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#71421 - 10/29/06 07:14 AM IWire...or any other commercial guys.....(especially retail....)
mahlere Offline
Member

Registered: 11/17/05
Posts: 514
Loc: New Jersey
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?

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#71422 - 10/29/06 07:31 AM Re: IWire...or any other commercial guys.....(especially retail....)
NJwirenut Offline
Member

Registered: 09/15/01
Posts: 816
Loc: Bergen County, NJ
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.

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#71423 - 10/29/06 07:48 AM Re: IWire...or any other commercial guys.....(especially retail....)
mahlere Offline
Member

Registered: 11/17/05
Posts: 514
Loc: New Jersey
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?

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#71424 - 10/29/06 07:58 AM Re: IWire...or any other commercial guys.....(especially retail....)
yanici Offline
Member

Registered: 09/03/06
Posts: 57
Loc: Atlantis
Mahlere, just curious if the building steel is grounded?

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#71425 - 10/29/06 08:07 AM Re: IWire...or any other commercial guys.....(especially retail....)
mahlere Offline
Member

Registered: 11/17/05
Posts: 514
Loc: New Jersey
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.

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#71426 - 10/29/06 08:08 AM Re: IWire...or any other commercial guys.....(especially retail....)
iwire Offline
Moderator

Registered: 01/05/03
Posts: 4343
Loc: North Attleboro, MA USA
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.
_________________________
Bob Badger
Construction & Maintenance Electrician
Massachusetts

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#71427 - 10/29/06 08:10 AM Re: IWire...or any other commercial guys.....(especially retail....)
mahlere Offline
Member

Registered: 11/17/05
Posts: 514
Loc: New Jersey
there is an idea

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

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#71428 - 10/29/06 08:30 AM Re: IWire...or any other commercial guys.....(especially retail....)
LarryC Offline
Member

Registered: 07/05/04
Posts: 776
Loc: Winchester, NH, US
 Quote:
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.


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

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#71429 - 10/29/06 08:40 AM Re: IWire...or any other commercial guys.....(especially retail....)
mahlere Offline
Member

Registered: 11/17/05
Posts: 514
Loc: New Jersey
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.

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#71430 - 10/29/06 09:07 AM Re: IWire...or any other commercial guys.....(especially retail....)
Dnkldorf Offline
Member

Registered: 12/12/04
Posts: 1091
Loc: nowhere usa
(missed the working with the two prong adapter)

Hmmm..



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

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