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Author Topic: New AV panel with ground path back to orginal building AC panel.  (Read 4670 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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Steve Swaffer

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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Nicholas Bailey

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Re: New AV panel with ground path back to orginal building AC panel.
« Reply #10 on: December 22, 2016, 06:41:58 am »

Thanks for all the replys guys!

I worked late and did not check the post last night.

Video path is LAPTOP > EXTRON SW 4 4K > EXTRON AUDIO DE-EMBEDDER 4K > EXTRON DA4 4k HDMI DA > CABLES TO GO HDMI POWER INJECTOR > WEST PENN 75 foot fiber HDMI > Panasonic Projector.

Resolution is limited to 720p on this path because of the churchs cheap laptop.  We have a Clickshare set at 1080p on input 2, and it also shows same issue.

Yesterday I did pull the old outlet off of he stucture that projector was mounted to and that did help some.

One think that I learned is this panasonic projectors AV connectors show a 1 ohm connection to the actual Chief projector mount.

We also got all of my 4 new outlets on the same phase as well.

What I still cannot explain is this back stage I have a QSC low power monitor amp and a K-Array powered 12 inch sub.

When the power sequencer powers up those two devices by triggering a Lowell RPC1 the video on projector will flash to black for a second, but the reshow the image.

If I run a extension cord from the sound booth to that location the problem disappears.

I know now that the video signal is being dropped in the Extron hdmi DA.  If I take my Sencore generator to the actual fiber cable connected to the projector it is solid.  If my path is SENCORE > DA > Fiber cable > PROJECTOR, it will make it flash with all AC back connected to the correct locations.

I am going back today with a new DA and a ground impedance meter and see what I can find out.

I can also make it fail some times when I run the projection screen up and down.

I have learned a ton reading this message board.


Nick Bailey
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Keith Broughton

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Re: New AV panel with ground path back to orginal building AC panel.
« Reply #11 on: December 22, 2016, 06:51:45 am »

If you removed the old receptacle from the structure, the new one is still connected to building steel, as is the projector frame.
Iso the NEW receptacle, as suggested earlier, AND iso the projector mounting bracket from the steel.
This way, the projector is grounded to the AV panel only.
Still, seems odd this would be a problem of grounding when using fibre but this kind of trouble shooting is taking one step at a time.
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Cailen Waddell

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Re: New AV panel with ground path back to orginal building AC panel.
« Reply #12 on: December 22, 2016, 07:39:02 am »

Is it possible you have a bad distribution amp?  Or that there are hdcp issues causing the DA to resync?

I'm not negating what sounds like a weird power issue contributing, but it also sounds like behavior for a loose/bad hdmi connection....  ugh I hate hdmi....   

Another thought:
I would be interested to know are there big power dips when you run the screen or turn on the qsc sub?   I'm wondering if a less than solid connection in the building wiring (or even on the pole) is causing a power dip that the DA is especially susceptible to?   


Sent from my iPhone using Tapatalk
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Stephen Swaffer

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Re: New AV panel with ground path back to orginal building AC panel.
« Reply #13 on: December 22, 2016, 11:16:51 am »

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.  ;)

I doubt it would pass inspection without 4 wires--- assuming there was an inspection 😀. I may sound cynical, but having 15 years experience as an electrical contractor, my cynicism is solidly grounded in experience.

Strictly by the NEC, EMT can be an EGC-the stick of emt in the middle of a run has to be bonded- where is its EGC?  But there are conditions-no concentric ko's, etc. probably shouldnt swerve too far though.

Where does the monitor amp and K sub get their power? Could there be a bootleg ground allowing a normal power on spike to be transferred to/through a building ground?
« Last Edit: December 22, 2016, 11:23:37 am by Stephen Swaffer »
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Mike Sokol

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Re: New AV panel with ground path back to orginal building AC panel.
« Reply #14 on: December 22, 2016, 02:53:32 pm »

Where does the monitor amp and K sub get their power? Could there be a bootleg ground allowing a normal power on spike to be transferred to/through a building ground?
Yep, a bootleg ground can indeed inject normal neutral currents into the EGC path and create all sorts of trouble, especially when gear draws a big current pulse on startup.

And I've seen a similar effect "popping noise in a sound system" when all the receptacles on a branch circuit were bonded together, but the EGC wire never made it to the service panel and wasn't bonded to neutral at all. So any single piece of gear could inject a big voltage spike into the grounds of everything plugged into that branch circuit.

Three light outlet testers are just about useless for identifying proper grounds. What are you using for a ground impedance tester?
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Frank DeWitt

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Re: New AV panel with ground path back to orginal building AC panel.
« Reply #15 on: December 22, 2016, 06:21:47 pm »


What I still cannot explain is this back stage I have a QSC low power monitor amp and a K-Array powered 12 inch sub.

When the power sequencer powers up those two devices by triggering a Lowell RPC1 the video on projector will flash to black for a second, but the reshow the image.

If I run a extension cord from the sound booth to that location the problem disappears.

Nick Bailey

I had a LED display (TV) that would flash to black for a second, but the reshow the image when a florescent light in the room behind it was turned OFF.  I wanted that TV and the CAT-5 to HDMI converter that feeds it on the same SurgeX power as the sound system and other TVs anyway so I ran new power. That fixed it.  I never did find out what caused it.
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Mike Sokol

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Re: New AV panel with ground path back to orginal building AC panel.
« Reply #16 on: December 22, 2016, 06:38:58 pm »

I had a LED display (TV) that would flash to black for a second, but the reshow the image when a florescent light in the room behind it was turned OFF.  I wanted that TV and the CAT-5 to HDMI converter that feeds it on the same SurgeX power as the sound system and other TVs anyway so I ran new power. That fixed it.  I never did find out what caused it.

Florescent lights with old-school ballasts will create a big voltage spike when turned off. This is due to the magnetic flux field collapsing, just like a relay coil. I know from personal measurement that this spike can reach 10 times the incoming voltage, so if the switch is tuned off at the peak of the 60 Hz waveform, you could create a spike reaching 1,000 volts or more. And it has a very fast rise-time, so that spike can sneak into all sorts of interconnected gear. 
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Mike Sokol
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Frank DeWitt

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Re: New AV panel with ground path back to orginal building AC panel.
« Reply #17 on: December 22, 2016, 06:55:18 pm »

Florescent lights with old-school ballasts will create a big voltage spike when turned off. This is due to the magnetic flux field collapsing, just like a relay coil. I know from personal measurement that this spike can reach 10 times the incoming voltage, so if the switch is tuned off at the peak of the 60 Hz waveform, you could create a spike reaching 1,000 volts or more. And it has a very fast rise-time, so that spike can sneak into all sorts of interconnected gear.

That ex planes it.  I am glad I put it on the surge-X circuit.  If I run into it again, it is a good excuse to switch to LED
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Nicholas Bailey

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Re: New AV panel with ground path back to orginal building AC panel.
« Reply #18 on: December 23, 2016, 07:36:02 am »

Thanks Again for all the help.

All the AC outlets were in spec.  I will shoot a pic of the vintage tool my boss brought me later.

I swapped out the HDMI DA and the problem still present.

I then found that Fiber connected to laptop would flash about 1 out of 5 times.

Move to the projector...  short cable and putting the DA up in the balcony was solid.

I then swapped in a 75 foot pro Extron hdmi cable and required Extron EQ.  That made the system solid.

The AC may have been a factor in why the fiber cable was giving me such trouble, I dont have to know why.

My though was the spick of the rpc1 tripping on the 10 foot run from the panel was causing the light source of the hdmi cable to drop momentary or get out of time.  Powering from the booth from a 100 foot extension cord reduced the spick.

I never was able to read move than a 3 volts from MIN to MAX on my fluke.


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 #19 on: December 23, 2016, 08:40:50 am »

I never was able to read move than a 3 volts from MIN to MAX on my fluke.

If you're referring to the voltage spike, a standard DMM will never be able to read it properly since it will be only milliseconds long, if that much. You'll need a digital storage scope set to trigger & save just above the normal AC voltage levels. Then you can catch and measure the voltage spike.
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Nicholas Bailey

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Re: New AV panel with ground path back to orginal building AC panel.
« Reply #20 on: December 24, 2016, 04:05:05 pm »

Thanks for the clarification.  My fluke was sampling at 100 ms.

Here is the pic of the vintage tool.


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 #21 on: December 25, 2016, 09:10:31 am »

Thanks for the clarification.  My fluke was sampling at 100 ms.

Here is the pic of the vintage tool.


Nick

Very cool. Here's my first Woodhead Ground Loop Impedance Tester that I got in the early 70's. I've been playing with grounds for a very long time.

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Re: New AV panel with ground path back to orginal building AC panel.
« Reply #21 on: December 25, 2016, 09:10:31 am »


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