out worries me. I got the impression he drove ground rods at the gondolas and lifted the real EGC.

there is an idea

we've done a ton of these for different stores, never a problem. this is the first time.

If you are feeding 3 different circuits of receptacles and are using a shared neutral, sounds like a "classic" harmonics problem. All of the new equipment use switching power supplies. The non linear current spikes are NOT canceling on the shared neutral. That explains the voltage difference between the neutral and ground at the receptacles.

The fix for that is to run dedicated neutrals for each circuit.


The hum bars come from AC current running on the video cable shields. Dedicated neutrals might fix that. Verify that the source for the video and audio feeds comes from the SAME subpanel that feeds the TVs. If there is a small difference in voltage between the grounds at the TVs and the Video source, current will flow on the video shields and give you the bars. Also verify the Audio and Video cables are not laying on top of the lighting fixtures or tied to conduits carrying power.

Larry C

Larry,

Dedicated neutrals is not an option at this point. Job was spec'd with shared neutrals, conduit sized accordingly.

They are all coming out of the same panel.

We did shut off 2 of the 3 phases on each setup. still had the problem.

I will check into the A/V cables. They are definately touching the conduits (and some MC) under the gondolas.

(missed the working with the two prong adapter)

Hmmm..



[This message has been edited by Dnkldorf (edited 10-29-2006).]

Larry separate neutrals, even if possible will not 'fix' this problem. The harmonics remain even on a two wire circuit, all that changes is the point where the harmonic current starts multiplying on a single neutral..with two wire circuit you have just moved the point to the panels feeder neutral which will still be supplying the equipment.

Also harmonics are becoming less of an issue, the European standards have forced the manufactures into dealing with the harmonics at each device.

If it anything like what I saw you have dozens of pieces of equipment interconnected via a switching device and spread out over a large area so much of the equipment ends up on different circuits.

Bob, I've seen this once before, and it ended up being the multiplexer, and cabling.

But the "working when pluged into a 2 prong", throughs my thought off.

Now it don't make sense, if it happens all over the country.

it's about 60 tv's spread out over around 2000 sq ft. different circuits. but they all interconnect into a central computer (so that the programming can be uploaded and changed remotely)

"turned everything on"

Have you tried bringing things on a few at a time and see if you can develop a pattern to the failure?
Different circuits, different types of equipment etc. Think about where the TV signal is coming from and where the TVs are being powered. You might be able to recreate the failure with 2 appliances and that makes it easier to get your head around. When I worked on power quality problems the real fix usually ended up being a regular article 250 issue, like a bad grounding connection somewhere. Usually they had tried lots of exotic things first.

Are the shields to the signal cable properly grounded at one end?

"Bob, I've seen this once before, and it ended up being the multiplexer, and cabling."

You will find the cabling and cable distribution is the problem, the bars are telling you this, their cable distribution has a problem, not your installation, in sayreville, we had a location with the same problem your describing, it turned out to be a faulted cable distribution amp, on a pole 1000 ft down the street.

That was my thinking Les, but what is with the "working with a 2 prong"

I'm lost on that.

i'm leaning towards their equipment or connection.

we turned everything off and on one circuit at a time, didn't make a difference.

they aren't using a cable signal from outside. They all run off a computer (for lack of a better term) that runs pre-programmed commercials for them on all the tv's.

there definately is a problem related to the difference between the neutral and the ground (1.0V) when they plug all the tv's in with 2 prong adapters, the problem disappears.

But it's a problem in the entire store.

I honestly don't know why the problem occurs when then use the ground prong.

I also, don't think that the voltage between the neutral and ground should be a problem.

In the past few years we've done setups like this for thousands of TV's, and have never had this problem.

However, this is the first time for this store and the first time with their equipment.

I bet Yanci just pinned this. I had the same problems when running a closed loop system running remote processors. Fixed it after going back thru everything and making sure the shields were only tied in once at the same place.

LarryC has pretty much spelled out the solution. The monitors AND the equipment feeding signals to them need to be fed out of the same panel. And separate neutrals for each circuit can only help.

A quick and dirty solution would be isolation transformers in the audio and video lines. Something like this:
http://www.kramerelectronics.com/indexes/item.asp?desc=569

not sure about the sheilding on the cables. we didn't install them. How would we check this. All their cables are pre-made with RCA jacks.

they are all fed out of the same panel. separate neutrals are not an option at this point. (both due to conduit fill and derating)

this job was installed as spec'd.

We were tempted to run a new ground directly from building steel to the CPU. Didn't know the effect of it and did not want to be on the hook for $100K in electronics.

To check for any accidental grounds to the shield, remove the cable from its source and then see if there is any continuity to ground. Should be open to ground.

BTW I've had to run a separate EGC all the way to the Grounding Electrode on some old main frame computers. Similar to an isolated ground. Maybe the way to ground the CPU.

A while back I had a simular problem in my CCTV set up, and through the advice here at ECN and some other sources eventualy corrected it. Heres an article about it: http://www.epanorama.net/documents/groundloop/video_isolation.html
Hope it all works out, but I would not promote the lifting of ground with a two/three prong adaptor.

Oh, another question, is the video feed grounded at all? Still thinking about those static shocks from the begining of the thread....

I understand they do not have a cable company feed, it's most likely, that their in store distribution amp is faulty and i bet if you measured from the cable shield to ground you would be supprised at the reading you get, this is a common problem with distribution amps, poor design, or bad instalations, just remove the cable from one of the sets and feed it with a isolated signal, i bet it clears up, there is AC getting into the signal distribution.

[This message has been edited by LK (edited 10-29-2006).]

Grounds- should either be everywhere if you are shielding for RF or single point if you are shielding for low frequency like less than 50 KHz.

The shells of the Video lines are either capacitor coupled or direct coupled to the earth ground of the power cord.

Check if the Audio-Video patch panels are grounded to the building steel, thru the mounting cabinet.

Replace the PC video source with an isolated video source and see if the lines disappear. PC's and video have not been a good mix in my experience.

Larry C

Hello everyone. From reading through this thread I have a question about the diagnosis of a faulty distribution amp from the video feed. Mahlere mentioned that using a two/three prong adapter corrected the problem. By isolating the equipment from the egc, how is that correcting the problem?

Let me just say to that the faulty distribution amp diagnosis makes sense to me, the two/three prong adapter however, it's a curve ball for me.

This really sounds like a Ground Loop (Circulating) Current problem - mainly due to the fact that the noise disappears when the 3 wire to 2 wire "Cheater" adapter is used.

My guess is the signal lines (A/V patch cords) are grounded at both ends - the Monitor end and the Distribution Amplifier end.
(BTW, by "Grounded At Both Ends", I mean each device has a physical connection to an EGC, which is also physically connected to the signal line cables' shield).

If this is true, the only way to eliminate the problem(s) will involve a single point grounding approach for the signal leads.

The A/V cables will need to have the shields + drains ground bonded at one end only (if using a coaxial cable with RCA jacks, for instance, the outer "Ring" of the connector).
In the case of multiple monitors at various locations, the "Star Point" for grounding the shields might best be done at the Distribution Amplifier.

The really interesting thing now will be how to isolate the ground connection of the signal leads at each monitor!

Another player in the creation of the noise may be from an additional (or sole) ground loop between the computer and the Distribution Amplifier.
If the computer links to the Amp via an RS 232 Serial Cable, that would be an additional ground loop path.
If connected via an Ethernet NIC, there should be an opto - isolator already on the interfaces, but something may have caused the unused pins of the RJ45 modular jack to be pulled to ground (???).

When the cheater adapter is placed in the AC Power Circuit's cord ... (for the monitor(s), right???)... the noise disappears. This points to a ground loop issue, most likely looping through the signal lines.

If it's not possible to eliminate the shields from being grounded at the monitors, then you will have to eliminate the grounding at the Amplifier ends of the signal lines and leave the monitor ends as they are, having the signal lines grounded at the monitors (only).

Contact the client's preferred vendor (for the A/V equipment), and get them to figure out how to isolate the shields from the grounded enclosure of the Amplifier (let them suggest and/or supply an isolation means for the signal lines).

The 1.0 Volts read N-G is normal - and a lot less than I would have expected! As Bob (Iwire) mentioned, it's the result of normal Voltage Drop in the branch circuitry.

Got a question about the noise (the interference, or lines, displayed across the monitors):

Do the "Lines" roll Horizontally (either Up or Down, maybe both), Vertically (to the Left or Right, maybe both), or do they roll diagonal?
As they roll, are the lines "solid" or "dashed"?
Lastly, is there audible noise present with the rolling lines?

Good luck man! Really wish I had 100% definite answers for you - it just really points towards loop currents in the signal lines.

Scott35

Scott,

I'll read that in the morning. My head hurts and I'm dizzy trying to understand that all right now

E57 - thanks for the link to the article.

I also just realized that I am a complete moron sometimes. The reason that I was getting a voltage between neutral and ground? very obvious....the 3 phases were not exactly the same draw, the neutral was simply carrying the unbalanced load of the 3 phases. Doh!

Doesn't solve the problem, but I'm kicking myself for not realizing that right away.

This same problem has occurred in other stores across the country.

The entire store had an issue with the grounding.

You also mentioned that you have not checked the transformer(s)

I have regrounded stores where misc. hard to define computer problems were happening. The procedure was to drive a new copper ground rod, sometimes 2, loop a run of # 1/0 between them, through the gutter beneath the panels, then bonded the ground bars and cans of each panel with bronze lugs on sanded surfaces back to the big 1/0, with #2 into each panelboard. This was also attached to bldg. steel. You have probably already read the thread about grounding transformers from a few days ago, and as somebody else said, this is probably an art. 250 violation. Not saying that it is not a more sophisticated/complex issue, but look at the obvious problems that might be causing your problem to go away when you use grounding adapters; if removing the electronics from the "reference point" i.e. earthing as the Brits call it, removes the symptoms, perhaps you have a lackadasical, casual, almost grounded service. Remember, the code is only the minimum standard...think about it...

[This message has been edited by Almost Fried (edited 10-29-2006).]

[This message has been edited by Almost Fried (edited 10-29-2006).]

I'd suggest trying a couple of simple tests:

1) What is the voltage difference between neutral and ground at the receptacles with everything plugged in and everything _off_. If everything is off, then there should be no voltage drop in the neutral, and any voltage between ground and neutral means another source for the problem.

2) With one of the TV sets unplugged from the power supply, but still connected to the signal cables, check for continuity between the ground pin on the plug and the electrical system ground. If you have such continuity, then this indicates that there is a parallel ground path via the signal cables. Loop area between the electrical system ground and the signal cable ground can pick up interference.

When you say 'RCA jack' that give me shivers for such a large distribution system. An RCA jack is a two terminal coaxial connector. Unless there is a separate ground strap, this means that the coax cable shield is being used as one of the signal conductors. From distant memory (confirm with other sources This is fine for a few feet in a home, but not suitable for a large system. Ideally the signal would be carried by two conductors that are isolated from ground, and then an over-all shield is grounded at _one side_ only, and is not expected to carry any signal current.

-Jon

I deal with this all the time in CCTV systems, problem is caused by current on the shield or ground of the signal cable. Most all equipment have the safety ground and signal ground tyed togeather. When you use an grounding adapter you lift the ground loop and isolate the equipment ground, no current can flow. Solutions, Make sure all equipment is served by the same panel, Use isolation transformers in the coax or signal lines. Make sure the signal line, or coax grounds DO NOT contact the bldg anyware in their run. An isolated ground ckt to all equipnent served to 1 ground point. Make sure the computer or whatever supplies the video to the displays is isolated and ONLY gets its ground from the isolated power cord. This applies to any signal distrubution equipment. Isolation transformers in the signal lines or fiber suppling the signal may be the easy way to correct the problem. Robert

"Let me just say to that the faulty distribution amp diagnosis makes sense to me, the two/three prong adapter however, it's a curve ball for me."

The curve ball is, when uses the adaper he removes the ground, and the cable stops suppling the AC fault from the distribution amp.

Ok that makes sense to me now LK. It's good to actually be able to say that too! (btw)

I'd work from the display back towards the source.
first check the display with a local test source, if clean, add the cable, and move the test source to the other end. If that is clean, add the DA without anything else connected except the test source and display under test. If that is clean, start adding other displays.
Finally, add the real source.

At the point where the problem appears, you can try adding a humbucker to the video line, and see if that cleans up the problem.

Another possible soultion might be using cat5 with baluns to isolate the video lines, instead of coax.

Mahlere, any update on the problem. Really curious if you know how it was resolved. Just twisting your arm a little!

[This message has been edited by yanici (edited 11-21-2006).]

Don't know. We determined that it wasn't in our wiring and was in the building or the A/V cables. Then we pushed it back to the engineers.

we determined that wasn't in our wiring...it was in the building or AV cables

I am curious if you ever opened up the transformer and source panel{s} to check if the N-G bond was there?

No to the transformer. We did open the source panels, everything was fine. What we did was run a temp line straight from the panel we fed out of, to the units (a couple of MC cables straight from the source so to speak) had the exact same issues.

So the problem was either in their A/V cables (since the entire store came out of these panels with no problems) or there was an issue with the neutral/ground feeding these panels.

Either way, it was out of our scope of work and was passed along to be addressed by those with more stripes on their shoulder than me.

For the record, I'm not taking the front cover off a tranformer while it's live...