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.
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?
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.
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.
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2 Tim 3:16-17
RSES CMS, HVAC Electrical Specialist
Member, IAEI
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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.
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.
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).
We aim to please....
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).
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
My guess it was doing it long before you got there.
Use the biggest hammer you like, pounding a square peg into a round hole does not equal a proper fit.
From your description, someone needs to go there and have a look at their electrical distribution....
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2 Tim 3:16-17
RSES CMS, HVAC Electrical Specialist
Member, IAEI
AOP Forum Rules:
My advice:
Don't let the smoke out
I can't think of any bad transformer that I have found where the secondary side failed. It has always been the primary even though the short was on the secondary side. I have never encountered one where the short caused enough load on the primary side to cause a line voltage breaker to trip. The failure has always been in the windings of the primary. Has anyone else had a different type of failure?
In residential I would think this less likely, but it can be a real issue in commercial (Think Superbowl) a Ground Fault Circuit Interrupter (GFCI) is designed to be very touchy - even moisture can cause enough leakage current (translates into load imbalance) to get it to trip.
Codes might be requiring GFCI even on an equipment circuit, if it goes outside, I dunno.
A GFCI will save your life though - I checked again this summer!
Jim A.
I never tested which side burnt up, but I've never had a transformer trip the primary that I am aware of. Granted most transformers I deal with are 24V and 100VA or less.
100VA at 24V = 100VA/24V = 4.167 amps continuous amps available on the secondary, thus many times these are 5A circuit breakers on the transformers with circuit breakers.
The conductors in the windings are so small that they will eventually be your fuse if you don't have one, assuming your smallest CB in the sub-panel was 15A.
I think it was coincidence, but something else likely happened at the same time, the homeowner plugged something they shouldn't have in while you were messing with their humidifier or a bird/string/branch/gopher/mouse crossed the wires in the main.
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Yeah...ran into the boss days later and he said I just got caught at the wrong place, wrong time. Still bothersome to me that 120+ worth of breakers are being fed by a 100. Oven and resistive heater being the chief consumers...but that's the way it's been for years.