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Author Topic: BX cable ground loops  (Read 17598 times)

Jeff Bankston

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Re: BX wire ground loops
« Reply #20 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.
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Mike Sokol

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Re: BX wire ground loops
« Reply #21 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.
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Mike Sokol
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Cailen Waddell

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Re: BX cable ground loops
« Reply #22 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.
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Jeff Bankston

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Re: BX cable ground loops
« Reply #23 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.
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Mike Sokol

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Re: BX wire ground loops
« Reply #24 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/

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.  ;)
« Last Edit: April 27, 2014, 10:43:25 am by Mike Sokol »
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Mike Sokol
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Kevin Graf

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Re: BX cable ground loops
« Reply #25 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.
« Last Edit: April 27, 2014, 11:51:59 am by Kevin Graf »
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Mike Sokol

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Re: BX cable ground loops
« Reply #26 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.  ;)
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Mike Sokol
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Kevin Graf

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Re: BX cable ground loops
« Reply #27 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.
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Mike Sokol

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Re: BX cable ground loops
« Reply #28 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)
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Mike Sokol
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Stephen Swaffer

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Re: BX wire ground loops
« Reply #29 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.
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Steve Swaffer

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Re: BX wire ground loops
« Reply #29 on: April 27, 2014, 05:45:53 pm »


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