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Author Topic: Ground Loop in Large Arena  (Read 8181 times)

David Sturzenbecher

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Ground Loop in Large Arena
« on: November 20, 2016, 08:21:33 PM »

Hello,
I am working with a large arena, built in 09-10, and they are currently complaining of a horrible hum when anything on the ground/floor level of the building is plugged into the sound system.

The arena has 4 main levels + a catwalk.  Current sound system has components on Level 1, 3, 4 (patchbay, control room mixing console)and in the catwalk with no noise problems within itself, but has fiber to all rack locations. 

Whenever anything (mixer, wireless mic receiver, etc)  is plugged into an outlet on the ground level (level 1) and connected to the sound system the noise manifests itself.   The customer tried to bypass the broadcast cabling and pulled a temp XLR from the control room (level 4) to the floor (level 1) and there was still hum/noise. Typically this noise can be eliminated with radial/Jensen transformers and ground lifts.   However, they are running into problems with intercoms which cannot be run through isolation.

Any hints to help troubleshooting this issue that has been present since day 1?   During my last visit I noticed that the power panel in the control room (and possibly all of the audio system panelboards) are setup with Isolated Grounds.  Could the connection of equipment on IG to a nonIG outlet on the floor cause this?  Or should I start hunting for bonded N-G's in sub panels?  Thanks!
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Mike Sokol

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Re: Ground Loop in Large Arena
« Reply #1 on: November 20, 2016, 09:36:31 PM »

Any hints to help troubleshooting this issue that has been present since day 1?   During my last visit I noticed that the power panel in the control room (and possibly all of the audio system panelboards) are setup with Isolated Grounds.  Could the connection of equipment on IG to a nonIG outlet on the floor cause this?  Or should I start hunting for bonded N-G's in sub panels?  Thanks!

I would run a long extension cord between the two areas and start measuring between the grounds. With a technical ground there should be close to zero volts between the two outlets. I've been in a few churches with a technical ground panel that was contaminated by an improper outlet EGC connection to building steel. I would start with a visual inspection of all receptacles and see where it leads you.

BTW: My tests show that ground loop hums can start with a ground difference as little as 1/10 of a volt, and become quite obnoxious with 1/2 to 1 volt ground loop differential. And I've personally measued up to 5 volts difference in building steel, so you can see how easy it is to create hum in a system if any of your EGC grounds contact building steel. Also note that any sub-panel that's mounted on building steel or connected via metallic conduit or MC metal sheathed wire will essentially bond your EGC ground to building steel, many times at multiple locations. So to maintain clean ground you'll probably need to have both a safety ground (green), plus a technical ground (green/yellow-stripe) going to each isolated-ground outlet. That's the thing to look for in a visual inspection.
« Last Edit: November 20, 2016, 10:56:46 PM by Mike Sokol »
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Keith Broughton

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Re: Ground Loop in Large Arena
« Reply #2 on: November 21, 2016, 06:54:43 AM »

As well as what Mike suggested, use an transformer on the audio inputs from the floor.

As for the com, how is the system set up?
Is there a single master station powering the system? Are there 2 powered stations in the system?
Is there a hum always or only when the com is connected to the cableing to the floor?
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Kevin Graf

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Re: Ground Loop in Large Arena
« Reply #3 on: November 21, 2016, 08:49:42 AM »

It might be a 'High Leg Delta' problem.
Jim Brown writes:

HIGH LEG DELTA
Figure A-6 shows a variation of the delta configuration that is widely used in North
America, especially in older mixed residential and industrial areas, and in rural areas.
One leg of the delta has a grounded center-tap that serves as the neutral for a singlephase
120/240VAC system, and 208 volts is available for certain industrial applications.
Also known as a Red Leg Delta, or Wild Leg, High Leg Delta systems must have that
phase conductor marked with orange tape, orange finish or similar. This marking is only
required where a connection is made and the neutral conductor is present.
The configuration can work for audio and video systems if there are no other loads on
the transformer. But EMC consultant Neil Muncy has taught us that when High Leg
Delta is used to feed multiple customers from the same transformer, the neutral currents
from one customer can circulate through another customer’s ground system.
When this happens, the neutral feed from the pole-mounted transformer may carry
relatively large neutral currents from those neighboring buildings.

The neutral current will find its way to earth through the system earth at the service entrance,
and in general, the better the earth electrode system, the greater the circulating
current will be! If the path to earth runs near audio equipment or wiring, the magnetic
fields produced by these currents can couple into system wiring, guitar pickups and
dynamic microphones without hum-bucking coils, and even the electronics of audio
gear. The result is hum and buzz that can be eliminated only by eliminating the field.
The hum component is 60 Hz, while the “buzz” consists of harmonics. When called in
to diagnose problems in a small recording studio complex in a renovated industrial
building, Muncy found a High Leg Delta power feed with 7A of neutral current finding
its return path via a water main running under the guitar isolation booth!
The solution is to use a transformer with a single-phase center-tapped secondary (Figure
1), to feed 120/240 v systems in the building, powering it from one of the ungrounded
240 volt phases. What matters is that the shared neutral feed to the building,
with the offending currents, must be eliminated.

http://www.audiosystemsgroup.com/SurgeXPowerGround.pdf
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Stephen Swaffer

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Re: Ground Loop in Large Arena
« Reply #4 on: November 21, 2016, 12:53:41 PM »

I would run a long extension cord between the two areas and start measuring between the grounds. With a technical ground there should be close to zero volts between the two outlets. I've been in a few churches with a technical ground panel that was contaminated by an improper outlet EGC connection to building steel. I would start with a visual inspection of all receptacles and see where it leads you.

BTW: My tests show that ground loop hums can start with a ground difference as little as 1/10 of a volt, and become quite obnoxious with 1/2 to 1 volt ground loop differential. And I've personally measued up to 5 volts difference in building steel, so you can see how easy it is to create hum in a system if any of your EGC grounds contact building steel. Also note that any sub-panel that's mounted on building steel or connected via metallic conduit or MC metal sheathed wire will essentially bond your EGC ground to building steel, many times at multiple locations. So to maintain clean ground you'll probably need to have both a safety ground (green), plus a technical ground (green/yellow-stripe) going to each isolated-ground outlet. That's the thing to look for in a visual inspection.

I would suspect a problem with the technical grounds.  It is possible that the technical grounds on the first floor are all connected together-but that someone overlooked a connection/wire to actually bond them to ground?  I would take a very close look at the neutral/ground/technical grounds in every panel on the first floor to make sure they all are connected as they should be-all of them isolated in every subpanel, but all correctly bonded together at ONE location in the service panel.
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Mike Sokol

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Re: Ground Loop in Large Arena
« Reply #5 on: November 21, 2016, 01:07:59 PM »

I would suspect a problem with the technical grounds.  It is possible that the technical grounds on the first floor are all connected together-but that someone overlooked a connection/wire to actually bond them to ground?  I would take a very close look at the neutral/ground/technical grounds in every panel on the first floor to make sure they all are connected as they should be-all of them isolated in every subpanel, but all correctly bonded together at ONE location in the service panel.

I consulted on a hum at a church recently that had installed a technical ground system perhaps 10 years ago, and the entire system was hum free until 2 years ago. After that they were living will all sorts of hum issues, and didn't have a clue as to why. I crawled around under the stage and found a single non-isolated outlet that had been added to power a small distribution amp under the stage. That single non-isolated outlet was in a metal junction box and bolted to building steel, effectively contaminating their nice clean technical ground. When I asked them when the offending outlet had been installed they told me it was right around the time they noticed all the ground-loop hum. Having an electrician install an isolated ground outlet corrected the problem. The real take-away is that this was under the stage and invisible during a normal room inspection. There was a trap-door for easy access under the stage, but without knowledge of a rogue receptacle under there, nobody would know to look. So it's entirely possible that someone hung a non-isolated outlet somewhere in an invisible spot, and that one connection to building steel can be causing the entire problem. Visual inspections are a highly underrated troubleshooting tool.

David Sturzenbecher

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Re: Ground Loop in Large Arena
« Reply #6 on: November 21, 2016, 04:07:59 PM »

I would suspect a problem with the technical grounds.  It is possible that the technical grounds on the first floor are all connected together-but that someone overlooked a connection/wire to actually bond them to ground?  I would take a very close look at the neutral/ground/technical grounds in every panel on the first floor to make sure they all are connected as they should be-all of them isolated in every subpanel, but all correctly bonded together at ONE location in the service panel.

I am pretty sure that the outlets on the ground level are not on any sort of technical ground setup. They are just regular outlets.... nothing special about them. That would pretty much be the problem right there then right?


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Mike Sokol

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Re: Ground Loop in Large Arena
« Reply #7 on: November 21, 2016, 04:21:23 PM »

I am pretty sure that the outlets on the ground level are not on any sort of technical ground setup. They are just regular outlets.... nothing special about them. That would pretty much be the problem right there then right?

Yeah, if the panel is built with an isolated ground, and the outlets are non-isolated, then that's your problem. But just how do you know the service panel has an isolated ground system? That would imply a 4th green/yellow wire in all the runs, or metallic conduit run to each of the outlet boxes, or BX armored cable with an EGC wire  build into the metallic armor plus an isolated green "ground" wire. Isolated or Technical grounding systems are only as good as the outlets they power. A single non-isolated outlet can corrupt the entire grounding system.

An isolated ground outlet will have the little triangle symbol on its face next to the ground contact.

« Last Edit: November 21, 2016, 04:36:41 PM by Mike Sokol »
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David Sturzenbecher

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Re: Ground Loop in Large Arena
« Reply #8 on: November 21, 2016, 10:39:17 PM »

Yeah, if the panel is built with an isolated ground, and the outlets are non-isolated, then that's your problem. But just how do you know the service panel has an isolated ground system? That would imply a 4th green/yellow wire in all the runs, or metallic conduit run to each of the outlet boxes, or BX armored cable with an EGC wire  build into the metallic armor plus an isolated green "ground" wire. Isolated or Technical grounding systems are only as good as the outlets they power. A single non-isolated outlet can corrupt the entire grounding system.

An isolated ground outlet will have the little triangle symbol on its face next to the ground contact.

Mike, I forgot to take a picture of the panelboard in the control room, but I recall a label it said it was setup as isolated ground panel.   This panel was a custom setup lyntec panel.   I can ask my contact at site to take pictures the next time he is back on site, of the panelboard itself and the outlets.
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Mike Sokol

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Re: Ground Loop in Large Arena
« Reply #9 on: November 22, 2016, 04:23:35 PM »

Mike, I forgot to take a picture of the panelboard in the control room, but I recall a label it said it was setup as isolated ground panel.   This panel was a custom setup lyntec panel.   I can ask my contact at site to take pictures the next time he is back on site, of the panelboard itself and the outlets.

The last two sites I consulted at with hum problems both had separate sub-panels that were "isolated technical grounds" one of which cost them $15,000 to install. However, the electricians wiring the receptacles didn't get the memo and used regular grounded outlets. So that's the first place to look.

ProSoundWeb Community

Re: Ground Loop in Large Arena
« Reply #9 on: November 22, 2016, 04:23:35 PM »


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