Grounding Rod

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On Sun, 24 Jul 2005 01:09:14 -0400, "CL (dnoyeB) Gilbert"

Distribution is 3 phase, but only a single phase is usually brought into a residential service.
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-john
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message wrote:

That's true; it isn't really two "phases" per sae. There are two wires coming to the house from the transofrmer on the pole, and the literal phase of the voltage/current is 180 degrees out of phase in each wire. When one wire is at max voltage in above ground, the other is at max below ground. Technically it's called split-phase since it's a transformer generated relatioship where each wire is 180 degrees out of phase with the other.
Here's a page that does a decent job of describing how it comes oritinally as 3 phase (120 degree phase related) and becomes a split-phase derived sigle-phase power. That's why so many people refer to it as two-phase. There ARE two "phases", but ... it's derived from single phase by splitting that phase.
HTH,
Pop
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Poop wrote:

Yeah? Where?
Shit your drawers again ya old bastard...
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On Sun, 24 Jul 2005 09:51:16 -0400, "Pop"

It can be proven that it is single phase, because the two winding halves are in series, and during each alternation, current flows through one half, past the center tap (neutral, common), and continues in the same direction through the other half. The voltages add. It they were out of phase, they would buck, and cancel. The total current always changes direction at the same time, and flows in the same, continuous direction during each half cycle. However, due to the center tapped common reference point, the voltages are instantaneously opposite in polarity in respect to neutral. They are not out of phase, because their periods are not derived from alternate zero crossings along the time domain.
I am sorry for the use of the term "phase inverter", in electronics, because the inversion is effectual, but not actual. "Phase splitter" is the more "politically correct" term, IMHO. "Voltage splitter" or "voltage divider" is actually the most correct term.
In AC power, for all practical purposes, it doesn't matter, whether it's time shifted or polarity inverted. But with complex signals, like music and voice signals, the positive waveform is not the same as the negative half of the waveform, and the waveform is non repetitive. Phase angle becomes meaningless except at a fixed fundamental frequency, anyway. But even with a fixed frequency (repetitive waveform), the positive and negative halves don't necessarily have to be congruent. Here, combing two opposite polarity copies of the signal will result in cancellation, but combining the same signal with itself, shifted 180 degrees in time and subtracting, may not, since the instantaneous absolute values of the two halves are not always the same.
This is why I consider the term, "out of phase", when used for home power, to be technically incorrect, even though it really doesn't matter, provided the two "phases" are equally loaded.
How's this: Take a single phase, invert it, and you have phase inversion, but still no phase shift. :-)
*Now* I'm *really* splitting hairs. :-)
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CL (dnoyeB) Gilbert wrote:

The power to most homes in North American practice is single phase 240 volt that has been center tapped to derive 120 volts from it. That system is called an Edison circuit and that is why it is used here. It was developed for use in converting Edison Electrics DC three wire circuits to AC. No matter what anyone tells you it is only single phase. The clearest evidence of which is that there is no way to wire a motor to it that will make the motor turn without the motor having a starting capacitor. The power on the supply side of the transformer that supplies most homes here is often three phase but only one phase is used to supply each transformer.
--
Tom Horne


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I agree with single phase but not that all AC motors have starting capacitors. There are series motors, repulsion induction motors, split phase motors, shaded pole motors, and probably others which start and operate on single phase power without capacitors. Except for the series motor, they all do use some sort of system to develop an out of phase magnetic field for starting. Once running only a single phase field is required. Incidentally, a 180 degree out of phase field, even if derived by phase shifting rather than inversion, will not start an induction motor. There are two phase motors which operate with a 90 degree out of phase power source but they are rather specialized. Don Young
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RBM wrote:

A megger is actually used to measure very high (insulation) resistance.
To check for the required 25 ohms ground resistance you could connect the ground rod to a hot through a lightbulb and measure the current to the ground rod and the voltage from the ground rod to a metal burried water pipe.
Ground rods are not noted for being a good ground which is why #6 Cu is the wire size used to connect them. Burried metal water pipe is good. Ground rods are a hedge for conversion of metal water service pipe to plastic.
Bud--
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Thanks for the replies all.
Never heard of a megger before. Learn new things everyday.
Bill
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Next subject-"growlers"....
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If the house is old it probably has copper water line coming into it. This is a better ground than ground rods so one isn't required. Also, put the ground wire back on the gas line...it should be bonded to ground.
Fred

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The gas line should be grounded, but not serve as the ground. Most gas companies will insulate the inside metal gas pipe at the meter connection from the underground metal gas pipe to reduce electrolytic caused corrosion.

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The ground rods are required in addition to a metallic water pipe

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This is Turtle.
i have a rule to go by on what size wire to use on ground rods replacements.
I get the grounding clam that will attach the wire to the clamp then to the rod. i look at the hole of the clamp where the wire attaches to the clamp and get a wire that will fill the hole completely. That is the size i use and sometimes called Big Wire. It will not hurt a think over sizing this wire.
TURTLE
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RBM is providing straight answers. This will expand on what he had posted.
Post 1990 codes says at least one ground rod (or equivalent and dedicated device such as earthing plate, Ufer ground, etc) close to breaker box must be installed. This earth ground (called a GEC in the code book), to earth AC electric, serves multiple electrical purposes. Its human safety purpose is defined by the National Electrical Code (NEC).
Two rods may be installed because, well, the NEC does not even properly define what is sufficient conductive earth. They provide a number (25 ohms) and don't even state how that conductivity should be measured. So many electricians just install two rods that also meet other NEC criteria such as separation between rods. Even if two rods don't make soil conductive enough, the code says an electrician need not do anything more.
The water pipe was once considered a sufficient earth ground. For numerous reasons, a water pipe is no longer sufficient as an earth ground. But water pipe must be bonded to breaker box to remove any electricity from those pipes. Electricity could be from inside the building OR electricity that could be coming from utility water pipes. The pipe must be 'bonded' to the breaker box to remove electricity. This water pipe bonding may also end up supplementing the earth ground. But code no longer views water pipe as sufficient earth ground (except in some rare cases that are irrelevant here).
Some jurisdictions require that a gas pipe also be bonded (not to be confused with earthing). That bonding requirement would be uniquely defined by the local gas company. In one example, the home had no earth ground. When a utility's street transformer failed internally, electricity used the gas pipe as an electrical conductor (because water pipe ground was compromised and dedicated earth ground was never installed). Fortunately no one was home when gas meter gaskets finally failed and the house exploded. Just one rare example of why proper earthing and proper bonding are important.
Absolute minimum since 1990 means a direct connection from breaker box to cold water pipe where pipe enters building AND a short connection from breaker box to single eight foot ground rod. This is minimum to meet code requirements (assuming the electrician had a meter such (as from http://www.leminstruments.com/cgi-local/SoftCart.exe/online-store/scstore/c-GEO_Earth-Ground_Testers.html?E+scstore or http://tinyurl.com/89dty ). Numerous and very good reasons why minimal earthing is enhanced. A number one reason - transistor safety. Earthing defines transistor protection.
Meanwhile, volts500 in the newsgroup alt.home.repair entitled "Grounding Rod Info" on 12 July 2003 wrote a description of how a house should be bonded. Earthing defined only to meet post 1990 NEC requirements; only for human safety. Even pre-1990 homes should be upgraded to meet these simple requirements. So simple that any homeowner can install after a trip to Home Depot or Lowes: http://tinyurl.com/hkjq
Bill Davis Jr wrote:

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Greetings,
Everyone here says that #6 is the size ground that you need. I think that it is important to point out that you actually only need a #8. The problem is that #8 wire is so small that if you use it you must protect it from physical damage by placing it into a proper conduit or by purchasing an armored grounding cable. I often use a #8 ground when there is no lug which can accept a #6 ground. Please note that just because a #6 ground will fit doesn't mean the connector is rated for it! This would be a code violation.
Hope this helps, William
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