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  1. #1
    Join Date
    Jan 2012
    Location
    Fresno, ca
    Posts
    43

    1 phase 460v cond. motor?

    Today I was zapped bt the chassis of a condensing unit. I changed a cc heater on this unit because it shorted out in a puddle of water. I energized this unit after installing the new cc heater and the cond fan grill zapped me. I removed the 24v control wires, so the unit was not running. I put one meter lead on the roof flashing and the other lead on the cond motor grille, the meter showed 260v. I shut down power and removed the cond motor leads. I rechecked for chassis voltage and found none. I ohmed this motor out: c-s 74, c-r 76, s-r 82. I picked up a new motor and installed it with no chassis voltage. I ohmed the new motor before I installed it and the resistances were very similar.

    How could a single phase motor ohm out all the same? The motor only uses 2 legs of power. L1 to common, L2 to run cap to run, other side of run cap to start.

    ???

  2. #2
    Join Date
    Feb 2009
    Location
    Tallahassee, FL
    Posts
    6,051
    You gotta ohm it to ground.

  3. #3
    Join Date
    Nov 2006
    Location
    Southeastern Pa
    Posts
    19,554
    Quote Originally Posted by jarganda83 View Post
    Today I was zapped bt the chassis of a condensing unit. I changed a cc heater on this unit because it shorted out in a puddle of water. I energized this unit after installing the new cc heater and the cond fan grill zapped me. I removed the 24v control wires, so the unit was not running. I put one meter lead on the roof flashing and the other lead on the cond motor grille, the meter showed 260v. I shut down power and removed the cond motor leads. I rechecked for chassis voltage and found none. I ohmed this motor out: c-s 74, c-r 76, s-r 82. I picked up a new motor and installed it with no chassis voltage. I ohmed the new motor before I installed it and the resistances were very similar.

    How could a single phase motor ohm out all the same? The motor only uses 2 legs of power. L1 to common, L2 to run cap to run, other side of run cap to start.

    ???

    I have a more important question.

    Why was the chassis not bonded to the panel? That overcurrent device should have opened on the fault.
    [Avatar photo from a Florida training accident. Everyone walked away.]
    2 Tim 3:16-17

    RSES CMS, HVAC Electrical Specialist

    AOP Forum Rules:







  4. #4
    Join Date
    Jan 2012
    Location
    Fresno, ca
    Posts
    43
    This unit was grounded. It's supplied with 3 leg @ 460 and 1 ground wire. The cond motor is stud mounted. I did check the motor from each lead to the motor frame and found no resistance.

  5. #5
    Join Date
    Mar 2010
    Location
    Morgan Hill Ca.
    Posts
    1,230
    Quote Originally Posted by jarganda83 View Post
    This unit was grounded. It's supplied with 3 leg @ 460 and 1 ground wire. The cond motor is stud mounted. I did check the motor from each lead to the motor frame and found no resistance.
    Well there you go then....
    If a day goes by and you have learned nothing, I hope you got a lot of sleep.

  6. #6
    Join Date
    Jan 2012
    Location
    Fresno, ca
    Posts
    43
    So.... the old motor was not grounded. Right? But, it was, I replaced it and now the is no more voltage on the cabinet.

    Maybe my meter is not powerful enough the detect that small of a resistance between the motor lead and the frame? But still be enough to pass voltage. Is that possible? I should also mention that the comp. has one leg with continuous power. So the comp. could be allowing some voltage to pass to the cond. fan motor when neither motor is running.

    What about the motor winding resistances? I've never seen a single phase motor ohm out like that. I would think that common-start and common-run have to add up to start-run. How can the not? The new motor runs under it's rated amps.

  7. #7
    Join Date
    Jan 2012
    Location
    Fresno, ca
    Posts
    43
    This unit is a Carrier
    M/n: 38YCC036630
    S/n: 1002E30967

  8. #8
    Join Date
    Mar 2010
    Location
    Morgan Hill Ca.
    Posts
    1,230
    Quote Originally Posted by jarganda83 View Post
    So.... the old motor was not grounded. Right? But, it was, I replaced it and now the is no more voltage on the cabinet.

    Maybe my meter is not powerful enough the detect that small of a resistance between the motor lead and the frame? But still be enough to pass voltage. Is that possible? I should also mention that the comp. has one leg with continuous power. So the comp. could be allowing some voltage to pass to the cond. fan motor when neither motor is running.

    What about the motor winding resistances? I've never seen a single phase motor ohm out like that. I would think that common-start and common-run have to add up to start-run. How can the not? The new motor runs under it's rated amps.
    You misinterpreted your meter reading in your post... "Found no resistance" IS a direct short", you should have stated that the resistance was greater than what your meter was capable of reading, unfortunately for many meters, this is a mere 200ohms.

    Just because your meter did not read any "CONTINUITY" does not mean the motor was good.

    I am also betting that the grounding on the line side of the machine has been compromised in one way or another. Betting it is not bonded at all. (If it was, you would not have gotten "zapped". Also there needs to be a separate ground wire from the chassis of the condenser fan motor to a KNOWN good ground.

    Several years ago there was a facilities guy knocked unconscious by getting shocked and falling and hitting his head on a gas line. I often use my Volt Alert during PM's to test the sheet metal before I pull them apart, especially if they have a history of tripping breakers.

    GT
    If a day goes by and you have learned nothing, I hope you got a lot of sleep.

  9. #9
    Join Date
    Jan 2012
    Location
    Fresno, ca
    Posts
    43
    Im sorry, I ment to say my meter read "OL" when I checked from the motor leads to the motor frame.

    This motor came from the factory with just 3 leads (common, start and run) and no ground wire.

    If the unit was grounded properly then all of the leaking voltage should have gone straight to ground and none to me? I know there is a ground wire coming in with the line voltage, it's mounted in a lug near the contactor. I didn't check where it's coming from, maybe I should...

  10. #10
    Join Date
    Mar 2010
    Location
    Morgan Hill Ca.
    Posts
    1,230
    Quote Originally Posted by jarganda83 View Post
    Im sorry, I ment to say my meter read "OL" when I checked from the motor leads to the motor frame.

    This motor came from the factory with just 3 leads (common, start and run) and no ground wire.

    If the unit was grounded properly then all of the leaking voltage should have gone straight to ground and none to me? I know there is a ground wire coming in with the line voltage, it's mounted in a lug near the contactor. I didn't check where it's coming from, maybe I should...

    Now we're getting to the meat and potatoes.

    A higher end meter very well may have been able to prove the motor partially grounded. What make/model meter were you using?

    I agree that the ground issue needs to be investigated further. My bet is that the breaker panel the power is coming from has lost continuity with the ground stake at some point. Either that or it was never completed (both equally plausible).

    For god sakes be careful....
    If a day goes by and you have learned nothing, I hope you got a lot of sleep.

  11. #11
    Join Date
    Nov 2006
    Location
    Southeastern Pa
    Posts
    19,554
    Quote Originally Posted by jarganda83 View Post
    This unit was grounded. It's supplied with 3 leg @ 460 and 1 ground wire. The cond motor is stud mounted. I did check the motor from each lead to the motor frame and found no resistance.
    The unit needs an electrically acceptable low impedance ground to the overcurrent device that protects the unit. ALL current should have flowed through the equipment grounding conductor, as opposed to you, wearing work boots and touching the fan grille. If you felt discomfort when touching the grille, the impedance of the ground path is too high.
    [Avatar photo from a Florida training accident. Everyone walked away.]
    2 Tim 3:16-17

    RSES CMS, HVAC Electrical Specialist

    AOP Forum Rules:







  12. #12
    Join Date
    Nov 2006
    Location
    Southeastern Pa
    Posts
    19,554
    Quote Originally Posted by jarganda83 View Post
    Im sorry, I ment to say my meter read "OL" when I checked from the motor leads to the motor frame.

    This motor came from the factory with just 3 leads (common, start and run) and no ground wire.

    If the unit was grounded properly then all of the leaking voltage should have gone straight to ground and none to me? I know there is a ground wire coming in with the line voltage, it's mounted in a lug near the contactor. I didn't check where it's coming from, maybe I should...
    If you got "zapped," then yes, the entire ground path from the service SHOULD be checked. Similarly, the connection in the unit disconnect should not rely on three sheet metal screws to ground the cabinet of the unit. A grounding conductor should run from the disconnect to a grounding lug somewhere in the unit power section.

    Note to all:

    when using screws for grounding or bonding, only a machine thread screw is acceptable. Typical sheet metal thread screws are not acceptable in the NEC.

    A ground lug, attached with one or more machine screws, is best.
    [Avatar photo from a Florida training accident. Everyone walked away.]
    2 Tim 3:16-17

    RSES CMS, HVAC Electrical Specialist

    AOP Forum Rules:







  13. #13
    Join Date
    Apr 2001
    Location
    Over Here
    Posts
    1,105
    Quote Originally Posted by timebuilder View Post
    The unit needs an electrically acceptable low impedance ground to the overcurrent device that protects the unit. ALL current should have flowed through the equipment grounding conductor, as opposed to you, wearing work boots and touching the fan grille. If you felt discomfort when touching the grille, the impedance of the ground path is too high.
    Great explanation.

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