Grounding

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  #1  
Old 05-12-04, 03:53 PM
Bruce L
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Grounding

why is the neutral and the ground tied together in a main panel? if current flow on a white wire and is tied to the bare ground wire in the panel, wouldn't this defeat the equipment ground purpose?

ALSO,

why is it require to have separate ground and neutral in a subpanel when the main panel's neutral and grounds are connected? wouldn't the amps who flow on the ground of a sub also flow on the ground at the main?

thanks for your help
bruce l
 
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Old 05-12-04, 04:12 PM
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The neutral and ground are tied together so that if a fault occurs at an outlet or at an appliance and the hot wire connects to the grounded metal junction box or to the metal shell of the appliance, the current will have a path back to the source, which will hopefully tcause the breaker to trip. An electric circuit needs a path back to the source.

In a sub panel the ground and neutral are kept separated so that no current flows on the ground side of the circuit. In an electric circuit current flows on every path back to the source. The amount of current that flows on each path depends on the numbers of paths, and the resistance of each path. If the wiring to the subpanel had the ground and neutral the same size, and if they were connected at the sub panel, then the current across each wire would be roughly equal.
 
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Old 05-12-04, 04:21 PM
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I can think of a real good reason to have the ground wires from all the outlets connected to the neutral at the main panel. Consider what would happen if you had, say, an electric drill that had some leakage due to a partial short to the metal case. As long as your neutral was connected everything would work fine. It's possible, however, for a wire nut somewhere in the line to loosen or otherwise go bad. Now, if you try to use that drill, YOU, could become the neutral lead and would be conducting the current with the possible electrocution hazzard involved. If you have a second wire, called the ground, also connected to the neutral at the panel, it would look like a lot lower resistance back to neutral, and would conduct the lion's share of the current, and possibly save your life.

Having a second return wire also allows your ground fault interruptors to work properly and stop the current flow when it detects a difference in current between the hot and neutral leads.
 
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Old 05-12-04, 04:37 PM
Bruce L
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cornfused

so the ground is simply a secnd return, besides the white? since sometimes the wirenut will come off and the white wire comes loose what happens if the ground comes off instead but you still have the white wire? will the white wire take the fault back to ground? and if so, whats the need for the ground? i guess the gorund is simply an extra protection but houses did fine in the early days without being grounded. all this ground wire is tied to the white wire sometimes and not tied in other times is confusing. its also confusing why if the ground and neutral are tied at the main panel and the amps on the white are also running on the ground at the panel, why isn't the amps also running on the ground of a recepticle?
 
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Old 05-12-04, 04:52 PM
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No. The ground is a ground. A return carries current back to the source. A ground does not carry current except under certain conditions, and then (hopefully) for only a very short period of time.

In a properly wired circuit the return will not see fault current.

There is noithing confusing about when the return and ground are tied together. They are tied tiogether at one location and one location only in a residence. This is usually at the main panel. Sometimes it is at the meter.

No, current does not flow on the ground at the point they are tied together. There are three wires coming from the utility. These three wires are two hot wires (110 volts each) and a return. There is no ground coming from the utility. The ground connection at the main panel goes to an earth ground, usually via the coolw water pipe entering the house and/or a metal rod inserted into the ground.

Under normal circumstances no current flows through the ground.


One other piece of information. Grounds are needed more and more in todays world. UPS devices and other surge supressors use the ground to dump the surge current. Computers (and other electronic equipment) use the ground for reference.
 
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Old 05-12-04, 04:57 PM
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You can always make the argument that people got by years ago without a safety ground, but people did get electrocuted back then, and that's why a safety ground system was installed in newer houses. As long as everything is working OK, the ground is a worthless wire. It conducts nothing. It's when things go wrong, and eventually murphy's law says it will, that the extra ground wire could save your life.

If you have a failure in the ground wiring system, you may never know it. Your electrical equipment would work fine. If there is an insulation problem with the equipment, however, you could now be in danger.

The worst shock I ever took in 30 years working around electrical equipment was from a piece of equipment without a 3rd grounding wire on the cord. There was a short between the hot wire and the equipment case. The switch was off on the equipment so the fuse didn't blow. I was carrying the equipment by it's metal handle and touched another large metal, well grounded, piece of equipment and stood there for a few seconds receiving a very nasty shock, and couldn't let go. I finally fell to the floor breaking my connection. If the other piece of equipment had a 3rd ground lead in it's electrical cord the breaker would have tripped, or the equipment fuse would have blown saving me from an almost fatal shock. Others haven't been so lucky in the past.

I know that some of the rules are confusing, but if you diagram the system on a piece of paper and create a hypothetical system fault and see what whould happen under certain circumstances, you will see the value of an extra ground wire that directly connects to the equipment case.
 
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Old 05-12-04, 05:08 PM
Bruce L
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this dont make sense "No, current does not flow on the ground at the point they are tied together". if the white wire is carrying the return amps from the load and it is in physical contact with the ground, then there has to be retun flow on the ground. it is the same thing as if the hot tounches the metal cabnet of a dryer, amps is flowing to the cabnet. and if the amps is flowing to the ground at the main how can it not be flowing on a recepticle? there is the physical connection between the white and the bare at the panel. how is it that there isnt also an electrical connection between the two? i have drawn these out on paper and have seen diagrams in books, but if 10 amps is flowing out the hot wite, goes to the load and returns 10 amps on the white wire, the 10 amps seems to be going to the ground also at the panel and if its on the panel how can it not also be on a recepticle since the two are physical and thereby electical connected? i'm sorry if im dum and iratating but im just trying to understand.

EDITED: ok, is there any webpage that gives a complete details of how groung works? i've googled this and can't find anything complete. i dont want to be a pain in the butt on this forum so if anyone can guide me to a good site, i'd be most grateful.

thanks for your replies,
bruce l
 

Last edited by Bruce L; 05-12-04 at 05:19 PM.
  #8  
Old 05-12-04, 05:33 PM
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Yes, the neutral wire returns to the main panel all the current that's flowing out on the hot wire. But, the ground wire is only connected to the neutral on ONE end of the circuit and that's at the main panel. No current can flow on the ground wire if it's only connected on ONE side. The ground system should be open circuited at the equipment end if all is well in the equipment. It's when there's a fault in the equipment that current should THEN flow in the ground system wires. Hopefully enough current will flow to trip a breaker and interrupt the circuit.
 
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Old 05-13-04, 11:45 AM
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Quote


One other piece of information. Grounds are needed more and more in todays world. UPS devices and other surge supressors use the ground to dump the surge current. Computers (and other electronic equipment) use the ground for reference.[/QUOTE]


Their are TVSS products that do not contaminate the ground line.
www.surgex.com
 
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Old 05-13-04, 11:49 AM
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EDITED: ok, is there any webpage that gives a complete details of how groung works? i've googled this and can't find anything complete. i dont want to be a pain in the butt on this forum so if anyone can guide me to a good site, i'd be most grateful.

thanks for your replies,
bruce l[/QUOTE]

An engineer can spend an entire lifetime on grounding. Keep trying, it'l eventually start making sense to you. Then start trying to apply grounding to the proffesional audio world. All hell breaks loose.
 
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Old 05-13-04, 11:50 AM
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Exclamation It's the semantics that are messing us up!

When you are up to your eyeballs in semantics it is a little hard to remember that the original objective was to clear the fault.

There is a proposal being considered by the code making panel to change the name of the Equipment Grounding Conductor (EGC) to Equipment Bonding Conductor. The purpose of the change is to clarify what the actual purpose of this conductor really is. The EGC connects the non current carrying conductive parts of the electrical system to the grounded tap of the electric utilities transformer winding. It is important to understand the difference between grounding and bonding.

The explanation in the code can be hard to read and it is worth remembering what the purpose of the code is.
90.1 Purpose.
(A) Practical Safeguarding. The purpose of this Code is the practical safeguarding of persons and property from hazards arising from the use of electricity.
(B) Adequacy. This Code contains provisions that are considered necessary for safety. Compliance therewith and proper maintenance will result in an installation that is essentially free from hazard but not necessarily efficient, convenient, or adequate for good service or future expansion of electrical use.
(C) Intention. This Code is not intended as a design specification or an instruction manual for untrained persons. .

250.4 General Requirements for Grounding and Bonding.
The following general requirements identify what grounding and bonding of electrical systems are required to accomplish. The prescriptive methods contained in Article 250 shall be followed to comply with the performance requirements of this section.
(A) Grounded Systems.
(1) Electrical System Grounding. Electrical systems that are grounded shall be connected to 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 operation.
(2) Grounding of Electrical Equipment. Non–current-carrying conductive materials enclosing electrical conductors or equipment, or forming part of such equipment, shall be connected to earth so as to limit the voltage to ground on these materials.
(3) Bonding of Electrical Equipment. Non–current-carrying conductive materials enclosing electrical conductors or equipment, or forming part of such equipment, shall be connected together and to the electrical supply source in a manner that establishes an effective ground-fault current path.
(4) Bonding of Electrically Conductive Materials and Other Equipment. Electrically conductive materials that are likely to become energized shall be connected together and to the electrical supply source in a manner that establishes an effective ground-fault current path.
(5) Effective Ground-Fault Current Path. Electrical equipment and wiring and other electrically conductive material likely to become energized shall be installed in a manner that creates a permanent, low-impedance circuit capable of safely carrying the maximum ground-fault current likely to be imposed on it from any point on the wiring system where a ground fault may occur to the electrical supply source. The earth shall not be used as the sole equipment grounding conductor or effective ground-fault current path.
If you read the above section carefully and patiently you will see that the conductor that the code calls the Equipment Grounding Conductor is doing two jobs. It connects the non current carrying conductive parts of the system to the main bonding jumper and in so doing it limits the voltage to ground on those parts. That is it's "grounding" role. It also connects them together and to the electrical supply source in a manner that establishes an effective fault current path. That is it's "bonding" role. When a fault occurs in a wiring system the leaking current is trying to return to the source. The source of the current in most home wiring systems is the utilities transformer. The round trip pathway for that fault current is from the transformer windings ungrounded end through one of the ungrounded current carrying conductor (hot) of the service entrance wire, through a fuse or circuit breaker, through the branch circuits ungrounded current carrying conductor, through the fault contact point, through the Equipment Grounding Conductor, through the main bonding jumper, through the grounded current carrying conductor (neutral) of the service entry conductors, and back into the transformer winding via the grounded tap of the transformer. The ability of the earth to conduct current between the Grounding electrode of the homes service equipment and the grounding electrode of the utilities transformer is so poor that the two grounds play no significant part in carrying enough current to trip the overcurrent protective device thus clearing the fault.
--
Tom Horne
 

Last edited by hornetd; 05-13-04 at 12:04 PM.
  #12  
Old 05-13-04, 11:56 AM
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grounding

you are absolutely correct Tom.

I switched to the using "bonding" about four years ago. for two years nobody knew what I was talking about so I switched back to "grounding". Maybe now I'll be able to make the switch again
 
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Old 05-13-04, 01:03 PM
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this dont make sense "No, current does not flow on the ground at the point they are tied together". if the white wire is carrying the return amps from the load and it is in physical contact with the ground, then there has to be retun flow on the ground. it is the same thing as if the hot tounches the metal cabnet of a dryer, amps is flowing to the cabnet.
Bruce,

You are making a very basic mistake here. You are confusing 'amps' and 'volts'.

Amps are a measure of current, quite literally the number of _electrons_ that flow past a location per unit time. The standard analogy that people use is with water in pipes. 'Amps' are analogous to 'gallons per minute'.

Volts are a measure of 'electromotive force', essentially the _force_ pushing electrons to move. In the water analogy, volts are equated with _pressure_.

Voltage tends to equalize in conductors, meaning that everything that is electrically connected together is at the same _voltage_...(note: this excludes resistance and magnetic effects; this is just an approximation).

When the 'hot' touches the metal cabinet, the _voltage_ of the metal cabinet is increased to that of the hot wire...but no _current_ is yet flowing. It is only when the circuit path from the hot cabinet to ground is completed that any current will flow. This path might be through a person, who would get shocked. But the point is that current doesn't flow without a circuit.

Ideally, no current will flow through the 'equipment grounding conductors'. (Or equipment bonding conductors, which IMHO is a better term; add me to the bandwagon..) The green wire is _connected_ to the neutral at your panel, so both are at the same _voltage_. But the green wire is not supposed to be part of the circuit, so no current flows through it. By connecting these two systems at a _single_ location, you make sure that no _circuit_ includes any of the ground wires, and you don't get current flow in them. The _voltage_ is the same, the current is not.

If the green (EGC, ground, whatever you want to call it) wire is connected to the white (neutral, return, whatever you want to call it) wire in _two_ locations, then you would have a 'parallel path', and current _could_ end up flowing in the EGC conductors.

When you have a 'hot to ground' fault, you get lots of current flow, possibly sparks, and the circuit breaker _should_ trip. When you have a 'neutral to ground' fault, then you get almost nothing in the way of excess current flow through your loads, and a portion of the current starts to flow in the ground wire system. A normal circuit breaker won't detect this, however a GFCI will detect this.

-Jon
 
  #14  
Old 05-14-04, 12:42 PM
DaveB.inVa
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A lot of your confusion is coming from the common misconception that "electricity tries to go to the earth/ground". This is simply not true. All electricity wants to do is get back to its source. In your case and most cases out there the source is the transformer... not the earth!

There are two different types of grounding that are commonly and quite dangerously mistaken for one another.

One is equipment grounding and the other is service grounding.

To start it is essential to have an idea about your electrical system. At the transformer there is basically a coil. From one end of the coil to the other you get 240v. There is also a centertap, just a midpoint of the winding. From the ends to this centertap you get 120v. The utility grounds this centertap by use of ground rods or a buttplate on the end of the pole. This centertap is also your neutral and the two ends of the coil are the hots.

At your house you bring in the 2 hots and the neutral. Here again at your mainpanel and at your main panel ONLY you are required to reground the neutral.

This is what you must know for all this to make sence.

Now first to service grounding. Its a little simpler and more direct to understand. Service grounding is bonding the neutral to the ground. It is done for 2 basic reasons and these basic reasons ONLY! #1. to stabilize the voltage to earth. This means if you bond one conductor to the earth by rods and such then the other two conductors (ie the hots) will have 120v each to the neutral and the earth. This is good because it gives you a reference for everything. If it wasnt reference capacitve coupling could drive the voltages up to a point to where insulation damage could occur. Now the second reason. #2. to dissipate lightning strikes and limit voltage due to contact from higher voltage lines. This is a no brainer. It gives the lightning somewhere to go and if a higher voltage line touches it it keeps everything around the same potential.

NOTE: The above section on service grounding did NOT say anything about tripping a breaker or saving your life. Why? The earth is not the greatest conductor. And without the neutral to carry back the fault current the earth in its self can not carry enough current to trip a breaker. This is why you cant drive a ground rod and connect your ground wire to it... unless it is bonded to the neutral back at the main service.

Equipment grounding. This isnt too hard either. You simply must bond the frames and such metallic stuff back to the neutral at the main service. This does two things. One, since your bonded to the earth at this point you can be pretty sure that the case on your drill will have just about the same voltage as the earth your standing on... an equipotential plane of sorts. No voltage gradients mean no shock and this is a good thing! But in case of a fault ie a hot touches the case the grounding conductor is there to safely carry this large current. It carries it back to the main service, where the ground is bonded to the neutral... and most of the current goes back to the transformer by way of the neutral (because the earth is a crappy conductor). All this happens fast and hopefully just as fast the breaker will trip. This is what you want to happen.

Both of these systems function together for good performance. You quite possibly could get by with out a ground rod. Your equipment would still be bonded to the neutral and the breaker would trip during a fault, but performance would suffer. The voltage on everything grounded to the neutral would rise in relation to the earth because the neutral isnt bonded to the earth! Its very important to have both functioning for your best interest.

The reason the neutral and ground cannot be bonded in about 99% of subpanels is simple as well. Remember that if you have current flowing on a wire that you have a voltage drop. Well at that subpanel if you were to wrongly ground your equipment to the neutral you will get a voltage drop due to the current flow on the neutral. That means that from the actual earth to this neutral that there will be a difference in potential. Youve just bonded all your equipment to that elevated voltage! That means all youre bonded equipment will have an elevated voltage in relation to the earth you are standing on. This elevated voltage is probably not going to be much. It all depends on the type of wire and amount of current flowing on it. Possibly on the order of a few volts. But you still dont want that to happen. So thats why you ground all your equipment to a grounding bar that is NOT bonded to the neutral. The independent grounding wire wont have any current flow over it. That means no voltage drop on it either and no difference in voltage between that ground bar and the earth your standing on. So when you ground your equipment to that bar you can be sure the frame of your refrigerator is at the same voltage as the earth its standing on!
 
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