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06-09-2012, 11:42 PM   #1
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How does grounding work?

Okay. I know enough to know it's important but I'm starting to get into the actual science and am trying to understand some things about grounding and how they relate to circuits as a whole.

Some background on my question:

I know that the hot comes in through the main, as does a neutral circuit. Those go to the sub panel where the hot ties to the hot bus bars, the neutral to a neutral, and the ground bus bar has a wire which ties into the neutral (bonding?) and also into something grounded to the earth. What I don't quite understand is basically... how does this whole thing work-- in English, not in scientific hooplah.

I get that current flows from the hot, through the object using it, and returns over the neutral line. When an abundance of energy (over the amount a breaker is specified to let through) is drawn, the breaker trips. I think, that a ground is around to serve a similar purpose, in that if there is a problem with the neutral return, the electricity follows the path of least resistance (I assume copper wire is less resistant than a human being) and so the electricity follows the ground, and in attempting to complete the circuit, lands on the ground, into the earth which I read is a source of an infinite energy dump. That said, over 20 amps on a 20 amp breaker could flow to the earth, however, once that point hits, the breaker trips, canceling out all power.

What I don't understand is why the ground ties into the neutral as well, and why as I read, "Electricity naturally flows to the earth." I know there's something about lightning and it's connection to Ben Franklin and all this, but I can't remember and google is at the point of a migraine-inducing headache to sift through all this and make heads or tails of it.

Anyways- if anyone can help to clarify this to me in relate-able analogies or something, that would be appreciated. I've tried understanding the wikipedia page on grounding but I just don't seem to understand how Electricity naturally wants to flow to the earth and so forth. The more I try to understand, the more my head starts hurting.

06-09-2012, 11:50 PM   #2
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Do not think that the ground carries any current. That is not its purpose. The ground rod tied into the panel is there more for lightning protection. It gives a lightning surge a path to ground. It is not a return path for the power you are using.
The ground wires in your house there for safety. If an item has a metal frame then the ground wire is connected to the frame. If the hot should come loose and contact the frame the ground provides a path for the return current to trip the breaker. Otherwise the metal frame becomes energized YOU can be the return path and get electrocuted.

 06-10-2012, 08:34 AM #3 Licensed Electrical Cont.     Join Date: Feb 2004 Location: NY State Posts: 7,724 Rewards Points: 1,798 This is a huge subject that cannot be fully covered easily, especially with regard to laypeople. Heck, I know electricians that are not completely clear on the subject of grounding/bonding Joe make very good and correct points. I will just make a couple of my own comments. Remember that grounding, and grounding electrodes have nothing to do with the functionality of a typical electrical circuit. Equipment grounding is simply a return path for FAULT CURRENT. It is NOT a "drain" or path for "extra current". Some of the things we hear are pretty crazy. Like Joe says, it is a path for stray (fault) current that is trying to flow where it shouldn't. There is talk of changing the term "equipment grounding" with something like "equipment bonding" which is far more accurate and much less confusing. Grounding has very little to do with the actual ground itself. Grounding electrodes, and the grounding electrode system have NOTHING to do with equipment grounding, contrary to what many people think. A ground rod is NOT a path for fault current, nor is it related to the round hole in any receptacle in your home. The GES is for lightning and HIGH voltage surges such that you might get from a utility line. __________________ Sometimes I feel like if I answer any more questions it is like someone trying to climb over a fence to jump off a bridge and me giving them a boost.
06-10-2012, 09:42 AM   #4
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Quote:
 Originally Posted by scyarch What I don't understand is why the ground ties into the neutral as well, and why as I read, "Electricity naturally flows to the earth."
Think of a 1.5 volt 'D' battery. Connect two wires from the battery to a light bulb. The bulb is powered by 1.5 volts.

That same voltage can also be 1001.5 and 1000 volts. Because the battery and bulb have a 'floating' ground. Not a problem when power comes from a battery. A major problem when the battery is replaced with a transformer powered from a 4000 volt primary.

The same light that sees only 1.5 volts can also see 1001.5 volts. All depends on which two points are used to measure voltage.

So that your house AC never sees more than 120 volts, tie an AC center point to earth. Called the neutral wire. Then your dryer that is powered by 240 volts wire to wire (or L to L) is also never more than 120 volts to earth (L to E).

Obviously, 120 volts going from a dryer to earth via the human body is safer than 240 volts.

Different voltages can exist on the same wire. These have different names such as longitudinal and metallic mode. Earth a breaker box neutral so that a floating ground does not exist. So that every wire relative to earth will not exceed 120 volts. Even for 240 volt appliances. Also earth so that a light bulb's hot and neutral wires are not floating at 4120 and 4000 volts.

06-10-2012, 09:52 AM   #5

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Quote:
 Originally Posted by westom So that your house AC never sees more than 120 volts, tie an AC center point to earth. Called the neutral wire. Then your dryer that is powered by 240 volts wire to wire (or L to L) is also never more than 120 volts to earth (L to E).
Actually, this center point is the utility transformer, not earth.

Quote:
 Originally Posted by westom Different voltages can exist on the same wire. These have different names such as longitudinal and metallic mode. Earth a breaker box neutral so that a floating ground does not exist. So that every wire relative to earth will not exceed 120 volts. Even for 240 volt appliances. Also earth so that a light bulb's hot and neutral wires are not floating at 4120 and 4000 volts.
I am not sure what you mean by "floating ground". To be honest, I am not sure that this whole paragraph means.
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 06-10-2012, 10:07 AM #6 E2 Electrician     Join Date: Apr 2012 Location: Litchfield, CT Posts: 5,655 Rewards Points: 2,970 Some pictures to show whats going on....
06-10-2012, 10:49 AM   #7
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Quote:
 Originally Posted by Speedy Petey I am not sure what you mean by "floating ground".
Hold a 1.5 volt 'D' battery in your hand. How many volts from the bottom of that battery to earth? It could be 5 volts. Or it could be 1000 volts. Without a defined connection, then any of those volts can exist.

If the bottom of the battery is 1000 volts to ground, then the top of that 1.5 volt battery is 1001.5 volts to ground.

Since above was new, then you will not even begin to understand it until a third reread. Anything understood in a first read was already known. This is completely new to you.

Voltage from the bottom of that battery to earth can be any voltage until it is specifically defined. We call that 'it can be any voltage' a floating ground. The concept is quite simple. And extremely difficult when first introduced.

06-10-2012, 11:11 AM   #8
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Quote:
 Originally Posted by westom Hold a 1.5 volt 'D' battery in your hand. How many volts from the bottom of that battery to earth? It could be 5 volts. Or it could be 1000 volts. Without a defined connection, then any of those volts can exist. If the bottom of the battery is 1000 volts to ground, then the top of that 1.5 volt battery is 1001.5 volts to ground. Since above was new, then you will not even begin to understand it until a third reread. Anything understood in a first read was already known. This is completely new to you. Voltage from the bottom of that battery to earth can be any voltage until it is specifically defined. We call that 'it can be any voltage' a floating ground. The concept is quite simple. And extremely difficult when first introduced.
and you think this helps the OP how?

 The Following 2 Users Say Thank You to stickboy1375 For This Useful Post: Code05 (06-12-2012), Speedy Petey (06-10-2012)
 06-10-2012, 11:42 AM #9 Member   Join Date: Dec 2011 Location: Livermore, California Posts: 272 Rewards Points: 264 westom, I hate to say it. But you're almost as bad at explaining things as I am. I believe the following is a simpler way to put it. Certain situations can induce a voltage in an object. The amount of voltage varies based upon the event, rubbing ones feet on a carpet vs a lightning strike. The object will remain at the induced voltage until it comes into contact with an object at a different voltage. When two objects of different voltages touch, current will flow until the voltage between the two equalize. Thus the reason you can get a static shock when you touch metal after rubbing your feet on a carpet. When it comes to your house wiring, the grounding electrode ensures that most induced voltages will discharge to the earth. Without the grounding electrode, it would be possible for an event to induce a voltage in the house's wiring. This induced voltage could then shock someone if they touch something connected to the house's equipment ground while also touching something grounded to the earth, such as a water pipe. Last edited by a_lost_shadow; 06-10-2012 at 11:45 AM.
06-10-2012, 12:59 PM   #10
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Quote:
 Originally Posted by stickboy1375 and you think this helps the OP how?
Because if the neutral is not earthed, then the neutral can be at 4000 volts. And the hot wire can be at 4120 volts. That is still 120 volts to the light bulb and TV. And 4000 volts to the human.

This is difficult because so many are taught about voltage at one point. Voltage is always about a potential difference between two points. A 1.5 volt "D" battery can be at 1001.5 volts and also at 1000 volts.

The OP asked what an earth ground does.
Quote:
 but I just don't seem to understand how Electricity naturally wants to flow to the earth and so forth.
An earth ground connection changes a floating safety ground from maybe 4000 volts to 0 volts. So that humans are not electrocuted.

That should have your complete attention. When did you last inspect your earth ground? The homeowner and nobody else is responsible for something that important.

06-10-2012, 01:06 PM   #11
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Quote:
 Originally Posted by Bookerc What's the difference between grounding electrode and equipment grounding?
Equipment grounding (the words used in the code) is safety ground. A third prong on a wall receptacle is a safety ground. So that any electrical fault is shorted out. Causing a circuit breaker to trip.

All safety grounds and neutral wires must meet at a common point. At a bus bar inside the main breaker box. Only then will safety ground trip a circuit breaker. No receptacle safety ground can connect to earth.

As described eariler, if the safety (equipment) ground is not connected to earth ground, then safety ground can float to thousands of volts. A connection from safety ground to earth ground addresses numerous anomalies. But a firstmost 'reason why' code requires it: so that safety ground does not float to thousands of volts. So that safety ground and earth ground have almost zero volts difference.

06-10-2012, 01:20 PM   #12
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Quote:
 Originally Posted by a_lost_shadow The amount of voltage varies based upon the event, rubbing ones feet on a carpet vs a lightning strike. The object will remain at the induced voltage until it comes into contact with an object at a different voltage. When two objects of different voltages touch, current will flow until the voltage between the two equalize. Thus the reason you can get a static shock when you touch metal after rubbing your feet on a carpet.
That is true. It explains how electricity flows. But earth ground has no relationship to static electric discharges. That static electric 'ground' is carpet beneath shoes. The electric circuit is out an arm, through furniture or a door handle, down to the floor, to connect to a 'ground' - the floor beneath shoes. Earth ground is nowhere in that circuit.

That static electric discharge example demonstrated how a 'floating' ground can obtain other voltages - even 18,000 volts found in static electric discharges. It does not explain what an earth ground does.

Last edited by westom; 06-10-2012 at 01:28 PM.

 06-10-2012, 02:00 PM #13 Civil Engineer   Join Date: Mar 2009 Location: Boston Posts: 5,414 Rewards Points: 4,408 The voltage potential on the neutral is maintained by the power company at close to earth potential because the neutral side of the transformers and wires on the power company side are typically connected to earth via grounding cables. Except under power company fault conditions, there should NOT be a floating neutral. There are certain specialized systems whereby the power supply is NOT grounded, deliberately, and the neutral then does float. So far as I know, this method of power delivery is not used for residential supply in the United States. Westom's concept of 4000 volts on the neutral side, and 4120 volts on the hot side, should never occur except under extreme conditions where the power company grounds all failed. Consider what would happen if such an event occurred. The ground at your home is designed to be at 25 ohms or less. At 25 ohms, with a 4000 volt neutral, there would be 4000/25 = 160 amps of current flowing through the neutral to ground, which would likely melt the neutral, unless the breaker tripped first. I do not belileve that the grounding rod installed in the typical residential installation is intended to prevent a floating power company neutral, that is the responsibility of the power company, and I do not think the grounding rod is intended to act as a safety feature for power company faults. I believe the previous posts noting that the function of the grounding rod is to dissipate lightning strike induced voltage is correct. Under normal conditions, and even ground fault conditions inside the residence, the majority of the fault current is carried back on the power company neutral, which has a much lower resistance than the grounding system at your house. You can compute the amount of current that actually travels through the house grounding system if you know the resistance of the house grounding system, and the effective resistance of the power company neutral. The common myth that electricity takes the path of least resistance is incorrect, electricity follows all possible paths, in inverse proportion to the resistance of the path. One of the consequences of this fact explains what happens during a normal ground fault on a piece of equipment in the house, assuming there is an independent equipment ground. Suppose the hot wire (the black one) loses insulation and contacts the metal case of the device. If the device lacks an independent equipment ground, and the neutral is not connected to the case (typical condition), the breaker will NOT trip because there is no current carrying path through the breaker. However, the case will be energized at 120 volts, and if a person touches the case, you can get a dangerous shock. If the case has an independent equipment ground, there will be a low impedance path back to the neutral bond at the main panel, and there will be a temporary large flow of current through the case, and back to the neutral. Note that only a small part of this current flows through the house ground, since the house ground is typically around 25 ohms. Most of the current flows back to the transformer via the power company neutral, and hopefully trips the breaker at the house. The fact that the neutral at the main panel is bonded to the house ground is essentially irrelevant in this scenario.
 06-10-2012, 03:15 PM #14 Member   Join Date: Jul 2008 Location: NW of D.C. Posts: 5,990 Rewards Points: 2,000 And if, at night, you carefully run a 100W incandescent bulb from the hot side of an inside non-GFCI outlet to the soil near your house using an extension cord, the bulb -will light -will not light -cannot say ? How 'bout for a 7-1/2W bulb?
06-10-2012, 03:42 PM   #15
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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.

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