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Author Topic: BX cable ground loops  (Read 17600 times)

Jonathan Johnson

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Re: BX wire ground loops
« Reply #10 on: April 25, 2014, 03:37:42 pm »

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.

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 [couldn't] figure out if it was our electrician or the inspector wanting it done that way. 

In many jurisdictions, inspectors (if not the NEC) require a separate equipment grounding conductor that bonds every metal box, regardless of the type of raceway (metallic, rigid, flexible, non-metallic, BX, etc.). This is based on the suspicion that raceway fittings may not be secure (and therefore exhibit poor conductivity); and for BX cable or flex conduit, the length and "gauge" of the jacket is determined by its unwound length -- the contact between turns of the spiral is not reliable. So they require a separate ground conductor. In a non-isolated-ground scenario, this EGC also provides the equipment ground for receptacles, fixtures, and other devices.

(There is merit to this; I myself had to deal with a bad ground issue in my company's office. I was able to resolve it by tightening the locknut on a conduit fitting entering a junction box.)

In that case, for isolated ground, you will need to run an additional EGC, which in North America is typically green with a yellow stripe.
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Jonathan Johnson

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Re: BX wire ground loops
« Reply #11 on: April 25, 2014, 03:50:18 pm »

It seems that the Code's (and inspectors') concern is for fire safety and shock prevention. Electrical noise is not seen as a safety issue.

However, considering some of the discussions we've had in this forum, it appears that even an electrical system that's wired "properly" according to code can experience significant ground-potential differences. Those differences can result in current flow through grounded conductors, which in certain scenarios could cause those conductors to overheat and start a fire. In other scenarios, the ground-potential differences could result in electric shock.

It's probably time for electricians to start taking electrical noise more seriously, as a symptom of a bigger problem and an indicator of hazardous conditions. It's going to take more education in electrical theory, because it's going to require solutions that aren't necessarily one-size-fits-all that can be addressed by a code written by a bureaucratic/political body, no matter how great its expertise.
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TJ (Tom) Cornish

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Re: BX wire ground loops
« Reply #12 on: April 25, 2014, 03:56:45 pm »

It's probably time for electricians to start taking electrical noise more seriously, as a symptom of a bigger problem and an indicator of hazardous conditions. It's going to take more education in electrical theory, because it's going to require solutions that aren't necessarily one-size-fits-all that can be addressed by a code written by a bureaucratic/political body, no matter how great its expertise.
That would certainly be wonderful, however the more-likely solution will be the better understanding of the pin-1 problem of A/V equipment manufacturers, and hopefully fewer devices that are susceptible.  Digital snakes are a good thing, too.
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Lyle Williams

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Re: BX cable ground loops
« Reply #13 on: April 25, 2014, 06:17:23 pm »

We're 100 years into this problem (ground noise issues impact radio receivers too) and at least 50 years into the version of the problem with existing power/ground topologies.

This will remain a niche issue as so few sites care about the problem.

... and digital will make it go away as a problem too.  :-)

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Mike Sokol

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Re: BX cable ground loops
« Reply #14 on: April 25, 2014, 09:40:23 pm »

... and digital will make it go away as a problem too.  :-)

I think I've identified 60 Hz jitter distortion on S/PDIF line caused by ground loop currents, but I'm not ready for peer review yet. I'll let you know when I have a full hypothesis to test.
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Mike Sokol
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Jonathan Johnson

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Re: BX cable ground loops
« Reply #15 on: April 25, 2014, 11:54:53 pm »

I think I've identified 60 Hz jitter distortion on S/PDIF line caused by ground loop currents, but I'm not ready for peer review yet. I'll let you know when I have a full hypothesis to test.

Guess we need to start working on that optical mixer.

Just don't use it in direct sunlight.
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Jeff Bankston

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Re: BX cable ground loops
« Reply #16 on: April 26, 2014, 02:35:29 am »

none of the electricians i ever worked with would have known about checking the grounds like you did. all electricians are trained to do is install electrical "stuff" according to plans or verbal instruction and of course according to the electrical. it takes a sound guy expert like you Mike or electricians that are also trained in supplying electricity to sound reinforcement equipment to know how to trouble shoot this stuff. just because a guy/gal has a journeyman card doesnt mean a thing when it come to sound equipment hum. we are not trained for that.
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Mike Sokol

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Re: BX cable ground loops
« Reply #17 on: April 26, 2014, 06:25:26 am »

Guess we need to start working on that optical mixer.

Just don't use it in direct sunlight.

So vampires need not apply for the FOH mixing position?
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Mike Sokol
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Mike Sokol

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Re: BX cable ground loops
« Reply #18 on: April 26, 2014, 06:36:28 am »

Just because a guy/gal has a journeyman card doesn't mean a thing when it come to sound equipment hum. We are not trained for that.

I've been pitching the idea of a "HumBusters" clinic for sound system installers and electricians, but no traction so far. If anyone knows of an electrical trade association that might want to host such a clinic, please let me know. I have a bunch of portable demonstrations which let me create and correct hum and shock situations right on a table in front of you. I've learned more in the last 4 years of experimenting with hum currents on the bench than I did over the last 40 years of fighting it in shows. There's just no time to figure out what's really wrong while you're setting up a gig. If you can make a quick random hum fix during setup, then you're a hero and move on. But my test rig allows me to inject repeatable amounts of ground loop current into sound systems, then figure out how to measure and correct it in the field. That's the only way to get good at this, I think.
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Mike Sokol
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Stephen Swaffer

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Re: BX wire ground loops
« Reply #19 on: April 26, 2014, 11:26:17 pm »

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.

Code requires boxes to be bonded to any existing EGC that is run with power conductors.  However, there is ans exception allowing isolated ground conductors to pass through boxes without being bonded.  Most inspectors would allow the green wire to serve as the isolated ground as long as there is another EGC that meets code. 

However, "BX" is not recognized by code.  Code recognizes AC (armored cable) and MC (metal clad cable).  My understanding is that usually BX refers to AC.  The armor of AC is recognized as an EGC.  The cladding on MC is not-but as usual there are the exceptions-some metal clad is made so that the cladding can be used an an EGC.  Bottom line is that you need to make sure that you are dealing with AC or approved MC, not just standard MC is you want to use the metal sheath as a grounding conductor.

Also, though it obviously is a ground and creates issues, simply being attached to building steel is not technically considered bonded.  Probably because the next trade in the space that needs to run something is likely to un attach the cable!

There is MC available with 2 grounding conductors made specifically for health care facilities and rest homes where a redundant ground is required.  If you really want to have fun with grounds try setting up a sound system in a facility like that!

I like the idea of a "Humbusters Clinic", but it seems like there are very few that really want to be skilled at there profession-at least out here.  Guys seem content to sit in the same classes code cycle after code cycle as long as they get their CEUs....
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Steve Swaffer

ProSoundWeb Community

Re: BX wire ground loops
« Reply #19 on: April 26, 2014, 11:26:17 pm »


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