Page 1 of 5 123 ... LastLast
Results 1 to 10 of 45

Thread: 3 phase wiring question

  1. #1
    Join Date
    Oct 2008
    Location
    Cincinnati, OH
    Posts
    57

    3 phase wiring question

    I have worked with single phase and 3 phase 4 wire but not that much with 3 phase 3 wire 240 volt. I was working at a warehouse with 3 phase 3 wire 240volt the other day. They called me out because the maintenance man had changed a 1000w HPS ballast in one of the outside fixtures and he could not get it to work. He said he was getting strange voltages from the 240volt time clock that was feeding the fixture.

    Now I have seen a 3 phase wild leg where the voltage to ground is 0 volts but on this one I was getting - 10 volt - 185 volt - 234 volts - to ground on each leg but getting 243 volts between each leg. I checked back at the main and got the same voltage. Checked a 3 phase compressor disconnect and got the same voltage. So now I'm scratching my head. I turned off the double pole breaker, in the 3 phase panel, feeding the outside light. The voltage changes through out the building - 85 volts - 156 volts - 122 volts - on each leg to ground.

    Now I'm thinking I have to stop listening to this maintenance man about this voltage thing and find the problem with the outside light HE FIXED. I take it apart and he's got the ground from the building connected to the ballast wire that is clearly marked lamp. One hot connected to the 240 volt marked feed for the ballast which it right. The common which you would hook up the other side of the 240 Volt is hooked to the lamp wire. And the other leg of the 240 volt is capped off and not used.

    To make a long story even longer, I hooked everything up right on the ballast and the light works. I just don't understand why having this ballast hooked up wrong would change the voltage through the whole building.

  2. #2
    Quote Originally Posted by delfadelfa View Post
    I have worked with single phase and 3 phase 4 wire but not that much with 3 phase 3 wire 240 volt. I was working at a warehouse with 3 phase 3 wire 240volt the other day. They called me out because the maintenance man had changed a 1000w HPS ballast in one of the outside fixtures and he could not get it to work. He said he was getting strange voltages from the 240volt time clock that was feeding the fixture.

    Now I have seen a 3 phase wild leg where the voltage to ground is 0 volts but on this one I was getting - 10 volt - 185 volt - 234 volts - to ground on each leg but getting 243 volts between each leg. I checked back at the main and got the same voltage. Checked a 3 phase compressor disconnect and got the same voltage. So now I'm scratching my head. I turned off the double pole breaker, in the 3 phase panel, feeding the outside light. The voltage changes through out the building - 85 volts - 156 volts - 122 volts - on each leg to ground.

    Now I'm thinking I have to stop listening to this maintenance man about this voltage thing and find the problem with the outside light HE FIXED. I take it apart and he's got the ground from the building connected to the ballast wire that is clearly marked lamp. One hot connected to the 240 volt marked feed for the ballast which it right. The common which you would hook up the other side of the 240 Volt is hooked to the lamp wire. And the other leg of the 240 volt is capped off and not used.

    To make a long story even longer, I hooked everything up right on the ballast and the light works. I just don't understand why having this ballast hooked up wrong would change the voltage through the whole building.


    He had power flowing through the lighting transformer, onto the grounding conductor, which was throwing off your reference point for your meter
    It depends on what the meaning of "is" is.

  3. #3
    Join Date
    Jun 2003
    Location
    NE (9.1 miles @5.07 Degrees from Winged Horses)
    Posts
    5,818
    You most likely were working with a 240 volt ungrounded system. The odd voltages to ground occured when he grounded one of the legs via the transformer on the ballast. An ungrounded delta will have odd voltages to ground even without the problem you encountered.

    A three phase wild leg will have a voltage of 208 to ground on one leg and 120 volt to ground on the other two.

    A grounded delta will have 240v to ground on two legs and 0v to ground on the third.
    Tom
    TBLO

  4. #4
    Join Date
    Aug 2007
    Posts
    2,754
    if phase to phase voltage is ok (which it seems to be), and you are getting strange voltages to ground, then you probably have no ground (you should check the grounded conductor, apparently the main bonding jumper is missing or loose), if the system has a ground detection system, then it is intentional.

  5. #5
    Join Date
    Oct 2008
    Location
    Cincinnati, OH
    Posts
    57
    Quote Originally Posted by nakulak View Post
    if phase to phase voltage is ok (which it seems to be), and you are getting strange voltages to ground, then you probably have no ground (you should check the grounded conductor, apparently the main bonding jumper is missing or loose), if the system has a ground detection system, then it is intentional.
    Phase to phase was constant 243 volts. This system did not have a grounded conductor, only a grounding conductor, which is the conduit.

  6. #6
    Join Date
    Jun 2003
    Location
    NE (9.1 miles @5.07 Degrees from Winged Horses)
    Posts
    5,818
    Your system is not grounded. Meaning at the transformer, or first disconnect one of the phases is not intentionaly grounded. For a Wye system the center point is intentionaly grounded. Delta systems may or may not have any leg grounded.

    Just because you have a conduit does not mean the system is grounded.
    Tom
    TBLO

  7. #7
    Join Date
    Oct 2008
    Location
    Cincinnati, OH
    Posts
    57
    Quote Originally Posted by ptonsparky View Post
    Your system is not grounded. Meaning at the transformer, or first disconnect one of the phases is not intentionaly grounded. For a Wye system the center point is intentionaly grounded. Delta systems may or may not have any leg grounded.

    Just because you have a conduit does not mean the system is grounded.
    I see what your saying, all the metal in the building is bonded to ground but the transformed itself is not bonded to ground so when I am testing phase to ground there is no path back to the transformer. Am I thinking about this right?

  8. #8
    Join Date
    Jun 2006
    Location
    Ohio
    Posts
    12,817
    Quote Originally Posted by delfadelfa View Post
    I see what your saying, all the metal in the building is bonded to ground but the transformed itself is not bonded to ground so when I am testing phase to ground there is no path back to the transformer. Am I thinking about this right?
    Yes, you are thinking correct. Does the system have a ground detector? It should. Either that, or the system is grounded but lost it somehow...
    I'll never get there. No matter where I go, I'm always here.

  9. #9
    Join Date
    Dec 2007
    Location
    NE Nebraska
    Posts
    18,599
    Quote Originally Posted by Smart $ View Post
    Yes, you are thinking correct. Does the system have a ground detector? It should. Either that, or the system is grounded but lost it somehow...
    As I was reading through posts this was on my mind. OP may have a corner grounded system and has lost the grounded phase someplace.

  10. #10
    Join Date
    Jun 2006
    Location
    Ohio
    Posts
    12,817
    Quote Originally Posted by kwired View Post
    As I was reading through posts this was on my mind. OP may have a corner grounded system and has lost the grounded phase someplace.
    ...lost the grounded phase? Not quite the same as lost the [corner] phase ground. You may have meant this, but I felt compelled to clarify. Having ~240V between all three line-to-line combinations is an indication no "phase" has been lost.
    I'll never get there. No matter where I go, I'm always here.

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •