Page 1 of 3 123 LastLast
Results 1 to 13 of 30
  1. #1
    Join Date
    Nov 2011
    Location
    St. Louis
    Posts
    3,482

    Can a shorted secondary on an xfmr travel back to primary breaker?

    I recall from electricity 101 that the primary and secondary windings of a transformer are not physically connected, that the magnetic current from the primary winding around the iron core is induced to the seconady windings.

    My question is this: If you had an unfused secondary circuit and a sustained (until the secondary of the transformer shot craps) dead short; would damage be possible to the primary side of the xfmr and possibly reach back to trip the breaker in the panel.

    That was a worse case scenario. More realistic: If you had a minor brief short in the secondary circuit and still had a good secondary winding (as evident by a few ohms in a 120/24 xmfr); would there be any possible way that could trip the breaker in the panel?

  2. #2
    Join Date
    Dec 2012
    Location
    columbus, OH
    Posts
    2,071
    I just fried one a couple weeks ago, no breaker problem. A 40VA transformer can handle 1.6 on the secondary. And since voltage is higher on the primary amps will be lower.

  3. #3
    Join Date
    Nov 2006
    Location
    Southeastern Pa
    Posts
    18,804
    A draw on the secondary will cause a draw on the primary, due to the coupling effect of the magnetic fields.

    So, yes. Pop goes the breaker.
    [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
    Feb 2013
    Location
    Phoenix, AZ
    Posts
    198
    It depends a lot on the size of the transformer. True, a shorted secondary will reflect back into the primary with increased current draw, but the transformer windings have resistance and this limits maximum (fault) current. Also, the iron core of the transformer will only support so much coupling before it 'saturates' and can pass no more, this is related directly to how much metal is there. So, a small transformer is limited both ways, the smaller the VA rating the higher the winding resistance and the less iron in the core. If a smaller transformer is on a 15 or 20 A circuit, it will probably not trip the breaker, at least not due to overcurrent. Of course, ideally it would be protected with a properly sized fuse, but this is not always the case.

    Now, even the small transformer will get very hot if the sec is shorted, and if it reaches the point where the enamel insulation on the windings melt, then this is a different story. What happens is the melting insulation allows the windings to short together, creating a dead short across the primary leads. Now that the winding resistance no longer limits current, then the total fault current of the branch circuit flows, possibly hundreds (and probably a lot more) of amps, this is what takes out the breaker or fuse.

    So, a momentary short on a small (100 VA or less) transformer would probably not trip a 20A or larger breaker. If it cooks long enough to short the primary windings, then it definitely would trip, but the transformer would obviously be toast.
    Last edited by DDC_Dan; 02-25-2013 at 09:18 PM.

  5. #5
    Join Date
    Nov 2011
    Location
    St. Louis
    Posts
    3,482
    So a tiny humidifier transformer, certainly 40vac or less, would have to have a complete meltdown to impact the primary voltage significantly.

    Assumed as much...but nice to have some science behind it.

    Thanks guys.

  6. #6
    Join Date
    Feb 2013
    Location
    Phoenix, AZ
    Posts
    198
    I happen to be sitting at the bench and I have a transformer handy. It's a 40VA 120/24V. Normal max secondary current would be 1.6A, and primary current about 0.3A (a 240V transformer would be about half that as you know). With secondary shorted the 'fault' current on the secondary was 11.6A with primary drawing 2.6A. No breakers were harmed in this experiment, and the transformer survived as well. Total shorted on time was only about 30 sec. (in two tests).

  7. #7
    Join Date
    Nov 2011
    Location
    St. Louis
    Posts
    3,482
    Quote Originally Posted by DDC_Dan View Post
    I happen to be sitting at the bench and I have a transformer handy. It's a 40VA 120/24V. Normal max secondary current would be 1.6A, and primary current about 0.3A (a 240V transformer would be about half that as you know). With secondary shorted the 'fault' current on the secondary was 11.6A with primary drawing 2.6A. No breakers were harmed in this experiment, and the transformer survived as well. Total shorted on time was only about 30 sec. (in two tests).
    Awesome! On demand documented lab results!

    So how about a 660/220 residential street transformer? If I shorted out the incoming lugs in my panel for a max of 5 seconds or so...

    j/k I have a good idea of what woud happen...

  8. #8
    Join Date
    Feb 2013
    Location
    Phoenix, AZ
    Posts
    198
    We aim to please....

  9. #9
    Join Date
    Feb 2013
    Location
    Phoenix, AZ
    Posts
    198
    Quote Originally Posted by hurtinhvac View Post
    So how about a 660/220 residential street transformer? If I shorted out the incoming lugs in my panel for a max of 5 seconds or so...

    j/k I have a good idea of what woud happen...
    Pics or it didn't happen!

  10. #10
    Join Date
    Dec 2012
    Location
    columbus, OH
    Posts
    2,071
    Quote Originally Posted by DDC_Dan View Post
    I happen to be sitting at the bench and I have a transformer handy. It's a 40VA 120/24V. Normal max secondary current would be 1.6A, and primary current about 0.3A (a 240V transformer would be about half that as you know). With secondary shorted the 'fault' current on the secondary was 11.6A with primary drawing 2.6A. No breakers were harmed in this experiment, and the transformer survived as well. Total shorted on time was only about 30 sec. (in two tests).
    11 amps wow. What kind of transformers you using? I shorted one for a fraction of a second before it didn't smell to good. I also have a nice paper weight.

  11. #11
    Join Date
    Feb 2013
    Location
    Phoenix, AZ
    Posts
    198
    It's made by Elk Products, I use them for powering up small ddc panels, you wouldn't normally find this one on a refrigeraton system (especially since it's a 120V primary).

  12. #12
    Join Date
    Feb 2013
    Location
    Florida
    Posts
    69
    Quote Originally Posted by hurtinhvac View Post
    Awesome! On demand documented lab results!

    So how about a 660/220 residential street transformer? If I shorted out the incoming lugs in my panel for a max of 5 seconds or so...

    j/k I have a good idea of what woud happen...
    The primary on your residential street transformer is most likely around 7500 volts. I hope you like yours extra crispy

  13. #13
    Join Date
    Nov 2011
    Location
    St. Louis
    Posts
    3,482
    The motivation for this thread was a simple humidifier call last week. I read power across the humidistat even in test mode. While there I wanted to make sure that was the only issue and went to verify water flow. The xfmr was one of those little guys that sits right on the humidifier power supply outlet box with the secondary taps exposed. So I broke loose the water line on the humidifier side and ran leads right to the solenoid from the xmfr, got my water flow when the solenoid opened and buttoned that piece of it back up. Then I went to verify my initial findings again and...nothing. Not across the humidistat, xmfr 2ndary, primary, disconnect, breaker nor lugs on the sub panel.

    I'm now hopelessly stuck in a classic "but it was working when you got here" mess. And it was. Somehow the breaker feeding the subpanel tripped. Not the breaker on the subpanel for the humidifier circuit I was concerned with, but the breaker feeding the subpanel. I just could not understand how jumping a low voltage solenoid could cause an event way back at the feeder panel. The examples above were extreme - at no time did I cause a short...just opened and closed a solenoid as the circuit ultimately does anyway.

    Homeowner was bent out of shape and kind of freaky anyway under the best of circumstances. Fretting away loudly about her plants in the green room (not even related to this part of the house and panel). Probably had a half dozen cats locked up in the back rooms somewhere. But she is the good doctor's wife and had the owner of my company on the phone within minutes. Ultimately I was sent away, puzzled and shamed and wondering what the hell could have happened. But before I left I did note that they had a hundred amp breaker in the feeder panel supplying a sub panel with a 60 amp for the electric furnace, a 60 for the oven, a 15 for the humidifier and maybe one more fifteen for something else. The tech dispatched as a second set of eyes quickly called an electrician in (feeder breaker kept tripping.

    Just wondering if my little 24v, miniscule amp circuit could have been the proverbial straw and why didn't it act up before, when the humidifier was operational? Or was it coincidence? All I know is I was the last one to touch it

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •  
Comfortech Show Promo Image

Related Forums

Plumbing Talks | Contractor Magazine
Forums | Electrical Construction & Maintenance (EC&M) Magazine
Comfortech365 Virtual Event