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  • #31
    Re: Electrical Grounding Question - Detached Garage

    Originally posted by BHD View Post
    (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 ).
    Thanks. That's what I was looking for. Anyone think of anything else?
    Last edited by jimboburnsy; 08-06-2009, 02:37 PM.

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    • #32
      Re: Electrical Grounding Question - Detatched Garage

      I agree with BHD's comments. It's not a perfect world, and the point of the equipment ground is to be as safe as possible in terms of handling fault conditions. So you have to consider loose/corroded connections just as he's described. In a perfect world a ground fault trips a breaker within milliseconds. Reality says that your fault itself may be a high resistance connection and never trip the breaker. It only takes 30-40 volts and relatively low current to create a very dangerous or fatal shock hazard.

      It's also true that the whole safety ground/equipment ground scheme was introduced in the mid to late 1950's and has evolved since then. It's still evolving. There's grounding and bonding differences in the requirements between the latest few revisions of the NEC and many local codes. So it's far from settled. I'm no expert on the practical vagarities of the codes, and wouldn't even be surprised if your friend's wiring was legal or possibly normal practice at some point in he past. I'm sure others here would know about this.

      But like you, if it was my house and pulling new wire was too hard/impractical, I would put GFCI's. Then later, when in the walls for other reasons (bound to happen, just a matter of time), pull the new romex. Even GFCIs aren't perfect... they really do go bad and the homeowner has to make a point of pushing the test button. I have to admit that I don't push that button very often. Of course I also don't test my water heater T&P very often or look to see if water is coming out of the pipe. Safety stuff that requires the homeowner to actually do something isn't really the best IMHO.

      The only thing I don't buy in BHD's excellent post is the idea that two ground rods will have much effect on the bird. Earth is high resistance. There's no danger from multiple ground rods.

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      • #33
        Re: Electrical Grounding Question - Detatched Garage

        Originally posted by Andy_M View Post
        I agree with BHD's comments. It's not a perfect world, and the point of the equipment ground is to be as safe as possible in terms of handling fault conditions. So you have to consider loose/corroded connections just as he's described. In a perfect world a ground fault trips a breaker within milliseconds. Reality says that your fault itself may be a high resistance connection and never trip the breaker. It only takes 30-40 volts and relatively low current to create a very dangerous or fatal shock hazard.

        It's also true that the whole safety ground/equipment ground scheme was introduced in the mid to late 1950's and has evolved since then. It's still evolving. There's grounding and bonding differences in the requirements between the latest few revisions of the NEC and many local codes. So it's far from settled. I'm no expert on the practical vagarities of the codes, and wouldn't even be surprised if your friend's wiring was legal or possibly normal practice at some point in he past. I'm sure others here would know about this.

        But like you, if it was my house and pulling new wire was too hard/impractical, I would put GFCI's. Then later, when in the walls for other reasons (bound to happen, just a matter of time), pull the new romex. Even GFCIs aren't perfect... they really do go bad and the homeowner has to make a point of pushing the test button. I have to admit that I don't push that button very often. Of course I also don't test my water heater T&P very often or look to see if water is coming out of the pipe. Safety stuff that requires the homeowner to actually do something isn't really the best IMHO.

        The only thing I don't buy in BHD's excellent post is the idea that two ground rods will have much effect on the bird. Earth is high resistance. There's no danger from multiple ground rods.
        Just FYI, here's a depiction of the "bootlegged ground" that I found at my friend's house. This is not from his house but the method is identical.



        I'm not great about testing the GFI's either. An electrician that I used in the past had a circuit tester with an LED lamp that drew 5 mA. That thing was great. I would probably test more often if I had one.
        Last edited by jimboburnsy; 08-06-2009, 02:55 PM.

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        • #34
          Re: Electrical Grounding Question - Detached Garage

          The only thing I don't buy in BHD's excellent post is the idea that two ground rods will have much effect on the bird. Earth is high resistance. There's no danger from multiple ground rods.
          I was not saying that there would be any danger from multiple ground rods, (as what they do is reduce the Resistance of the ground to earth).
          and in a 4 wire system they help to equal out the ground plane

          but what I was saying (depending on the stray voltage in the ground) the ground is not nessarly all at one potential thus one could find current between ground rods that are some distance apart,

          and depending on where you are and connected to a neutral and where your grounding out at you could find a voltage potential difference between them, and it could be enough to feel the current.

          (it is the same reason why they put in a equipotential plane in/under dairy barns and swimming pools)

          (going back up to my original example of a tool with a metal case, if some one has a large property and the neutral and ground at the first point of disconnect is made, that is where the neutral and the ground is of equal potential, now say you have a circuit that is on the opposite side of the lot, and your working in your (say your) shed, and you have the ground and the neutral connected at the plug as this discussion is referring to, and your shed has concrete floor with a pipe embedded in it for a vice stand, there could be enough "stray voltage" to feel a shock from the case of the tool to the vice, in this illustration, (I am not saying one nessarly would but there is that possibility),

          this is also why the ground is all from that one point as to keep the potential difference in the ground equal, and is not being referenced from different points,

          thus the statment of the bird and the wire, (yes there both conected at one point but one location some distance away may not hold the same potential thus landing on that section of wire (earth) for the bird may not nessarly be free of shock hazard),

          ~~~~~~~~~~~~~~~~~~~~~~~~~~

          In the case of the detached garage, it becomes a unit in it self,

          Now if there is any other metal that connects the two buildings (water pipes, phone, TV, etc) the system is to be treated as sub panel type set up, with the neutral and the ground keep separate from the original place of disconnect, (or that is my understanding),

          this video has some good info on stray voltage, http://www.mikeholt.com/strayVoltageVideo.php
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