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Thread: Floating Neutral

  1. #1
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    Floating Neutral

    Today I happened to work on a circuit in a single phase panel fed from 25 kVA 480/240V XFMR. The voltage from neutral to ground was 11.2 volts. I suspect X0 has not been connected to building steel.

    Tomorrow, an outage is scheduled at 6:00 AM to shut down and install the GEC.

    My question is: What is the best way to explain to the customer why this needs to be corrected?

    I told the manager that, "This electrical system is abnormal. It has what we call a floating neutral. This can result in equipment damage and may be a hazard to personnel."

    My answer is generic. Is there anecdotal evidence to back it up?

    I think saying that it is a code violation is insufficient or even immaterial (to a manager). What have you out there said to explain the importance of retrofitting the GEC?

  2. #2
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    You could see a system bonding jumper installed but no grounding electrode system or grounding electrode conductor?

  3. #3
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    In an extreme condition, while the line-to-line and line-to-neutral voltages will remain consistent, the overall voltages to ground could become almost anything (say, from a primary-to-secondary fault) and possibly exceed equipment and insulation ratings.

    You might also bring up the fact that the NEC requires the bond.
    Code references based on 2005 NEC
    Larry B. Fine
    Master Electrician
    Electrical Contractor
    Richmond, VA

  4. #4
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    Quote Originally Posted by chris kennedy View Post
    You could see a system bonding jumper installed but no grounding electrode system or grounding electrode conductor?
    I did not open the XFMR, but my belief is that neither the SBJ or GEC has been installed.

    Quote Originally Posted by LarryFine View Post
    In an extreme condition, while the line-to-line and line-to-neutral voltages will remain consistent, the overall voltages to ground could become almost anything (say, from a primary-to-secondary fault) and possibly exceed equipment and insulation ratings.

    You might also bring up the fact that the NEC requires the bond.
    Good thought, Larry. I did not conceive of consequences of fault conditions. Thank you.

  5. #5
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    At a division of GM where I used to work a unistrut grid was constructed to support utilities and a suspended ceiling for a new cafeteria/restaurant. The person who wired the transformer neglected to bond the transformer to building steel. The result was the entire grid was giving people shocks!

  6. #6
    Quote Originally Posted by jeremysterling View Post
    Today I happened to work on a circuit in a single phase panel fed from 25 kVA 480/240V XFMR. The voltage from neutral to ground was 11.2 volts. I suspect X0 has not been connected to building steel.
    First off, this voltage reading alone does not indicate a lack of bond. How far away from the transformer was your voltage reading taken? What is the amperage in the neutral? What is the amperage in the Ground (probably not zero)?

    Even though we don't normally consider conductors to be resistive, they are, and you will have some voltage drop across the neutral between the point of bonding and the point of load. Because the ground has very little amperage, it won't have this voltage drop for the same distance, and that is why you will have a voltage difference between the two conductors.
    Quote Originally Posted by jeremysterling View Post
    I told the manager that, "This electrical system is abnormal. It has what we call a floating neutral.
    It is not correct to call this a floating neutral (which is much more serious). A floating neutral is a break in the neutral between the source and load.

  7. #7
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    Quote Originally Posted by Rick Christopherson View Post
    ... It is not correct to call this a floating neutral (which is much more serious). A floating neutral is a break in the neutral between the source and load.
    I don't agree. I have never heard the term "floating neutral" used to describe a break in the neutral. That would be an "open" neutral and a "floating" neutral is one where there is no system bonding jumper.
    Don, Illinois
    "It is the first responsibility of every citizen to question authority." B Franklin

  8. #8
    Quote Originally Posted by don_resqcapt19 View Post
    I don't agree. I have never heard the term "floating neutral" used to describe a break in the neutral. That would be an "open" neutral and a "floating" neutral is one where there is no system bonding jumper.
    No, a missing bond causes the whole system to float. The difference in voltage between ground and neutral will also be the same offset voltage for the phases too. Everything floats at the same amount. For example, if your gnd-neut voltage is 20 volts, then your gnd-A voltage will be 140V and gnd-B will be 100V. However, Neut-A and Neut-B will still remain 120V

    A true floating neutral has no immediate tie to the rest of the system (except via the loads), and as such, its voltage relative to the phases will vary, depending on the load between the phases and the neutral. In this case, your neut-A will be 100V and Neut-B will be 140V. (But your gnd-A and gnd-B will likely remain at 120V because the bond is still present upstream from the break in the neutral.)

    An open neutral is a different condition, and doesn't become a floating neutral unless there are multiple loads (on opposing phases) sharing the same neutral, but still no return path back to the system, except via the loads through the opposing phases.
    Last edited by Rick Christopherson; 10-14-10 at 03:03 PM.

  9. #9
    I forgot that I have a diagram showing a simple a floating (unbonded) system. If I were to redraw this a a floating Neutral, the L! and L2 would remain locked to Ground, but the Neutral would be the "Float".

    Oops, it is not clear. This is a Floating System, not a Floating Neutral.

    Attached Images Attached Images  

  10. #10
    This is what a floating neutral would look like


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