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Old 06-10-2012, 02:44 PM   #16
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How does grounding work?


Westom,
You're correct, that first paragraph was just to explain the idea of induced voltages. I realized that I didn't really encounter the concept of induced voltage/current until I started to study EMT. So I figured a basic description with examples might help. As I side note, am I correct in the assumption that you work in telecommunications dealing with long distance runs?

Daniel,
I had the same inkling that the grounding electrode wouldn't really be needed for induced voltages. Given the short distances back to the utility pole (save in rural areas), not much should be able to induce a significant voltage save lightning. But when I did a bit of googling, a number of folks were talking about it being the primary purpose, and lightning style discharge being a secondary purpose. Taking a SWAG, I think the rules for a grounding electrode probably came about due to commercial installations with badly behaving/designed equipment that induced significant voltages to ground over time. Then someone smart remembered that some home owners are idiots, a percentage of whom also play with equipment that could induce voltages.

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Old 06-10-2012, 02:55 PM   #17
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How does grounding work?


All of the posts are giving accurate information but I'm getting a headache trying to follow the higher level ones. I have included the pic of a center tapped transformer which is feeding your house and it may be easier to follow.

1) Forget about earth, ground, and that little earth/ground symbol on the neutral tap of the transformer for a moment and only pay attention to the 3 connections coming out of the right of the transformer which goes into your house.

2) Forget about how electricity wants to flow to earth. It doesn’t apply to your question except for lightning.


In a perfect world all you need are the 3 connections (two hots and the neutral) going to your house which you indicated that you understand, good. Throw in bad wire connections, faulty appliances, and some lightning and now you have a need for an Equipment Grounding Conductor (shock prevention) and Grounding Electrode Conductor (lightning/high voltage protection rod driven into the ground). Both of these safety ground connections are tied to the neutral and provide a zero voltage reference, not for the operation of your house but mainly for your safety. These are just the basics but it should answer your questions. In reality, the neutral will provide shock protection but it is safer to have a separate conductor for this purpose. The little earth/ground symbol on the neutral of the transformer is showing the electric company ground point and the ground rod and water pipes at your house.

Added info after reading your post again. You can't depend on the earth, as in soil, to have a low enough resistance to trip a breaker, it depends on moisture content and distance. That is the purpose of the EGC which is a low resistance conductor wired directly to the neutral.



How does grounding work?-split-2520phase-2520small.jpg

Last edited by zappa; 06-10-2012 at 03:34 PM.
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Old 06-10-2012, 02:58 PM   #18
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How does grounding work?


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Originally Posted by a_lost_shadow View Post
I think the rules for a grounding electrode probably came about due to commercial installations with badly behaving/designed equipment that induced significant voltages to ground over time. Then someone smart remembered that some home owners are idiots, a percentage of whom also play with equipment that could induce voltages.
Electrical systems that are grounded shall be connected to the earth in a manner that will limit the voltage imposed by lightning, line surges or unintentional contact with higher-voltage lines and that will stabilize the voltage to earth during normal operations.


This is why we install grounding electrodes...
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Old 06-10-2012, 03:05 PM   #19
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How does grounding work?


Cossack,
Here's how the equipment ground can provide safety.

Imagine that someone is using an old metal cased drill to hang some metal water pipe. He's been going at it for awhile so his hands are sweaty. Additionally, the internal wiring on the drill has begun to give out causing the hot wire to come into contact with the drill case.

If the drill has an equipment ground, then as soon has he plugs the drill it will energize the case. This will case current flow from the hot, through case, into the equipment ground, back to main panel, and finally back out the utility's neutral service wire. There's essentially no resistance from the electrical wiring, so the current will skyrocket and the circuit breaker will trip immediately.

If the drill doesn't have an equipment ground, then again when he plugs the drill in it will energize the case. However, without an equipment ground there's no path to complete the circuit. Thus the case remains energized. The drill will even work, and our poor sap won't notice a thing. Until he grabs the metal pipe. At that point, current will flow from the energized case, into the poor sap's hand, though his body, out his other hand to the metal pipe, from the metal pipe to the main panel, and finally back out the utility's neutral service wire.

In this case, human body can have a resistance as low as 500 Ohm. So assuming standard 120. Using V = IR, we find that the current can only get up to 120 V / 500 Ohm = 0.24 Amps. Thus the circuit breaker doesn't think anything is wrong, and our poor sap gets a nice electrocution. Depending on the source, AC currents from 0.06 to 0.1 amps are needed across the torso to lead to death.
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Old 06-10-2012, 03:10 PM   #20
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How does grounding work?


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Originally Posted by stickboy1375 View Post
Electrical systems that are grounded shall be connected to the earth in a manner that will limit the voltage imposed by lightning, line surges or unintentional contact with higher-voltage lines and that will stabilize the voltage to earth during normal operations.


This is why we install grounding electrodes...
But making Stupid Wild Ass Guesses are so much more fun. But since lightning comes before stabilizing the voltage during normal operations, I'll concede that lightning is primary and induced voltage is secondary.

Last edited by a_lost_shadow; 06-10-2012 at 03:12 PM. Reason: lightning != lighting
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Old 06-10-2012, 03:24 PM   #21
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How does grounding work?


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But making Stupid Wild Ass Guesses are so much more fun. But since lightning comes before stabilizing the voltage during normal operations, I'll concede that lightning is primary and induced voltage is secondary.
I would say lightning and accidental contact from higher voltages are a tie...
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Old 06-10-2012, 03:27 PM   #22
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How does grounding work?


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Consider what would happen if such an event occurred. The ground at your home is designed to be at 25 ohms or less. ... I do not belileve that the grounding rod installed in the typical residential installation is intended to prevent a floating power company neutral,
A real world example.


They ignored intermittently dimming lights. Did not inspect their earth ground. When the transformer neutral completely failed, electricity began using gas pipes as an electrical conductor. Because the house earth ground had been compromised and ignored.

Eventually a gasket in the gas meter failed. Fortunately nobody was home when the house exploded.

Cossack, this event answers your question - for the entire house. If asking about a third prong in a wall receptacle; that is completely different and not earth ground. a_lost_shadow's example of a power drill defines a safety ground in a wall receptacle. A safety ground so that an electrical fault will trip a circuit breaker. An example of grounding doing another safety function.

Earth ground's primary purpose is to keep neutral from 'floating'. So important that every transformer is also earthed. Home earthing has has other purposes. But if a neutral is not earthed, then it could easily float to 2000 or 15,000 volts.

Only a homeowner is responsible for his earth ground. A homeowner should periodically inspect that earth ground. A connection so important that even a Mil-Standard says how often (in years) it must be inspected.

Provided are pictures of another important earth ground. A homeowner should also inspect this ground:
http://www.tvtower.com/fpl.html
Especially since copper thieves are so active.

These grounds have other functions. We are currently only discussing the original , more important, and basic ground functions. We have yet to discuss earthing for lightning and other equivalent transients.

How can copper wire and dirt be so complex?

Last edited by westom; 06-10-2012 at 03:36 PM.
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Old 06-10-2012, 07:18 PM   #23
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How does grounding work?


Are there any books out there you all can recoemnd that covers all this?
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Old 06-10-2012, 09:23 PM   #24
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How does grounding work?


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I am glad to see this discussion, but I am also having trouble understanding.
This is why engineers should stick to their trade, and leave the real world every day electrical to real world folks.

Most of what is in this thread is completely useless to a lay person, but I am sure makes the poster feel very important.
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Answers based on the 2008 & 2011 NEC.
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Old 06-10-2012, 09:33 PM   #25
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How does grounding work?


I think it is important to know both HOW and WHY grounding should be done. It would be both interesting and helpful to read further on this. Don't electricians have to know some theory as well?
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Old 06-10-2012, 09:45 PM   #26
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How does grounding work?


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I think it is important to know both HOW and WHY grounding should be done. It would be both interesting and helpful to read further on this. Don't electricians have to know some theory as well?
Its very simple as far as the NEC is concerned... anything above that is just, well... above and beyond, and not necessary... it really is this simple, we drive two ground rods and move on to the next job....even if we know more, it doesn't matter, we only drive two rods and move on...
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Old 06-10-2012, 09:46 PM   #27
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How does grounding work?


Soares is a good start

http://www.amazon.com/Soares-Book-Gr.../dp/1890659576
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Old 06-10-2012, 09:49 PM   #28
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How does grounding work?


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Originally Posted by Speedy Petey View Post
This is why engineers should stick to their trade, and leave the real world every day electrical to real world folks.

Most of what is in this thread is completely useless to a lay person, but I am sure makes the poster feel very important.
I love how all the most useless information is displayed upon the people that have no idea the relationship of the ground port on a receptacle and a ground rod in the earth...
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Old 06-10-2012, 10:03 PM   #29
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How does grounding work?


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and you think this helps the OP how?
Quote:
Originally Posted by stickboy1375 View Post
Electrical systems that are grounded shall be connected to the earth in a manner that will limit the voltage imposed by lightning, line surges or unintentional contact with higher-voltage lines and that will stabilize the voltage to earth during normal operations.


This is why we install grounding electrodes...
Your real good at quoting article 250(a)(1).....but I don't think you really understand what it means.....

westom's anology actually made quite a bit of sense......

May I suggest watching this video...it illustrates quite well what westcom was talking about.



The problem we are getting into now is trades vs engineers....

The trades know the rules and how to build to those rules....but in many cases, don't really understand the how and why of those 'rules'..

While engineers understand those rules...but sometimes strugle with how to physically implement those rules.

Case in point....my FIL can design a supersonic airplane....but would struggle to buck a single rivet in the plane.....

In a perfect world, there should never be any current flow to ground.

In a lightning strike....the voltage is typically imposed on all lines....including ground....the degree of which is determined by how close the hit and where exactly it is.

The since most of us are on 'the ground', it is preferable that any surges be directed to that potential.

As mentioned above...grounding is a long and distinguished subject.....that is still evolving....another case in point....the Ufer ground that is now code but yet was discovered in WWII.....and it took over 50 years to become a standard?

And the trades still argue about twist vs no twist....
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Old 06-10-2012, 10:05 PM   #30
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How does grounding work?


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Originally Posted by Cossack View Post
I am glad to see this discussion, but I am also having trouble understanding. Maybe explain what happens if there is no third ground wire as in older wiring. In old houses there is usually just a hot and neutral wire yet everything seems to work fine.
First thing is to understand that earth grounding has to do with equipment protection and consists of your GES (grounding electrode system) on the NEC side of the meter and the utility grounding on the NESC side of the meter (line side).

The equipment grounding conductor (egc) that is utilized in your homes wiring or 3rd wire as you called it ... is there to protect humans from electrocution from energized metal and other bonded conductive parts of the premise wiring system..
The egc never carries current unless a ground fault occurs to bonded metal . In which case the fault current uses the egc as a path to return to the transformer center tap or midpoint. This low impedance/resistance path allows large amounts of current to flow on the 'fault circuit' and that current must pass thru the circuit breaker protecting that branch circuit. The breaker will open and the fault will be cleared and the bonded metal deenergized.

Some diargams to illustrate
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How does grounding work?-bonding-diagram-1.jpg   How does grounding work?-grounded-ground-grounding-1.jpg  

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