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Electrical Grounding Question - Detatched Garage

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  • #16
    Re: Electrical Grounding Question - Detatched Garage

    woussko. on your test you said to drive two ground rods in a straight line. how else can that be done? not sayin nothin now just askin, just askin. lol. breid

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    • #17
      Re: Electrical Grounding Question - Detatched Garage

      breid1903

      Sorry I didn't see your post until now. To answer the question I don't think you can do it any other way. I just put that in my post.

      By the way I did that test and in my case I had ground rods at the 4 corners of my property. I did find some stray current flow, but not enough to worry about it. When I tried reading resistance from ground rods about 75 feet apart that got me to thinking it would be a good idea to actually drive several close together and connect them.

      I wish I could find a way to drive 50 feet of 4" iron well casing and use that as a good ground here. There's no way to setup the equipment and local laws ban such stuff.

      Comment


      • #18
        Re: Electrical Grounding Question - Detatched Garage

        NEC 2005

        250.4(A)(5) Effective Ground-Fault Current Path.

        Electrical equipment and wiring and other electrically conductive material
        likely to become energized ....ya-da, ya-da, ya-da.... It shall be capable
        of safely carrying the maximum groundfault 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
        considered as an effective ground-fault current path.



        Now isn't (for purposes of an individual service) the "source" the meter
        socket? And if yes, then doesn't the ground need to be carried back to
        the ground point at the service entrance?
        "When we build let us think we build forever. Let it not be for present delight nor for present use alone. Let it be such work that our descendants will thank us for, and let us think, as we lay stone upon stone, that a time is to come when these stones will be held sacred because our hands have touched them, and that men will say, as they look upon the labor and wrought substance of them, "See! This our fathers did for us."
        John Ruskin (1819 - 1900)

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        • #19
          Re: Electrical Grounding Question - Detatched Garage

          I'm not an electrician but I have an NEC. Seems pretty clear that for a detached building like this a separate ground rod is required per code, so I'm not quite getting the basis for the original debate. I would think that if you don't have the rod, you don't pass. My understanding is that the rod is really there mostly to provide a lightning path. Any ground fault in a branch is going to go back to the neutral line in the panel, anyway, no?

          As mentioned, in NEC '05 you bring 3 wires from the main panel. So I have always assumed that you have to connect the safety ground bus to the neutral at the subpanel. Seems like this is the only way to have a ground fault current path back to the service. The earth ground won't do it, and isn't allowed as pointed out by Bob D. Yes? No?

          But, per earlier posts, in NEC '08 you have to bring the safety ground over from the main panel as well. Under this code, do you connect your new ground rod and the safety ground bus to the neutral bus at the subpanel? I would think so. But the other side of the coin is, I was taught that you only connect safety ground to neutral at the main service entrance. Now I'm not really sure what good that fourth wire does, unless it's a "belt-and-suspenders" thing. It does seem goofy that you would connect the safety grounds together for two separate buildings. I'm not seeing the benefit there at all. Hopefully someone can clear this up for me?

          Finally, what about water service? In the original post, it stated "no plumbing". But what if you did have water service in the outbuilding that's tapped off the main house water supply? That one is bonded to the safety ground bus in the main panel. Must you/should you ground the water service in the outbuilding? To the fourth wire from the main panel or to the subpanel safety ground bus? I would think you would bond the safety ground from the outbuilding panel to the water service - but not sure.

          Thanks,

          -Andy
          Last edited by Andy_M; 08-01-2009, 07:04 PM.

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          • #20
            Re: Electrical Grounding Question - Detatched Garage

            Originally posted by JamesE View Post
            The ground rod must be driven at the sub under '05 code and '08 will require the addition of a ground pulled from the main. The ground rod is in fact for grounding the sub at the garage. The addition of the ground wire required for '08 compliance will ensure that all grounds of the system are at the same potential. There is no current flow through any ground anywhere at any time. It's a safety ground we require in this country. Many other countries do not require this. Current flows from leg to leg or from leg to nuetral (or vise versa depending upon electron flow or hole flow theory). Not through the ground.
            This is sort of correct but for the wrong reasons.

            A ground rod DOES NOT "ground" anything in the sense of circuit grounding.

            A 3-wire feeder was allowed in previous versions of the NEC, it was not mandatory or the only way to install a feeder. It was allowed under very specific circumstances. A 4-wire feeder was always allowed.
            See 250.32 for details.

            A grounding electrode was also always required for any detached structure served by a feeder. If only a circuit is/was run a grounding electrode was not required.

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            • #21
              Re: Electrical Grounding Question - Detatched Garage

              For me, with what I have experienced and learned, the more grounds the better

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              • #22
                Re: Electrical Grounding Question - Detatched Garage

                Silly layman's question...

                If the sub-panel at the garage is fed from the main panel but grounded seperately, isn't that setting up conditions for ground loops?

                Wouldn't the separate rod at the garage need to be bonded to the main bus?

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                • #23
                  Re: Electrical Grounding Question - Detatched Garage

                  A ground rod at a detached structure IS bonded to the main service through the feeder serving the structure.

                  What would be non-compliant would be a 3-wire feeder; two hots and a neutral, and a ground rod, with the grounds and neutrals kept separate.
                  The ground rod would be serving little to no purpose in a case like this.
                  A ground rod does NOT provide a ground as in a circuit or feeder ground.

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                  • #24
                    Re: Electrical Grounding Question - Detatched Garage

                    >> "If the sub-panel at the garage is fed from the main panel but grounded seperately, isn't that setting up conditions for ground loops?"

                    When I think of ground loops, I think of electronics. Those are actual circuit grounds that carry current.

                    What we're talking about in terms of the fourth - green or bare - wire here is a safety ground. It carries none of the circuit current, unless there is a ground fault. In our power system, current flows in the red and black wires (for 240VAC), black and white (120VAC) or red and white (also 120VAC). You don't even need the safety ground for system operation. It only comes into play in case a hot wire, say, comes loose and puts voltage on something that a person could touch. If the safety ground is connected to that now-hot thing, be it a metal cover plate or metal part of an appliance or whatever, the safety ground provides a low-resistance path back to the service panel, where it is conected to the white neutral. The "fault current" is then more likely to go through the safety ground back to the service than it is to go through you. You live to fight another day.

                    The red, black and white are three taps off the power company transformer. The red and black are the end taps and are 240 volts apart. The white is the center tap, and is thus 120 volts from either red or black. But these are transformer taps, their absolute potential is not necessarily referenced to anything. It might be and often is by the utility company, but doesn't have to be. Hence the ground rod, which is tied to the white wire. In my view the most important function for the ground rod is that it provides a path for lightning. Better for any lightning hitting your house to find the ground rod than to run around the house looking for a path to ground. That could be very disconcerting.

                    If you disconnected the rod, all your electric gizmos would still work, because, as previously mentioned, the whole safety ground system doesn't carry any system current during normal operation. You can now also see why it's important to wire an outlet in the correct polarity. AC generally doesn't care about polarity. But the fault current path created by the safety ground system for devices plugged into grounded outlets will only work as intended if the polarity of the outlet is connected properly. Again, it only matters when there is a ground fault.

                    If you have more than one ground rod, it is likely that you will measure a small potential between them. This is the closet thing to your 'ground loop' question. But it doesn't matter. At most it just changes the reference point a bit... the circuit (if its working ok) just operates on the three wires, red, black and white. Any currents that flow in the safety ground wire between rods will be quite small since the earth is a pretty poor power supply. Too bad about that... if it wasn't the case we could connect up some ground rods and solve the nation's energy shortage.

                    At least that's my understanding of how it works.
                    Last edited by Andy_M; 08-04-2009, 11:30 PM.

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                    • #25
                      Re: Electrical Grounding Question - Detatched Garage

                      Originally posted by Andy_M View Post
                      >> "If the sub-panel at the garage is fed from the main panel but grounded seperately, isn't that setting up conditions for ground loops?"

                      When I think of ground loops, I think of electronics. Those are actual circuit grounds that carry current.

                      What we're talking about in terms of the fourth - green or bare - wire here is a safety ground. It carries none of the circuit current, unless there is a ground fault. In our power system, current flows in the red and black wires (for 240VAC), black and white (120VAC) or red and white (also 120VAC). You don't even need the safety ground for system operation. It only comes into play in case a hot wire, say, comes loose and puts voltage on something that a person could touch. If the safety ground is connected to that now-hot thing, be it a metal cover plate or metal part of an appliance or whatever, the safety ground provides a low-resistance path back to the service panel, where it is conected to the white neutral. The "fault current" is then more likely to go through the safety ground back to the service than it is to go through you. You live to fight another day.

                      The red, black and white are three taps off the power company transformer. The red and black are the end taps and are 240 volts apart. The white is the center tap, and is thus 120 volts from either red or black. But these are transformer taps, their absolute potential is not necessarily referenced to anything. It might be and often is by the utility company, but doesn't have to be. Hence the ground rod, which is tied to the white wire. In my view the most important function for the ground rod is that it provides a path for lightning. Better for any lightning hitting your house to find the ground rod than to run around the house looking for a path to ground. That could be very disconcerting.

                      If you disconnected the rod, all your electric gizmos would still work, because, as previously mentioned, the whole safety ground system doesn't carry any system current during normal operation. You can now also see why it's important to wire an outlet in the correct polarity. AC generally doesn't care about polarity. But the fault current path created by the safety ground system for devices plugged into grounded outlets will only work as intended if the polarity of the outlet is connected properly. Again, it only matters when there is a ground fault.

                      If you have more than one ground rod, it is likely that you will measure a small potential between them. This is the closet thing to your 'ground loop' question. But it doesn't matter. At most it just changes the reference point a bit... the circuit (if its working ok) just operates on the three wires, red, black and white. Any currents that flow in the safety ground wire between rods will be quite small since the earth is a pretty poor power supply. Too bad about that... if it wasn't the case we could connect up some ground rods and solve the nation's energy shortage.

                      At least that's my understanding of how it works.
                      Andy,

                      I was actually thinking about electronic gremlins like video and audio interference, "ghost" channel changes, wireless internet problems, garage doors opening spontaneously, etc. I realize that's farfetched, hence silly layman question.

                      Now, if you lift the main ground then you are creating conditions for another kind of ground loop, yes? The kind where a short in one appliance could create a potential in the casing of anything else grounded on that circuit...

                      Comment


                      • #26
                        Re: Electrical Grounding Question - Detatched Garage

                        If what you mean by "lift the main ground" is to break the path from the safety ground to the neutral (at the service panel) then yes, you could end up with just what you describe. A major (THE major, I think) point of the entire safety ground system is to provide a low-resistance path for the fault current to get back to the service neutral. If you have the branch circuit safety grounds all tied together at the ground bus but don't go back to the neutral, you've created a hazard rather than a safety system, just as you said. So yeah, as you recognized, you don't want to defeat the safety ground path back to service neutral. Sort of similar logic says you don't want to connect safety ground to neutral anywhere except back at the service entrance.

                        On the other hand, if by "lift the main ground" you mean disconnect the bare (or green) wire that goes to the ground rod, but you still have the ground bus connected to the service neutral, you will still have the path for the fault current. But you no longer necessarily have earth reference. In reality I strongly suspect that most utility companies tie their transformer to earth before you see it at your service entrance. The biggest consequence of a disconnected ground rod is I think, that you don't have a good path for lightning to get to earth. Of course, the utility company doesn't want the lightning getting into their transformer and maybe taking down every building on that transformer...

                        As for problems like cable TV interference, I know the cable TV people like to tie their coax shield to your home's safety ground. I can't resist playing with it and I've never seen it make a lick of difference in terms of the quality of reception, but then again my personal experience isn't much of a sample size. I do know from experience for sure that noisy grounds are sometimes very subtle. I think that if you're getting poor reception and the earthing helps, it's probably because there is some noise getting on the coax shield from the environement. I don't think that multiple ground rods are likely to create a problem, but that's just an opinion and sure doesn't mean that it couldn't happen.

                        I think that wireless internet problems, spontaneous things happening with garage doors, etc.... well, I've learned the nothing's impossible... but those things are let's say, highly improbable. Most of your electronics are actually pretty well isolated from the power line, as it all is run off of power supplies (or wall transformers) that do a decent job of isolating the DC that the circuits use from the AC line. Big spikes and surges can get through (hence the need for surge protection) but little bits of power line noise are accounted for and taken care of in the power supply design and EMI filtering. Sometimes you hear stories about crazy things happening like garage doors flapping away on their own etc., that get attributed to things like we're discussing. But almost always, in my experience, there's another, less exotic but far more reasonable cause, recognized as such or not.

                        Comment


                        • #27
                          Re: Electrical Grounding Question - Detatched Garage

                          Originally posted by Andy_M View Post
                          If what you mean by "lift the main ground" is to break the path from the safety ground to the neutral (at the service panel) then yes, you could end up with just what you describe. A major (THE major, I think) point of the entire safety ground system is to provide a low-resistance path for the fault current to get back to the service neutral. If you have the branch circuit safety grounds all tied together at the ground bus but don't go back to the neutral, you've created a hazard rather than a safety system, just as you said. So yeah, as you recognized, you don't want to defeat the safety ground path back to service neutral. Sort of similar logic says you don't want to connect safety ground to neutral anywhere except back at the service entrance.

                          On the other hand, if by "lift the main ground" you mean disconnect the bare (or green) wire that goes to the ground rod, but you still have the ground bus connected to the service neutral, you will still have the path for the fault current. But you no longer necessarily have earth reference. In reality I strongly suspect that most utility companies tie their transformer to earth before you see it at your service entrance. The biggest consequence of a disconnected ground rod is I think, that you don't have a good path for lightning to get to earth. Of course, the utility company doesn't want the lightning getting into their transformer and maybe taking down every building on that transformer...

                          As for problems like cable TV interference, I know the cable TV people like to tie their coax shield to your home's safety ground. I can't resist playing with it and I've never seen it make a lick of difference in terms of the quality of reception, but then again my personal experience isn't much of a sample size. I do know from experience for sure that noisy grounds are sometimes very subtle. I think that if you're getting poor reception and the earthing helps, it's probably because there is some noise getting on the coax shield from the environement. I don't think that multiple ground rods are likely to create a problem, but that's just an opinion and sure doesn't mean that it couldn't happen.

                          I think that wireless internet problems, spontaneous things happening with garage doors, etc.... well, I've learned the nothing's impossible... but those things are let's say, highly improbable. Most of your electronics are actually pretty well isolated from the power line, as it all is run off of power supplies (or wall transformers) that do a decent job of isolating the DC that the circuits use from the AC line. Big spikes and surges can get through (hence the need for surge protection) but little bits of power line noise are accounted for and taken care of in the power supply design and EMI filtering. Sometimes you hear stories about crazy things happening like garage doors flapping away on their own etc., that get attributed to things like we're discussing. But almost always, in my experience, there's another, less exotic but far more reasonable cause, recognized as such or not.
                          Question,

                          Recently, while helping a friend install some can lights in his new fixer-upper, I checked his outlets with a simple circuit analyzer. He appeared to have grounded power throughout the house. While in the attic I noticed one of the runs (that was clearly identifiable as the feed for an outlet that had just tested "correct") was old two-wire romex with the black sheathing. Hmm... When I got back downstairs I pulled the outlet cover and saw that someone had bootlegged the ground by running a jumper between the ground screw and the neutral screw. The story was the same all around the house. My problem was that beyond telling my friend that he actually had wiring (I assume) designed to fool an inspector I was unable to precisely enumerate why this is bad.

                          Little help...

                          Comment


                          • #28
                            Re: Electrical Grounding Question - Detatched Garage

                            Grounding and neutrals are misunderstood by a great many, including many electrical inspectors. A neutral (with the white conductor) is a current carrying conductor. It is not ever to be used as a ground. A ground wire (bare with no insulation on it) is to only be used as a grounding conductor. It is never to be swapped in and used as a current carrying conductor. It is, in essence, a safety valve so that if there is a short in the wiring, it has a safe way to carry that current to ground. The neutral and the ground bar are bonded together in the main panel -- NOT any other place in the system. What your friend has is clear grounding violation which should be corrected.
                            Cheers,
                            Jim Don

                            Comment


                            • #29
                              Re: Electrical Grounding Question - Detatched Garage

                              Originally posted by JimDon View Post
                              Grounding and neutrals are misunderstood by a great many, including many electrical inspectors. A neutral (with the white conductor) is a current carrying conductor. It is not ever to be used as a ground. A ground wire (bare with no insulation on it) is to only be used as a grounding conductor. It is never to be swapped in and used as a current carrying conductor. It is, in essence, a safety valve so that if there is a short in the wiring, it has a safe way to carry that current to ground. The neutral and the ground bar are bonded together in the main panel -- NOT any other place in the system. What your friend has is clear grounding violation which should be corrected.
                              Cheers,
                              Jim Don
                              I completely agree that this needs to be corrected. If it were my house I would disconnect the jumper from every outlet and replace as many breakers as possible with GFCI breakers (using GFCI outlets otherwise) and mark them all as "no equipment ground". Since this is not my house I told him he needed a pro opinion but I couldn't explain exactly why the condition was bad.

                              I understand that neutral is neutral, ground is ground and never the two shall meet between the main and the lightbulb, but what I'm looking for is an explanation of what the specific danger in that condition is.

                              In other words, if you connect the ground terminal on an outlet to the neutral terminal for whatever reason - how does that damage human property or human tissue? Is it a matter of possibly electrifying the casing of an appliance plugged into that outlet? Anything else?

                              Comment


                              • #30
                                Re: Electrical Grounding Question - Detached Garage

                                (this is my understanding of the situation),

                                On a grounded tool with a metal case, then it is electrified to the same potential as the neutral current, IN other words voltage is passing to the case,

                                you say well the ground is at the same potential as the neutral any way so there is in a sense the same as the bird setting on the wire effect, yes that is true in a perfect world, there should be not a problem. If both are equal.

                                but what happens if the neutral some were has a corroded or lose connection and there is resistance in the line, your hanging on to the metal case and you come in contact with a solid ground, (with less resistance than the wire) say a copper water pipe,

                                then you may become the current carrying conductor, as the current then passes through the case of the tool, then you, getting to a path of less resistance.

                                the Idea of the ground is to remove you from becoming any form of a current carrying conductor,

                                with a ground on the metal case of the tool, it is like putting a shield around the current carrying conductors, and giving a path of current flow if any thing ever goes wrong, and a place to flow besides the operator if some how current is transfered to the case,

                                in the case of the neutral and the ground (earth) is hooked together you just energized the case of the tool, but if some thing happens to the neutral you can have a lot of electrical power flowing into that case, (If you lose your neutral on a 220 volt system,
                                you could have up to 220 volts flowing in that neutral wire (depending on how unbalanced the system is), as the power tries to balance it self by flow into the neutral and to the other side of the transformer leg).

                                and having a poor neutral connection in the panel or on the pole is not that uncommon.

                                the simple of it is you just connected your self to a current carrying conductor, by bonding them at the plug. (and your safety then depends nearly entirely on the quality of the neutral wire and it connections)
                                with a true ground (wire) system you have put in a safety back up system (in the event any current does make it to the grounded case), a free low resistance path of flow to reduce the possibility of you becoming the path of flow.

                                (also as talked about earlier in other posts, the ground (earth) is not nessarly the same
                                potential at all locations, as if you take two ground rods and drive them some distance apart you may find there is a potential difference between them, so the bird on the wire effect may not be total applicable ).
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