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Author Topic: New AV panel with ground path back to orginal building AC panel.  (Read 5102 times)

Nicholas Bailey

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New AV panel with ground path back to orginal building AC panel.
« on: December 21, 2016, 07:22:20 am »

Guys, 

I am working on a install in a older country church.  The electricans had
put in a new panel to service the sound booth, amp rack, and LCD.  They skipped a step and the outlet that was installed on the metal structure that the projector is flown from was wired into the orginal building AC.  We did get them to come back and put a outlet serviced off the new panel on the same metal structure. They left that outlet wired to the old building panel mounted as well.

Could both outlets mounted to the same structure which is creating both grounds to have
continunity together cause problems.

I am chasing a issue of the projector randomly dropping the HDMI signal.

I also learned that the new AC at the projector and the booth are on different phases.  I read 247 volts from hot at projector to hot in sound booth.

Other strange things is during testing I put a UPS in the booth on the AV rack.  When that rack was powered from the UPS I could turn off light switches off and make the sound system pop.   With the UPS taken out of the AC path, I was not getting the audiable pop, but the image on the projector would blank
and reappear.

I will be back onsite today to do more tests.

I look forward to your comments.

Nick B

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Mike Sokol

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Re: New AV panel with ground path back to orginal building AC panel.
« Reply #1 on: December 21, 2016, 08:19:44 am »

Guys, 

I am working on a install in a older country church.  The electricians had
put in a new panel to service the sound booth, amp rack, and LCD.  They skipped a step and the outlet that was installed on the metal structure that the projector is flown from was wired into the original building AC.  We did get them to come back and put a outlet serviced off the new panel on the same metal structure. They left that outlet wired to the old building panel mounted as well.

Could both outlets mounted to the same structure which is creating both grounds to have
continuity together cause problems.

I am chasing a issue of the projector randomly dropping the HDMI signal.

I also learned that the new AC at the projector and the booth are on different phases.  I read 247 volts from hot at projector to hot in sound booth.

Other strange things is during testing I put a UPS in the booth on the AV rack.  When that rack was powered from the UPS I could turn off light switches off and make the sound system pop.   With the UPS taken out of the AC path, I was not getting the audible pop, but the image on the projector would blank
and reappear.

I don't have much experience (yet) with ground loop currents affecting HDMI/SDI video, but I do know that with VGA (Analog) video it can create some pretty nasty hum bars on the video screen. But I'm sure that sufficient ground loop current can make a digital signal lose clock and blank the screen. If you have a basic clamp ammeter ($50 at a big box store) that can read down to 10 mA resolution (most of them can), then simply clamping it around the exterior of the HDMI cable will tell you if there's significant current. In the audio world, XLR inputs with the "pin-1" problem will hum with as little as 10 to 100 mA, but again I don't know how much is tolerated by HDMI. You can also disconnect the connector of the HDMI cable feeding the projector and measure the AC voltage between the shield of the HDMI connector and the chassis of the projector. Again, in the analog XLR world as little as 100 mV (1/10 volt) can create enough ground loop current to hum. But I don't know how this relates to HDMI video.

The projector being on a different AC power leg (phase) shouldn't have anything to do with this. And I believe there are companies that make HDMI isolation transformers for just such situations. Probably cheaper to get an HDMI balun than to spend days troubleshooting where the ground loop current is actually coming from. And remember, there can be a lot of current flowing around in building steel, so even grounding to two different structure points in the same building can result in a few volts difference, which often turns into a few amperes of loop current.

Don't know about the UPS pop, but that's grist for another think.
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Nicholas Bailey

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Re: New AV panel with ground path back to orginal building AC panel.
« Reply #2 on: December 21, 2016, 09:09:55 am »

I don't have much experience (yet) with ground loop currents affecting HDMI/SDI video, but I do know that with VGA (Analog) video it can create some pretty nasty hum bars on the video screen. But I'm sure that sufficient ground loop current can make a digital signal lose clock and blank the screen. If you have a basic clamp ammeter ($50 at a big box store) that can read down to 10 mA resolution (most of them can), then simply clamping it around the exterior of the HDMI cable will tell you if there's significant current. In the audio world, XLR inputs with the "pin-1" problem will hum with as little as 10 to 100 mA, but again I don't know how much is tolerated by HDMI. You can also disconnect the connector of the HDMI cable feeding the projector and measure the AC voltage between the shield of the HDMI connector and the chassis of the projector. Again, in the analog XLR world as little as 100 mV (1/10 volt) can create enough ground loop current to hum. But I don't know how this relates to HDMI video.

The projector being on a different AC power leg (phase) shouldn't have anything to do with this. And I believe there are companies that make HDMI isolation transformers for just such situations. Probably cheaper to get an HDMI balun than to spend days troubleshooting where the ground loop current is actually coming from. And remember, there can be a lot of current flowing around in building steel, so even grounding to two different structure points in the same building can result in a few volts difference, which often turns into a few amperes of loop current.

Don't know about the UPS pop, but that's grist for another think.

Thanks for the reply.

The HDMI cable is a fiber cable.

I am wondering if there is a bootleg ground on the old wiring and that current is flowing through the steel and effecting the ground on the new panel.


Nick
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Mike Sokol

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Re: New AV panel with ground path back to orginal building AC panel.
« Reply #3 on: December 21, 2016, 09:22:55 am »

Thanks for the reply.

The HDMI cable is a fiber cable.

I am wondering if there is a bootleg ground on the old wiring and that current is flowing through the steel and effecting the ground on the new panel.

Well a fiber cable can't carry any loop currents, so that's out. However, get the electrician to open up the sub-panel and make sure the green "ground" screw isn't inserted in the Neutral bar bonding it to the ground of the box. Also, the entire sub-panel is probably bolted to building steel, so unless it has a separate (isolated) ground bus, then it can be tricky to get all EGC's at the same voltage. However, as long as all your A-V gear is powered from the same sub-panel, this usually isn't a problem. That being said, if this is an industrial building with heavy equipment, there can be a lot of electrical noise on the steel of the building itself. The NEC won't care about this because the voltages are so low as to be safe for humans, but they can create substantial loop currents between A-V gear introducing all sorts of nasty effects.
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Nicholas Bailey

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Re: New AV panel with ground path back to orginal building AC panel.
« Reply #4 on: December 21, 2016, 09:59:29 am »

Well a fiber cable can't carry any loop currents, so that's out. However, get the electrician to open up the sub-panel and make sure the green "ground" screw isn't inserted in the Neutral bar bonding it to the ground of the box. Also, the entire sub-panel is probably bolted to building steel, so unless it has a separate (isolated) ground bus, then it can be tricky to get all EGC's at the same voltage. However, as long as all your A-V gear is powered from the same sub-panel, this usually isn't a problem. That being said, if this is an industrial building with heavy equipment, there can be a lot of electrical noise on the steel of the building itself. The NEC won't care about this because the voltages are so low as to be safe for humans, but they can create substantial loop currents between A-V gear introducing all sorts of nasty effects.

The building is a wood contruction and sub panel is mounted to to the brick
ouside wall and a single conduit connects it to the main.
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Tom Burgess

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Re: New AV panel with ground path back to orginal building AC panel.
« Reply #5 on: December 21, 2016, 10:22:57 am »

Nicholas,

What are the brands and models of the HDMI source, the fiber HDMI cable, and the projector.  Are there any other terminations / adapters / wallplates / etc. in the run?  What is the total length of the fiber run?  What is the total length of the non-fiber run, if there is one?
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Stephen Swaffer

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Re: New AV panel with ground path back to orginal building AC panel.
« Reply #6 on: December 21, 2016, 12:39:02 pm »

What wiring method is used to gt power to the projector?  EMT, MC, romex (surely not?)  It might be fairly easy to get set up an isolated ground receptacle which which allow the receptacle ground to be at the same potential as the subpanel ground.  But with a fiber HDMI cable, I doubt you have a ground loop issue.

Mike mentioned not having a green bonding screw installed in the subpanel.  With the disclaimer that I know nothing about the installation or the qualifications of the electricans, keep in mind that Mike is assuming that the subpanel has 4 wires (or at least 3 wires plus a conduit EGC-but to be an EGC it has to meet certain requirements).  If the subpanel only has three wires it really can't be made right by removing or installing that screw-but removing it could be potentially hazardous.  In any case, that particular shortcut can cause all kinds of noise and other issues.
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Keith Broughton

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Re: New AV panel with ground path back to orginal building AC panel.
« Reply #7 on: December 21, 2016, 03:29:45 pm »

outlet that was installed on the metal structure that the projector is flown from was wired into the orginal building AC.  We did get them to come back and put a outlet serviced off the new panel on the same metal structure.
So let me see if I have this straight...
The ground connection to the new AV panel is run straight back to the old panel. (nice)
You have an outlet from the AV panel connected  to the steel structure.
You also have an outlet from the old panel mounted to the same metal structure.
So if those are both metal boxes, you now have the new ground connected to the building structure.
Does anyone else think this is problematic?
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Stephen Swaffer

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Re: New AV panel with ground path back to orginal building AC panel.
« Reply #8 on: December 21, 2016, 06:16:14 pm »

From a code/safety standpoint, no-if it is indeed structural steel you can use it as a ground.

From a technical standpoint, in my mind, it would raise a red flag-especially with a metallic path from AV booth/gear to projector.  If the fiber actaually isolates things, I wouldn't expect it to be a problem-but I would try to find a way to eliminate it as the culprit.  Hence the suggestion to install an ISO ground receptacle, if it can be done reasonably painlessly.

The other option I have used is I built a short extension cord with a PVC box/GFCI receptacle that has no through ground connection.  Carefully built to eliminate any possible safety issue,  it allows me to break a ground loop for testing purposes.
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Mike Sokol

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Re: New AV panel with ground path back to orginal building AC panel.
« Reply #9 on: December 22, 2016, 12:12:35 am »

Mike mentioned not having a green bonding screw installed in the subpanel.  With the disclaimer that I know nothing about the installation or the qualifications of the electricans, keep in mind that Mike is assuming that the subpanel has 4 wires (or at least 3 wires plus a conduit EGC-but to be an EGC it has to meet certain requirements).  If the subpanel only has three wires it really can't be made right by removing or installing that screw-but removing it could be potentially hazardous.  In any case, that particular shortcut can cause all kinds of noise and other issues.
In general, if this were a new sub-panel, could it be installed without a fourth "ground" wire and still pass inspection (not be a code violation)? I know that this sort of thing must have been common in pre 1960's wiring, but by the time I was reading and using my first NEC code book in 1978, that lack of a dedicated EGC wire was a code violation, at least in the industrial environment I was working in. But I really didn't work on residential wiring at the time, so maybe I missed something.

I've found ground loop currents in a Texas church and quickly discovered that the metallic conduit itself was the only EGC and apparently to code when it was installed perhaps 15 years ago. Don't know if that was a county or city thing, but the electricians there said it was common wiring practice in all the commercial buildings in the area.

I guess the adage "Assume Nothing" should be written on every electrical panel.  ;)
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Mike Sokol
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Re: New AV panel with ground path back to orginal building AC panel.
« Reply #9 on: December 22, 2016, 12:12:35 am »


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