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Author Topic: Grounded and Grounding.  (Read 8123 times)

Nitin Sidhu

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Grounded and Grounding.
« on: November 17, 2013, 08:03:44 am »

Please dont kill me for asking.

My understanding so far, Grounded is the Neutral. The grounding cable (green) is a dedicated backup cable also connected to the Neutral at the service entry, in case of the equipment chassis going live, this cable will carry voltage/current back to the neutral.

So where and why, is the actual earth connection (a rod going into the soil) ?

This is with regard to single phase systems only.


Tx a ton!
Sidhu
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Mike Sokol

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Re: Grounded and Grounding.
« Reply #1 on: November 17, 2013, 08:56:33 am »

You're mostly correct. The safety ground wire and neutral are bonded together back at the service panel, along with the earth ground rod. That 3-point connection of Neutral-Ground-Earth form a ground-plane very close to earth potential, mostly for lightning protection. Remember the ground rod could have an impedance of 100 ohms to dirt/earth and still be within code, so a separate ground by itself really doesn't "ground" your gear. The green safety ground wire is not allowed to carry any load current, only fault currents, so it must remain separated from the neutral/return all the way back to the service panels G-N-E bonding point.

I drew a diagram of a typical 120/240 split phase power system from the line transformer all the way to the wall outlets with typical series resistance of the wiring. I'm on a gig right now, but will post the graphic later today.
« Last Edit: November 17, 2013, 09:04:16 am by Mike Sokol »
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Kevin Graf

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Re: Grounded and Grounding.
« Reply #2 on: November 17, 2013, 11:25:49 am »

Those are recent NEC nomenclature, more or less they mean:

Grounded Conductor. A system or circuit (current carrying) conductor that is intentionally grounded. (Neutral)

Grounding Conductor, Equipment (EGC) (Safety Ground). The conductive path(s) that provides a ground-fault current path and connects normally non–current-carrying metal parts of equipment together and to the system grounded conductor or to the grounding electrode conductor, or both.

************************************
As to that old 25 Ohm business , it's now an:

Exception: If a single rod, pipe, or plate grounding electrode has a (measured) resistance to earth of 25 ohms or less, the supplemental electrode shall not be required.

Very, very few Ground Rods are ever measured!  It requires expensive test  equipment and a lot of time, so it's quicker and cheaper to just drive a second Ground Rod.  (yes there are less expensive and faster ways to measure it, but many inspectors won't accept that test)

From an audio point of view Ground Rods don't add anything good and may detract from the audio quality.
« Last Edit: November 17, 2013, 11:45:09 am by Kevin Graf »
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Mike Sokol

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Re: Grounded and Grounding.
« Reply #3 on: November 17, 2013, 01:03:16 pm »

Very, very few Ground Rods are ever measured!  It requires expensive test  equipment and a lot of time, so it's quicker and cheaper to just drive a second Ground Rod.  (yes there are less expensive and faster ways to measure it, but many inspectors won't accept that test)
A "Fall of Potential" ground test is the only one I know of that's universally recognized, but it is very complicated to perform and requires that the neutral and safety ground in the service panel are disconnected from the ground rod. That forces an existing facility to be shut down for hours for the FoP measurement, which complicates matters. Here's a pretty good tutorial on the technique. http://www.ospmag.com/issue/article/Falling-Into-a-Better-Grounding-Testhttp://www.ospmag.com/issue/article/Falling-Into-a-Better-Grounding-Test
I have a few of the more modern clamp-on ground impedance measuring tools that use high frequency test currents, but they may not be approved as a ground rod impedance certification. You're correct that driving a second ground rod is quick and cheap and doesn't require a ground impedance measurement, so most new installations that I've seen lately just add the second ground rod and skip the measurement.

Quote
From an audio point of view Ground Rods don't add anything good and may detract from the audio quality.
I've seen a number of studio installations where multiple ground rods were driven all around the building and power transformer isolation was added in an attempt to create quiet grounding, with sometimes disappointing results. There's lots of currents circling in the earth-ground around a building, so something that works today might not work next month when the water table changes. Far better to use a single-point grounding system with careful shield lifting on one end of the cable and audio transformer isolation for problem gear. Remember to use the best audio isolation transformers you can afford since there's a lot of cheap junk out there with very poor bass response.

On a side note, I've seen a few studios set up with "balanced power" (neutral and hot each at 60 volts AC from the ground) and not sure it's worth all the expense. We have such a system at the University where I teach (on a $300K console) so the added expense of balanced power was a small percentage of the room cost. But I'm not convinced of the advantages for this or any other studio. How about a new thread on balanced power? Do any of you guys have that sort of power system installed?
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Steve M Smith

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Re: Grounded and Grounding.
« Reply #4 on: November 17, 2013, 01:31:52 pm »

On a side note, I've seen a few studios set up with "balanced power" (neutral and hot each at 60 volts AC from the ground) and not sure it's worth all the expense. We have such a system at the University where I teach (on a $300K console) so the added expense of balanced power was a small percentage of the room cost. But I'm not convinced of the advantages for this or any other studio. How about a new thread on balanced power? Do any of you guys have that sort of power system installed?

In the UK we have that for construction site power tools.  Our normal supply is 240 volts but all construction tools are 110 volts powered from step down transformers with a centre tapped ground.  This gives a maximum potential of 55 volts to ground to minimise the effects of electric shock.

In this respect it is a good idea.  I'm not sure if it has any benefits for reducing system noise in audio though.


Steve.
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Jonathan Johnson

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Re: Grounded and Grounding.
« Reply #5 on: November 18, 2013, 01:34:17 am »

Regarding ground potential: my brother's farm has an electric fence for cattle containment. We found that the fence seemed ineffective, and measurement between the fence and ground showed a voltage far lower than the voltage across the
  • and [-] terminals of the fence charger.


What we figured out was that since the grounding rod for the fence charger (connected to the [-] terminal) was inside the barn, about 10 feet (3 meters) away from the exterior wall, the soil surrounding the grounding rod was extremely dry. Saturating that soil with water significantly improved the effectiveness of the electric fence.

We probably should relocate the grounding rod, but for now he just periodically waters it. Due to the impedance/resistance of dry soil being greater than that of the human body, you can be shocked when you touch the grounding rod if the soil is too dry.
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Nitin Sidhu

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Re: Grounded and Grounding.
« Reply #6 on: November 18, 2013, 03:07:23 am »

You're mostly correct. The safety ground wire and neutral are bonded together back at the service panel, along with the earth ground rod. That 3-point connection of Neutral-Ground-Earth form a ground-plane very close to earth potential, mostly for lightning protection. Remember the ground rod could have an impedance of 100 ohms to dirt/earth and still be within code, so a separate ground by itself really doesn't "ground" your gear. The green safety ground wire is not allowed to carry any load current, only fault currents, so it must remain separated from the neutral/return all the way back to the service panels G-N-E bonding point.

thank you. It severs well to understand that the grounding rod mostly serves as lightning protection, and would not help protect from fault currents.

How do I deal with venues where I am not getting a ground (no potential diff between the live and ground). Often, I have noticed, if the ground is not connected, i get a stray current in my grounding wire.

I must also state that in India, we run 220v.

Regards,
Sidhu
« Last Edit: November 18, 2013, 03:09:45 am by Nitin Sidhu »
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John Roberts {JR}

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Re: Grounded and Grounding.
« Reply #7 on: November 18, 2013, 10:37:50 am »

Can you find GFCI or RCD breakers there. When no ground path is available, the GFCI looks for leakage from hot to anywhere other than neutral.

JR
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Nitin Sidhu

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Re: Grounded and Grounding.
« Reply #8 on: November 18, 2013, 11:16:35 am »

Can you find GFCI or RCD breakers there. When no ground path is available, the GFCI looks for leakage from hot to anywhere other than neutral.

JR

Hey JR! Thank you for your time.

interesting, would you mean something like this ?
http://www.anchor-world.com/mcbs/rccb-gold.html

2 models availalbe, Listed here :
http://www.anchor-world.com/catalog/mcb/Switchgear-Protection-DevicesPricelist.pdf

I have experimented, and a very interesting phenomenon is, running a cable, touching the live (insulation's only), even if floated on both ends, has an induced voltage.

I have also seen, that often, if the grounding wire is not connected, often guitarists tend to get shocked from their vocal mic, when singing n playing their strats simultaneously.  This I now attribute to my finding of the stray voltage in the grounding wire, if the grounding wire has not been terminated.

I refuse to power up my rig if the metering does not check out. But have been studying the subject of AC largely now.

What has lately started disturbing me, is that While I now understand the function of the grounding wire, it seems to me that I cannot run my system if the grounding wire is not connected correctly at the service.


Regards,
Sidhu
« Last Edit: November 18, 2013, 11:19:20 am by Nitin Sidhu »
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John Roberts {JR}

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Re: Grounded and Grounding.
« Reply #9 on: November 18, 2013, 12:52:18 pm »

GFCI and RCD systems will protect humans by cutting off power. This may not be a good situation if your system is constantly tripping off, due to flaky back line gear, or whatever.  It will take a process to weed out the killer gear you encounter, before show time.

The main function of a separate safety ground is to trip a fuse or breaker when there is a hot path to a product chassis by sinking full current back to the panel.

GFCI and RCD will be more sensitive and turn off power at lower levels of leakage current (mA vs Amps).

Good luck.

JR

 
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Re: Grounded and Grounding.
« Reply #9 on: November 18, 2013, 12:52:18 pm »


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