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Sound Reinforcement - Forums for Live Sound Professionals - Your Displayed Name Must Be Your Real Full Name To Post In The Live Sound Forums => AC Power and Grounding => Topic started by: Mike Sokol on April 25, 2014, 07:34:02 am

Title: BX cable ground loops
Post by: Mike Sokol 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? 
Title: Re: BX wire ground loops
Post by: g'bye, Dick Rees 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"...
Title: Re: BX wire ground loops
Post by: Mike Sokol 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.
Title: Re: BX wire ground loops
Post by: Cailen Waddell 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....
Title: Re: BX wire ground loops
Post by: g'bye, Dick Rees on April 25, 2014, 08:07:58 am
This hum stuff is getting really interesting.

Hmmmm....
Title: Re: BX wire ground loops
Post by: Mike Sokol 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.
Title: Re: BX cable ground loops
Post by: Cailen Waddell 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.
Title: Re: BX cable ground loops
Post by: Mike Sokol 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?
Title: Re: BX cable ground loops
Post by: Cailen Waddell 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
Title: Re: BX cable ground loops
Post by: Mike Sokol 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...
Title: Re: BX wire ground loops
Post by: Jonathan Johnson 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.
Title: Re: BX wire ground loops
Post by: Jonathan Johnson 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.
Title: Re: BX wire ground loops
Post by: TJ (Tom) Cornish 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.
Title: Re: BX cable ground loops
Post by: Lyle Williams 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.  :-)

Title: Re: BX cable ground loops
Post by: Mike Sokol 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.
Title: Re: BX cable ground loops
Post by: Jonathan Johnson 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.
Title: Re: BX cable ground loops
Post by: Jeff Bankston 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.
Title: Re: BX cable ground loops
Post by: Mike Sokol 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?
Title: Re: BX cable ground loops
Post by: Mike Sokol 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.
Title: Re: BX wire ground loops
Post by: Stephen Swaffer 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....
Title: Re: BX wire ground loops
Post by: Jeff Bankston on April 27, 2014, 12:02:12 am


However, "BX" is not recognized by code. 
BX was used long ago but was outlawed about 20 years ago iirc. BX has a steel flex condiut with brown paper wrapped around each insulated colored condutor with about a 16 gauge bare aluminum ground wire. iirc the thin aluminum ground wire is why it was no longer allowed. ac cable has a steel conduit with clear plastic wrapped around the wires and a green insulated ground the same guage as the current carrying conductors. mc calbe is identical except the outer metal conduit is aluminum. yes the mc/ac letter are opposite the metal the condiut is made of go figure. mc/ac cable is available with a second seperate green/yellow ground wire for use with i.g. circuts. in cali you must have a standard green ground wire in every conduit even if it has an iso ground wire. never bond the iso to the box or ground buss in the panel. always bond the iso ground to an isolated buss bar thats insulated from the panel. then run a wire from the iso bus bar to the cold water pipe. in cali they were allowing an iso ground to be tied to the nearest cold water pipe insted of the standard 5 foot rule which is still in effect for the standard service ground.
Title: Re: BX wire ground loops
Post by: Mike Sokol on April 27, 2014, 05:43:07 am
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.

I wonder if an inspector would accept something like a SureTest Analyzer check on each metal box to confirm the ground impedance is sufficiently low via the armored cable. The SureTest analyzer states that the measured ground impedance should be under 1 ohm, but I'm not sure if that would be a substitute for a separate "dirty" EGC.

For those of you who've never used one, these GLIT's (Ground Loop Impedance Testers) will not only calculate the ground impedance of the EGC going back to the service panel's bonding point, they'll also predict the voltage drop for 15 and 20 amp loads on the current conductors. That's really handy for troubleshooting very long branch circuits that may have too much voltage drop for certain pieces of sound gear. For instance, last year at the University where I teach they were having trouble with an SSL console that wouldn't boot up cleanly every time and would exhibit strange lock-ups at times. Plant maintenance swore there couldn't be anything wrong with the electrical supply since this was powered by a dedicated UPS in the electrical room down the hall, plus a power conditioner in the gear closet right next to the console. I was suspicious when I metered the voltage at the receptacle as under 105 volts while running under full load. My SureTest predicted a drop down to 100 volts at 20 amps, and since the console draws around 15 amps steady-state (it's a BIG console) and way over 20-amps on startup current, tech support at SSL was sure this was the problem. I stepped off the length of the 12-gauge cable run for the branch circuit and calculated around 250 ft. Yikes.... Voltage drops for branch circuits are generally calculated on 100 ft runs, so instead of the allowed 6% voltage drop we were seeing around 15% drop under load. However, I found a second unused run of 10-gauge to the same room, so I had the plant electrician swap over to the heavier run of 10-gauge cable, which reduced the voltage drop so that the console was now seeing 114 volts at full load. Haven't had a single case of console bootup or lockup problem since then, so I'm sure the low voltage from the too-long branch circuit run was the problem. I don't know how often branch circuit wires are over-sized in a facility, or how the NEC really thinks about that situation, but that's one more interesting field of study. This is just one more example of an un-obvious wiring problem that caused a lot of headaches. But understanding impedance theory and Ohm's law are great tools to help figure out the problem.
Title: Re: BX cable ground loops
Post by: Cailen Waddell on April 27, 2014, 07:48:13 am

BX was used long ago but was outlawed about 20 years ago iirc. BX has a steel flex condiut with brown paper wrapped around each insulated colored condutor with about a 16 gauge bare aluminum ground wire. iirc the thin aluminum ground wire is why it was no longer allowed. ac cable has a steel conduit with clear plastic wrapped around the wires and a green insulated ground the same guage as the current carrying conductors. mc calbe is identical except the outer metal conduit is aluminum. yes the mc/ac letter are opposite the metal the condiut is made of go figure. mc/ac cable is available with a second seperate green/yellow ground wire for use with i.g. circuts. in cali you must have a standard green ground wire in every conduit even if it has an iso ground wire. never bond the iso to the box or ground buss in the panel. always bond the iso ground to an isolated buss bar thats insulated from the panel. then run a wire from the iso bus bar to the cold water pipe. in cali they were allowing an iso ground to be tied to the nearest cold water pipe insted of the standard 5 foot rule which is still in effect for the standard service ground.

Why would you run the ISO ground from the ISO buss bar to cold water?  Don't you want to run back to the transformer xo where the ground neutral bond is?  I'm confused how the ISO ground would provide a return path for a fault current if it only ties to the cold water pipe.
Title: Re: BX cable ground loops
Post by: Jeff Bankston on April 27, 2014, 09:16:21 am
Why would you run the ISO ground from the ISO buss bar to cold water?  Don't you want to run back to the transformer xo where the ground neutral bond is?  I'm confused how the ISO ground would provide a return path for a fault current if it only ties to the cold water pipe.
you can but i still does the same thing. the transformer might be 4 floors below or abouve the panel like i'v seen in some old hirises. the transformers primary ground is the cold water pipe and secondly building steel unless the building is a concrete skeleton structure like i'v worked in here in Los Angeles. the main service and every transformer must be tied to the cold water pipe no matter what. and why not ? , its an exelent ground. the cold water pipe in the ground and is connected to the main water system and is filled with water. you also must have a ground rod or rebar grid if the ground is too dry and that is a secondary backup ground.
Title: Re: BX wire ground loops
Post by: Mike Sokol on April 27, 2014, 10:34:51 am
Golly... I thought it was all "BX" but now I see modern versions of "AC" cable. http://seatekco.com/bx-cable/ (http://seatekco.com/bx-cable/)

Why is it called "BX".

Per Jim Dollins, VP of Product Development for AFC Cable Systems, the term "BX" stands for "Product "B" - Experimental."

Apparently, back when the product was first developed, the first manufacturer of this product had only one product at the time: "Product "A"". So when they developed this product they didn't know what to call it. Somebody suggested "Product "B" with the suffix "X" added as the product was at that time experimental.

More to study on this thing.  ;)
Title: Re: BX cable ground loops
Post by: Kevin Graf on April 27, 2014, 10:49:09 am
External water pipes, rebar grids (Ufer's) and rods in the dirt are all part of the Grounding Electrode Conductor system (GEC).
Safety Grounds or Protective Earths or Isolated Safety Grounds are all part of the Equipment Grounding Conductor system (EGC).
The Grounding Electrode Conductor system (GEC) and the Equipment Grounding Conductor system (EGC) have little to do with each other.
As they are only connected together at one point in the service entrance/main breaker box.
Title: Re: BX cable ground loops
Post by: Mike Sokol on April 27, 2014, 11:04:08 am
The Grounding Electrode Conductor system (GEC) and the Equipment Grounding Conductor system have little to do with each other.
As they are only connected together at one point in the service entrance/main breaker box.

Well, "supposed" to be connected together at only one point.  ;)
Title: Re: BX cable ground loops
Post by: Kevin Graf on April 27, 2014, 11:56:12 am
Well only one official connection! But the EGC may be bonded to the metal building framing any number of times and that framing can make easy contact to the Ufer or water pipe.
Title: Re: BX cable ground loops
Post by: Mike Sokol on April 27, 2014, 12:08:45 pm
Well only one official connection! But the EGC may be bonded to the metal building framing any number of times and that framing can make easy contact to the Ufer or water pipe.

And it's exactly those extra bonding points that causes ground loop hum if you have gear with the pin-1 problem.  8)
Title: Re: BX wire ground loops
Post by: Stephen Swaffer on April 27, 2014, 05:45:53 pm
I don't know how often branch circuit wires are over-sized in a facility, or how the NEC really thinks about that situation, but that's one more interesting field of study.

The NEC is primarily concerned with safety.  A #12 wire protected by a 20 A breaker will not cause a fire-even if it is way too long and causes excessive voltage drop-the voltage drop will cause heating, but it will be spread out over the entire length of wire.  Iir correctly, there is a fine pint note giving a recommended allowable voltage drop.  Meeting the minimum standard of the code doesn't always mean a well designed job.  (For example, code only requires separate circuits for bathrooms, laundry and 2 in the kitchen-technically all of the rest of the receptacles and lights in a home could be on the same circuit-but the home owner would not be happy!)

Using the cold water pipe for a ground is OK-but one qualifier that should be used is that it is metallic :).  Of course that should be obvious, but so should a lot of other things in audio work!  Most new construction around here has all non metallic plumbing-I was surprised to see CPVC pipe for a fire sprinkler system a few days ago.  The possibility that a repair to a metallic system will be made with non-metallic is very real, so even a metallic system is not always a good ground.  See the pic for a good example-the kind plumber followed the instructions of the Bell telephone company to not remove the ground in this case.
Title: Re: BX cable ground loops
Post by: Kevin Graf on April 27, 2014, 06:38:36 pm
While I'm not sure that it was ever in the NEC code book, using a internal metallic water pipe as a Safety Ground (EGC) to retrofit older two wire systems, was an accepted practice in many jurisdictions for a long time period.

One danger with metallic water pipes is that they may be carrying 'lost neutral' currents (yours or your neighbors). Plumbers have been killed.
Title: Re: BX cable ground loops
Post by: Jeff Bankston on April 27, 2014, 06:39:02 pm
Well only one official connection! But the EGC may be bonded to the metal building framing any number of times and that framing can make easy contact to the Ufer or water pipe.
unless its a wood frame of concrete frame(skeleton) building. if its an old concrete building like a few i worked in we would have to bust concrete(not allowed) to find rebar so the 2 grounding points are the cold water pipe and a ground rod. on new buildings we cadweld a ground wire to the rebar right below the main disconnect.
Title: Re: BX cable ground loops
Post by: Jeff Bankston on April 27, 2014, 06:48:56 pm
While I'm not sure that it was ever in the NEC code book, using a internal metallic water pipe as a Safety Ground (EGC) to retrofit older two wire systems, was an accepted practice in many jurisdictions for a long time period.

One danger with metallic water pipes is that they may be carrying 'lost neutral' currents (yours or your neighbors). Plumbers have been killed.
we are never allowed to use a branch water pipe as the main ground. the main ground must be connected to the main water pipe within 5 feet of where it enters the building. this is to prevent it from being disconnected at any time during normal operations. plumbers know this and if there is a problem on the other side of the water pipe the power for the building is shut down. the only ground we are allowed to connect to a branch copper water pipe is an I.G. ground. if its a steel frame building i usually put a clamp on a beam/collumn , crimp a connector on the wire and bolt it to the beam clamp.
Title: Re: BX cable ground loops
Post by: Kevin Graf on April 27, 2014, 07:33:59 pm
My post was about the EGC Safety Ground system. The main external ground GEC is what is connected to the external water pipe.

Not just any rebar can be used as a Ufer ground system (GEC).
By the way Herbert G. Ufer was the consultant's name that developed the system.

http://en.wikipedia.org/wiki/Ufer_ground
Title: Re: BX cable ground loops
Post by: Jeff Bankston on April 27, 2014, 09:49:21 pm
My post was about the EGC Safety Ground system. The main external ground GEC is what is connected to the external water pipe.

Not just any rebar can be used as a Ufer ground system (GEC).
By the way Herbert G. Ufer was the consultant's name that developed the system.

http://en.wikipedia.org/wiki/Ufer_ground
i know but i didnt use the name Ufer because i didnt know if anyone here would know what i was talking about. we have always called it "The Ufer". i rarely hear it called the "concrete encased electrode" even though thats whats printed on the blue prints. are you fur going to lunch now ?


p.s. for those that dont know concrete retains a lot of water.

 i also installed a lenght of bare 4/0 in a long trench(dont remeber the lenght) encased in concrete once per the blueprints. they filled the trench half way up with concrete and we placed the 4/0 on it and kept it in the center as they poured the rest. thats the only time i ever did one of those grounds.
Title: Re: BX cable ground loops
Post by: Mike Sokol on April 27, 2014, 10:04:08 pm
p.s. for those that dont know concrete retains a lot of water.

That's also why a lot of musicians have been shocked while standing on a concrete floor in their basement and playing an ungrounded electric guitar. Concrete can be VERY conductive.
Title: Re: BX cable ground loops
Post by: g'bye, Dick Rees on April 27, 2014, 10:35:38 pm
That's also why a lot of musicians have been shocked while standing on a concrete floor in their basement and playing an ungrounded electric guitar. Concrete can be VERY conductive.

Lightning hit a church bell tower here a few years ago.  The moisture in the limestone flashed to steam and sent large chunks of stone flying all over the parking lot.
Title: Re: BX cable ground loops
Post by: Craig Hauber on April 28, 2014, 01:12:54 am
we are never allowed to use a branch water pipe as the main ground. the main ground must be connected to the main water pipe within 5 feet of where it enters the building. this is to prevent it from being disconnected at any time during normal operations. plumbers know this and if there is a problem on the other side of the water pipe the power for the building is shut down. the only ground we are allowed to connect to a branch copper water pipe is an I.G. ground. if its a steel frame building i usually put a clamp on a beam/collumn , crimp a connector on the wire and bolt it to the beam clamp.
Found one of those water-pipe ground clamp setupsnear the service entrance on a small commercial-job where we were upgrading the panel.  It was a valiant attempt, but the clamp was fastened to 1" PEX  :-)
(wish I had snapped a pic)
Title: Re: BX cable ground loops
Post by: Jeff Bankston on April 28, 2014, 01:34:49 am
Found one of those water-pipe ground clamp setupsnear the service entrance on a small commercial-job where we were upgrading the panel.  It was a valiant attempt, but the clamp was fastened to 1" PEX  :-)
(wish I had snapped a pic)
some people are just .......... ! that reminds me of a guy srcewing a ground pigtail to plastic wiremold type stuff.
Title: Re: BX cable ground loops
Post by: Mike Sokol on April 28, 2014, 07:34:25 am
some people are just .......... ! that reminds me of a guy screwing a ground pigtail to plastic wiremold type stuff.

I'm not sure I can say any more than this photo from RV.net showing a "dirt ground".
Title: Re: BX cable ground loops
Post by: Jeff Bankston on April 28, 2014, 07:52:31 am
I'm not sure I can say any more than this photo from RV.net showing a "dirt ground".
yeah but you gotta admit , its dirt cheap !
Title: Re: BX cable ground loops
Post by: Cailen Waddell on April 28, 2014, 08:27:46 am
So not to be a broken record here, but I am still having trouble understanding the cold water ground from an IG buss. Where is the ground neutral bond?  And where is the bond to the primary ground? 

It would seem to me, if you have an IG panel, it has to run to the regular ground buss bar in the main disconnect panel, at the same point of the ground neutral bond for the building.  In the case of system fed with an isolation transformer and having an isolated ground system, it would feed to the Xo on the transformer where the neutral originates. If at the point a cold water ground is appropriate, then fine, but an isolated ground is really an isolated ground path for fault current, and if you just tag your IG buss over to a cold water pipe, there is no guarantee a fault current would find that ground neutral bond and the breaker would trip?

That's not really a question I guess, but that's me explaining it the way in understand it.  Please correct me.

 
Title: Re: BX cable ground loops
Post by: Stephen Swaffer on April 28, 2014, 08:51:37 am
I agree-there should be one point in the system that the neutral and any and all GEC are bonded together-and at that point the IG should also be connected.  Where this happens can vary-but the most convenient and common is in the panel or enclosure that contains the service disconnect.

I think code does allow bonding to a GEC in another location, however as I mentioned earlier simply meeting code is not always the best design.  Especially with an IG-consider why you are using it-I personally would want to see a good preferably copper path back to my main bonding point- jumping to another point on the GEC kind of defeats the purpose of an IG.
Title: Re: BX cable ground loops
Post by: Kevin Graf on April 28, 2014, 09:03:37 am
i know but i didn't use the name Ufer because i didn't know if anyone here would know what i was talking about. we have always called it "The Ufer". i rarely hear it called the "concrete encased electrode" even though thats whats printed on the blue prints. are you fur going to lunch now ?

It turns out that the NEC code book doesn't use the word 'Ufer' ether.
------------------------------------
A good article on the subject:

"What Is A 'Ufer' Ground?"
'Concrete-encased Grounding Electrodes
'

Published: April 2013
By Michael Johnston

- See more at: http://www.ecmag.com/section/codes-standards/what-ufer-ground#sthash.7jDp4WT9.dpuf
http://www.ecmag.com/section/codes-standards/what-ufer-ground
Title: Re: BX cable ground loops
Post by: Jonathan Johnson on April 28, 2014, 07:54:28 pm
If I were to wire a building -- and I am NOT an electrician, inspector, or engineer, nor do I own a recent copy of the NEC -- I would have one common bonding point for all of these systems where available:

Note that there may be non-metallic portions of utilities that interrupt electrical continuity between disparate metallic portions. (That's a fancy way of saying Joe the Plumber patched the copper pipe with a length of plastic.) In that event, there should be a jumper wire bypassing the insulative portion to electrically bond the two metal portions.

Because utility pipes and electrical raceways and boxes may be directly attached to building structure, as well as the primary EGC that bonds every box, this represents secondary bonding points that under certain circumstances can cause ground loop currents. (Except for EGC-to-box bonds, the secondary bonding points to structure and utilities can be interrupted by other trades so does not constitute a valid safety ground connection.) Therefore, for equipment that is sensitive to ground loops, the isolated ground system provides a safety ground that minimizes those ground loop currents.

Notes:
EGC = Equipment grounding conductor
GEC = Grounding electrode conductor
Title: Re: BX cable ground loops
Post by: Mike Sokol on April 28, 2014, 09:15:21 pm

  • separate GECs to separate utilites (gas, water, sewer, telco, CATV, coffee, beer, etc.) where they enter the building


How do you get utilities to run coffee and beer into your building? That sounds GREAT!  8)
Title: Re: BX cable ground loops
Post by: Jeff Bankston on April 28, 2014, 09:56:54 pm


Note that there may be non-metallic portions of utilities that interrupt electrical continuity between disparate metallic portions. (That's a fancy way of saying Joe the Plumber patched the copper pipe with a length of plastic.) In that event, there should be a jumper wire bypassing the insulative portion to electrically bond the two metal portions.

thats why the main service groung has to be connect to the first 5 feet of the main water pipe that enters the building. the water pipe that connects to the city water supply has to be either copper , cast iron , steel and thats the law , there are no exceptions. after the first 5 feet you can use whatever you like as long as it meets code.

you only want 2 grounding points unless the engineer has a reson for more. too many grounding points can cause hum in equipment.

the reason all gas lines are bounded together and grounded to the first 5 feet of the main water pipe is to try to prevent static buildup which could cause and explosion if theres a leak. it is also suppose to prevent a spark if an earthquake breaks a pipe. a steel frame building with metal studs also provides a "mechanical ground". concrete and wood frame building dont do anything to provide a mechanical ground.
Title: Re: BX cable ground loops
Post by: Stephen Swaffer on April 28, 2014, 10:42:22 pm
the water pipe that connects to the city water supply has to be either copper , cast iron , steel and thats the law , there are no exceptions.

I do not doubt that that is the case in LA.  Where I live, only about half my customers-resi or commercial-have city water.  Those that have rural water receive water from a system of pvc piping, almost all if not all new wells use pvc or pex.  In the city, I have to be aware that the incoming line might be lead (yep-really) and since lead will melt too easily, it cannot be used as a ground.

Title: Re: BX cable ground loops
Post by: Jeff Bankston on April 28, 2014, 11:03:43 pm
I do not doubt that that is the case in LA.  Where I live, only about half my customers-resi or commercial-have city water.  Those that have rural water receive water from a system of pvc piping, almost all if not all new wells use pvc or pex.  In the city, I have to be aware that the incoming line might be lead (yep-really) and since lead will melt too easily, it cannot be used as a ground.
doesnt matter. an arc can blow a hole in copper , steel or cast iron.