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Grounded and Grounding.

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Nitin Sidhu:
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

Mike Sokol:
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.

Kevin Graf:
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.

Mike Sokol:

--- Quote from: Kevin Graf on November 17, 2013, 11:25:49 AM ---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)

--- End quote ---
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.

--- End quote ---
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?

Steve M Smith:

--- Quote from: Mike Sokol on November 17, 2013, 01:03:16 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?

--- End quote ---

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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