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Author Topic: BX cable ground loops  (Read 18175 times)

Mike Sokol

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BX cable ground loops
« on: April 25, 2014, 07:34:02 am »

I was just called in to consult at a large church that had a hum in their sound system that would come and go when the HVAC compressors were running or stopped. The have three large consoles for FOH, Monitors, and Recording, and small mixers for two keyboard stations. This was a Southern Baptist Ministry, so lots of SPL from lots of monitors. They had been fighting this hum for the last two years and had already spent $30K on a separate "technical isolation" subpanel, another $50K on a huge isolation transformer feeding the entire building. The PoCo was trying to talk them into spending $50K on a new transformer on the street since they found the voltage was dropping to 110 volts when the HVAC kicked in, and claimed that the low voltage was making the sound gear hum. The pastor was even considering installing a generator outside since a big show had brought in their own genny and power distro, and there was no hum.

I found the problem within the first hour of arriving on site yesterday. Here's what I did.

I had a nagging feeling when I saw BX armored cable running to a pair of outlet boxes on the floor under the monitor console. A little experimenting suggested the nexus of the hum was from this console since muting inputs didn't stop the noise. I pulled out my SureTest Ground Impedance Tester and the voltage and polarity looked OK at first blush. But testing the ground impedance gave me some strange numbers. While the Hot and Neutral wires showed a normal 1/2 of an ohm impedance back to the panel, the Ground showed less than 1/10 of an ohm, suggesting there was a secondary ground path in parallel, perhaps through the BX cable metal exterior. I looked in the wiring closet and saw that the BX cables feeding all the stage outlets were tie-wrapped to the metal studs.

Opening up a few boxes confirmed my suspicion that these were standard non-isolated ground 20-amp outlets. Of course, the strap on the receptacle was bonded to the metal box cover, which was bonded to the metal box, which was bonded to the BX cable armored cover, which was bonded to the building steel all over the place from nylon tie wraps. That completely contaminated the grounds of all the sound gear, and when there were any large loads placed on the power system from the HVAC, the voltage differentials in the building steel were cross-connected into the various sound gear power outlet grounds. And we should all know about the pin-1 problem with a lot of sound gear that will cause it to hum with even 1/10 of a volt difference between XLR cable ends.

I confirmed this was the problem by lifting the metal cover off of the outlet feeding the monitor console, and saw that there was now a normal voltage difference between the BX armor shield and the safety ground wire in the outlet. The ground impedance increased to 1/2 ohm which matched the hot and neutral wire impedance, and the hum current went away completely from that console.

All of the previous expense of installing a technical ground panel and building isolation transformer was wasted. And I'm sure that putting in a larger transformer on the street would have done nothing to fix the hum problem either. All that's needed for the fix is for the electrician to install isolated ground receptacles for everything connected to the sound power distro panel, and I'm pretty sure the hum problem will be history. Of course, the guys charging $30,000 for the isolated ground panel install should have checked all the outlets for isolated grounds. I really don't think that technical ground panel was needed at all and simply separating all the outlet grounds from building steel via isolated receptacles was all that was really needed.

Cool stuff, eh? 
« Last Edit: April 26, 2014, 08:08:02 am by Mike Sokol »
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Mike Sokol
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g'bye, Dick Rees

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Re: BX wire ground loops
« Reply #1 on: April 25, 2014, 07:59:12 am »

I was just called in to consult on a large church that had a hum in their sound system that would come and go when the HVAC compressors were running or stopped. The have three large consoles for FOH, Monitors, and Recording, and small mixers for two keyboard stations. This was a Southern Baptist Ministry, so lots of SPL from lots of monitors. They had been fighting this hum for the last two years and had already spent $30K on a separate "technical isolation" subpanel, another $50K on a huge isolation transformer feeding the entire building. The PoCo was trying to talk them into spending $50K on a new transformer on the street since they found the voltage was dropping to 110 volts when the HVAC kicked in, and claimed that the low voltage was making the sound gear hum. The pastor was even considering installing a generator outside since a big show had brought in their own genny and power distro, and there was no hum.

I found the problem within the first hour of arriving one site yesterday. Here's what I did.

Cool stuff, eh?


Mike...

I hope they paid you an amount equal to that which they paid the previous "experts"...
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Mike Sokol

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Re: BX wire ground loops
« Reply #2 on: April 25, 2014, 08:05:42 am »


Mike...

I hope they paid you an amount equal to that which they paid the previous "experts"...

I charged them my full day-rate before I walked in the door, and didn't feel bad about only working for an hour and charging for a full day. And they were smiling when they handed me the check on the way out. It's not what you do, it's what you know that counts.

This hum stuff is getting really interesting.
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Cailen Waddell

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Re: BX wire ground loops
« Reply #3 on: April 25, 2014, 08:07:44 am »

Mike - when they isolate the ground with the new outlets - to meet code - won't they have to add a 4th wire to ground the box?  You know those boxes were already grounded through conduit and structure, but I seem to remember hearing this from an electrician one time....
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Re: BX wire ground loops
« Reply #4 on: April 25, 2014, 08:07:58 am »

This hum stuff is getting really interesting.

Hmmmm....
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Mike Sokol

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Re: BX wire ground loops
« Reply #5 on: April 25, 2014, 08:34:15 am »

Mike - when they isolate the ground with the new outlets - to meet code - won't they have to add a 4th wire to ground the box?  You know those boxes were already grounded through conduit and structure, but I seem to remember hearing this from an electrician one time....

Nope, not according to my local electrician for downtown Baltimore who's a code guy as well. The outlet box and metal cover is "grounded" by the BX cable metal sheath that's bonded to building steel, and the outlet itself is "grounded" by the green safety wire fed from the panel. The iso-receptacle is what keeps those two grounds separated.  I'm interested to know if there are any sparky's on this forum who can confirm that's code for other districts.
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Cailen Waddell

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Re: BX cable ground loops
« Reply #6 on: April 25, 2014, 08:38:19 am »

Thanks mike - in our AHJ, in a theater we just opened, the outlet boxes on the isolated technical ground had 2 ground wires, one for the receptacle and one for the box.  It was solid conduit back to the breaker panel.  I counselor figure out if it was our electrician or the inspector wanting it done that way. 

Your way in Baltimore makes perfect sense.  Glad it was an easy fix.  Sometimes it's the little things.
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Mike Sokol

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Re: BX cable ground loops
« Reply #7 on: April 25, 2014, 08:51:33 am »

Thanks mike - in our AHJ, in a theater we just opened, the outlet boxes on the isolated technical ground had 2 ground wires, one for the receptacle and one for the box.  It was solid conduit back to the breaker panel.  I counselor figure out if it was our electrician or the inspector wanting it done that way. 

Your way in Baltimore makes perfect sense.  Glad it was an easy fix.  Sometimes it's the little things.

What's the color code for the wires feeding the tech ground and the box ground?
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Cailen Waddell

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Re: BX cable ground loops
« Reply #8 on: April 25, 2014, 08:55:43 am »

One is green and one is green with yellow stripe. Pretty sure green with yellow stripe is tech ground
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Mike Sokol

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Re: BX cable ground loops
« Reply #9 on: April 25, 2014, 09:29:03 am »

One is green and one is green with yellow stripe. Pretty sure green with yellow stripe is tech ground

I had something similar at a church in Texas last year where the metallic conduit itself was the ONLY ground. Of course, it was strapped to building steel all over the place and code compliant for that district. So instead of rewiring the entire building I was able to install Whirlwind ISO-2 boxes in the appropriate XLR lines between the small FOH mixer and amp rack in the closet. 

That's two ways to skin the same cat (sorry, cats). You can either clean up all the power grounds in the building, or you can add audio transformer isolation between gear that's plugged into different power outlets. Whatever is cheaper works for me...
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Mike Sokol
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Re: BX cable ground loops
« Reply #9 on: April 25, 2014, 09:29:03 am »


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