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  1. #14
    Join Date
    Jan 2012
    Location
    Northern california
    Posts
    65
    If its a stinger leg transformer. You can still reverse any two legs to reverse motor on a three phase motor. Unless I'm missing something in the conversation.

  2. #15
    Join Date
    Jan 2012
    Location
    Syracuse, NY
    Posts
    13
    Ran into this two years ago on a Carrier rooftop (forgot exactly which model, remember serial being from late 80s) with electric heat. Building service was 208Y/120. RTU nameplate was 460V. RTU had a dedicated step up transformer for 480V service.

    First service call (no cooling, summer) the fuse block for the unit (not the electric heat) was completely toasted, absolutely fried. I still have it to this day to show people just how violent the arcing must have been. There was a non-fused disconnect at the RTU and a fused disconnect after the transformer in the electrical/mechanical room. Open up the disconnect after the transformer and the A and C phases were fused but the B phase was bypassing the disconnect by being attached to the ground lug of the disconnect. I think, "!!!!WHAT?!?" So I open up the transformer and the B phase lug has a 10GA green conductor ran straight to a grounding rod. After doing some research I learned that this was a corner grounded delta. Had to replace one of the two fuses in the disconnect but replaced both to be safe. AB, BC, and AC were all around 490V or so. A and C to ground were also 490V and B to ground was 0V. After understanding the corner ground this made sense. So, I replaced the fuse block and the fuses and the unit ran fine for the rest of the summer. We figured that the insulation of the fuse block must have broken down or something of that nature. There was no other explanation at the time.

    Second service call (too little heat, fall) I find the fuse block, contactor, and conductors of one of the two heaters equally as toasted as the units fuse block a few months prior. Again, after investigation, no explanation as to the cause. Replaced fuse block and contactor on both heaters, both heaters running fine. Do further research that night and find out that electrical distribution and control components utilized on corner ground systems can not be "slash rated" such as "120/240" "277/480" If it is a 480V system the components must have a solid "480V" rating. I believe this is because with a 480Y/277 system the potential from any terminal to ground will only be 277V but when you have a corner grounded system the potential from the A and C terminals to ground will be 480V. Luckily the contactors I had in the van were rated up to 600V and were even slightly over sized for the heater KW. I went back the next day and checked the compressor and evap fan contactors. The evap fan contactor had been recently replaced with a 480V contactor and both compressor contactors were rated up to 600V. Too bad I'll never know what the the original fuse blocks and contactors that burned up were rated for because they were completely toasted.

    Great learning lesson.

  3. #16
    Join Date
    Mar 2007
    Location
    wedged in freezer shelf
    Posts
    6,788
    Quote Originally Posted by VTP99 View Post
    A phase top
    B phase bottom left
    C phase bottom right

    Yes that B phase is grounded and will read zero volts to ground but she is still HOT.
    You'll never forget the first time
    I remember going WTF too
    “If You Can Dodge A Wrench You Can Dodge A Ball”

  4. #17
    Join Date
    Jan 2011
    Location
    Boston, MA
    Posts
    459
    so I don't have a "first time", how does one safely work with grounded B phase?

  5. #18
    Join Date
    Oct 2009
    Location
    God's country - Shenandoah Valley, VA
    Posts
    362

    Safety

    First,
    Remember that it's the same voltage to ground as it is between phases. If it's 480v between phases, then it's 480v to ground.

    Second,
    Treat the grounded phase like you would a "live" neutral that's carrying current: don't be the path that makes or breaks continuity of it.

    Third,
    As pointed out, all components must be rated for the actual voltage to ground.

    Fourth,
    No fusing in the grounded phase.

    There's probably more in the NEC, but it's too late to dig it up tonight.

    Buenas noches, amigos.

  6. #19
    Join Date
    Jul 2000
    Location
    Dallas,Texas
    Posts
    4,963
    This comes up from time to time. Here was the last discussion with some code references. http://hvac-talk.com/vbb/showthread....grounded+delta

  7. #20
    Join Date
    Mar 2005
    Posts
    664
    I called the power company out . and to be completely honest I still don`t completely understand it. Nuetral in a single phase makes perfect sense to me , ie negative on the sine wave yet still carrying current, this just blows my mind, there has been other really long and informative discussions here in the past, I have to put my hand up and say I am still not 100% on it stan
    Keep it simple to keep it cool!

  8. #21
    Join Date
    Nov 2006
    Location
    Southeastern Pa
    Posts
    18,618
    Quote Originally Posted by VTP99 View Post
    Yes i'm still here but that 3 phase motor is toast.
    If the voltages between the phases are correct, you should be able to change direction and have no toasting of the motor.

    Maybe there is a more detailed explanation of the sequence of events???
    [Avatar photo from a Florida training accident. Everyone walked away.]
    2 Tim 3:16-17

    RSES CMS, HVAC Electrical Specialist

    AOP Forum Rules:







  9. #22
    Join Date
    Jul 2000
    Location
    Dallas,Texas
    Posts
    4,963
    On "corner grounded delta", B phase is grounded. Si if you want to revese rotation you can swap A and C phase with no damage to the motor.

  10. #23
    Join Date
    Jul 2000
    Location
    Dallas,Texas
    Posts
    4,963
    Quote Originally Posted by stanbyyourword View Post
    I called the power company out . and to be completely honest I still don`t completely understand it. Nuetral in a single phase makes perfect sense to me , ie negative on the sine wave yet still carrying current, this just blows my mind, there has been other really long and informative discussions here in the past, I have to put my hand up and say I am still not 100% on it stan

    Perhaps some of your difficulty comes from the misconception of the neutral.
    It is not the negative of the sine wave. It is derived from a center tap of the winding on the transformer. A 240 volt winding tapped mid way provides 120 volts from tthe center tap to either end....240 volts to L1 and L2.

    Now if you ground that point in the system it will be at zero potential to ground and 120 volts to the ends of the winding or L1 and L2. If you don't ground the neutral it will drift and not be a stable voltage.

    Same principle with corner grounded delta. If you ground B phase, it will be at zero potential to ground or earth. But it will still be at 240 or 480 volts potential to the other phase conductors.

  11. #24
    Join Date
    Mar 2010
    Location
    Southeast Michigan
    Posts
    40
    Quote Originally Posted by wolfdog View Post
    On "corner grounded delta", B phase is grounded. Si if you want to revese rotation you can swap A and C phase with no damage to the motor.
    Why only A and C swap any two phases to change direction does not matter.

  12. #25
    Join Date
    Mar 2005
    Posts
    664
    Wolfdog thank you for reply ,right you are, just did a little refresher. A person could spend a lifetime studying electrical engineering and power distribution , whats even more amazing is how a technician grows technically in other areas, even electrically, some things get foggy with time over a 20 year period, good example. regards stan
    Keep it simple to keep it cool!

  13. #26
    Join Date
    Nov 2006
    Location
    Southeastern Pa
    Posts
    18,618
    Quote Originally Posted by wolfdog View Post
    On "corner grounded delta", B phase is grounded. Si if you want to revese rotation you can swap A and C phase with no damage to the motor.

    Unless the motor is grounded where it shouldn't be, a grounded B phase should behave just like the A and C phases.

    That's why I am asking.
    [Avatar photo from a Florida training accident. Everyone walked away.]
    2 Tim 3:16-17

    RSES CMS, HVAC Electrical Specialist

    AOP Forum Rules:







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