# How many Ohms would I read on a transformer? Basic question



## emtnut (Mar 1, 2015)

JasonCo said:


> I have a bunch of questions on my homework that my teacher was unable to go over in class because of time restraints. One of the questions read:
> 
> Using an ohmmeter on the transformer-winding connection points, as shown in figure 216.34, ____ ohms would be read between X1 or X2 or H1 or H2 and the metal transformer enclosure.
> 
> ...


If you had to make a logical choice ... what would YOU say ?


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## JasonCo (Mar 23, 2015)

If I put my meter on X1 and then put it on the metal frame, I'd get 0?

I'm just unsure, pretty green. I don't know in what cases I'd get answer A, B, C. Was wondering if someone could give me a scenario for each if possible.

Edit: I don't even know if point X1 at the end of the coil is even making contact with the transformer enclosure. This information isn't given, and I don't even know if it even matters for the answer. I'm totally lost


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## NC Plc (Mar 24, 2014)

Hint: If you read 0 ohms of resistance between X1 and the metal frame of the transformer you shouldn't energize that transformer.


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## JasonCo (Mar 23, 2015)

NC Plc said:


> Hint: If you read 0 ohms of resistance between X1 and the metal frame of the transformer you shouldn't energize that transformer.


So I should read infinity then? Because I don't think my answer is 2 to 20. I just don't know. I've been at this homework for the last 4 hours after working 11 hours today and I'm just burned out. Trying to think logically here, just struggling right now :/


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## 480sparky (Sep 20, 2007)

0 ohms is a dead short. 

It should (theoretically) read infinity.


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## JasonCo (Mar 23, 2015)

Thank you so much. So in what scinario would I get 2 to 20. Lets say my leads are on X1 and H2, both are on different coils.


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## telsa (May 22, 2015)

JasonCo said:


> Thank you so much. So in what scinario would I get 2 to 20. Lets say my leads are on X1 and H2, both are on different coils.


None.

:laughing:


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## Wirenuting (Sep 12, 2010)

JasonCo said:


> Thank you so much. So in what scinario would I get 2 to 20. Lets say my leads are on X1 and H2, both are on different coils.


Look for a 1/2 cooked squirrel laying across the 2 windings. That might give you 2 - 20 ohms between primary and secondary.


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## emtnut (Mar 1, 2015)

JasonCo said:


> Thank you so much. So in what scinario would I get 2 to 20. Lets say my leads are on X1 and H2, both are on different coils.


Between X1 and X2 , and between H1 and H2 you would read between 2 and 20 ohms.

Think of the wiring of the coils for the high and low side ... which one would read closer to 2 ohms (less coils) and which one would read closer to 20 ohms (more coils)


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## emtnut (Mar 1, 2015)

Wirenuting said:


> Look for a 1/2 cooked squirrel laying across the 2 windings. That might give you 2 - 20 ohms between primary and secondary.


Made me laugh ... I've seen a few of those, especially this time of year:laughing::laughing:


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## Meadow (Jan 14, 2011)

B, infinity or more exactly several dozen megaohms


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## Southeast Power (Jan 18, 2009)

JasonCo said:


> I have a bunch of questions on my homework that my teacher was unable to go over in class because of time restraints. One of the questions read:
> 
> Using an ohmmeter on the transformer-winding connection points, as shown in figure 216.34, ____ ohms would be read between X1 or X2 or H1 or H2 and the metal transformer enclosure.
> 
> ...


Since we know nothing about the transformer, I would have to say ZERO ohms. 
You are reading a coil of wire.
Dont assume any impedance reactance unless they specifically ask for that.


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## dmxtothemax (Jun 15, 2010)

It is of course infinity ! (Ideally).
It could not be zero ohms
As that would be a short !


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## Bad Electrician (May 20, 2014)

NC Plc said:


> Hint: If you read 0 ohms of resistance between X1 and the metal frame of the transformer you shouldn't energize that transformer.


 Depends on the transformer and the distribution voltage you want X1 can be grounded. One would have to see the drawing to make a definitive answer.

H1 - H2 should be open.


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## Southeast Power (Jan 18, 2009)

dmxtothemax said:


> It is of course infinity ! (Ideally).
> It could not be zero ohms
> As that would be a short !


Yup, a short.
Open would be a bad transformer coil.


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## mpetro (Jan 6, 2016)

Wirenuting said:


> Look for a 1/2 cooked squirrel laying across the 2 windings. That might give you 2 - 20 ohms between primary and secondary.


Free dinner!


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## glen1971 (Oct 10, 2012)

JasonCo said:


> I have a bunch of questions on my homework that my teacher was unable to go over in class because of time restraints. One of the questions read:
> 
> Using an ohmmeter on the transformer-winding connection points, as shown in figure 216.34, ____ ohms would be read *between X1 or X2 or H1 or H2 and the metal transformer enclosure*.
> 
> ...


The "or's" are saying the results will be the same no matter which one you pick and measure to the transformer frame... Thus the answer is "B"....



jrannis said:


> Since we know nothing about the transformer, I would have to say ZERO ohms.
> You are reading a coil of wire.
> Dont assume any impedance reactance unless they specifically ask for that.


Re-read the OP's original question.. They aren't asking for winding resistance, or how to determine if a transformer has a failed winding.. They are looking to determine the normal reading from a winding to ground to look for a dead short...


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## oliquir (Jan 13, 2011)

it depens on the kva on transformer, the bigger it is the lower the resistance is. i have a 1.5kvs in my hand and primary at 240v is under 1 ohm


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## NC Plc (Mar 24, 2014)

Bad Electrician said:


> Depends on the transformer and the distribution voltage you want X1 can be grounded. One would have to see the drawing to make a definitive answer.
> 
> H1 - H2 should be open.


Fair enough man. Under what parameters would you want X1 grounded?


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## JasonCo (Mar 23, 2015)

Thanks for all your help, I understand a bit more now. So if X1, X2, H1, or H2 is on one lead and the other lead is on the transformer metal, you'd read infinity. If a lead is on X1 and a lead is on H1, you'd read 0. If a lead is on X1 and a lead is on X2, you'd read 2 to 20. Is this all correct? Just theoretically speaking, if the question gives you no other information, just wants you to understand the theory of it and that's all, would everything I just said be correct?


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## John Valdes (May 17, 2007)

Put your meter on the ohm function and touch the two meter leads together.
This is continuity or a dead short.
Take the two leads apart from each other and this is an open circuit.

This problem you were given was very simple and only is a continuity check and ohms had nothing to do with it unless the instructor wanted you to come up with a value.

Sorry I see he did give you a value.
But use the method I posted above to see whats a circuit and what not a circuit.
On a very big transformer you may not read any resistance even though it has resistance. Your meter is not able to read those values sometimes as they are way to low.
So by putting leads together and taking leads apart is all you need to know about reading and measuring ohms. Open and closed with a number/value for closed.
For now.


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## Bad Electrician (May 20, 2014)

NC Plc said:


> Fair enough man. Under what parameters would you want X1 grounded?


In any application where you want the secondary grounded, I have seen X1 or X2 grounded.


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## don_resqcapt19 (Jul 18, 2010)

JasonCo said:


> Thanks for all your help, I understand a bit more now. So if X1, X2, H1, or H2 is on one lead and the other lead is on the transformer metal, you'd read infinity. *If a lead is on X1 and a lead is on H1, you'd read 0.* If a lead is on X1 and a lead is on X2, you'd read 2 to 20. Is this all correct? Just theoretically speaking, if the question gives you no other information, just wants you to understand the theory of it and that's all, would everything I just said be correct?


That combination should also read infinity.


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## JasonCo (Mar 23, 2015)

don_resqcapt19 said:


> That combination should also read infinity.


Really appreciate your help! Just got through with class and had asked my teacher about that and you both are saying the same thing. Appreciate your help, would have missed like 5 questions on my quiz if I had thought it was 0! Thanks again


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## Meadow (Jan 14, 2011)

Bad Electrician said:


> Depends on the transformer and the distribution voltage you want X1 can be grounded. One would have to see the drawing to make a definitive answer.
> 
> H1 - H2 should be open.



Ditto with H2.


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## Southeast Power (Jan 18, 2009)

JasonCo said:


> Really appreciate your help! Just got through with class and had asked my teacher about that and you both are saying the same thing. Appreciate your help, would have missed like 5 questions on my quiz if I had thought it was 0! Thanks again


ohms would be read between X1 or X2 or H1 or H2 *and *the metal transformer enclosure.


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## billn (Aug 31, 2011)

For any ONE terminal (H1 or H2 or X1 or X2) to the enclosure will read as an essentially open circuit (infinity).

Let's analyze what the resistance across X1 and X2 is likely to be. Assume that the transformer is sized for a 200 Amp load. The voltage is immaterial - for an approximate determination, all that matters is the coil resistance and the current. 

If the coil resistance is 1 ohm, the the power dissipation of the coil at full load equals: (I^2)R = 200 x 200 x 1 = 40,000 watts. That is a heck of a lot of heat, so the coil resistance has to be much smaller than 1 ohm. Even at 1 milli-ohm, the coil would have to dissipate 20 watts.

The actual dissipation is somewhat different, since it depends on both the DC coil resistance and the coil impedance (mostly inductance for a coil).


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## glen1971 (Oct 10, 2012)

jrannis said:


> ohms would be read between X1 or X2 or H1 or H2 *and *the metal transformer enclosure.


Since you keep insisting on this please explain your reasoning... Double check the OP's original post - none of the terminals are directly connected to the case...


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## power (Feb 27, 2012)

Provided I am understanding your explanation correctly, it would read infinite. There is no connection between a transformer's windings and ground until it's wired in an orientation that grounds the system. Wye (star) systems are grounded at the starpoint, but NOT until the electrician performs that operation. Center-tapped grounded Delta are the same situation. There will be no reference to ground until the electrician wires the three (3) individual windings in the necessary orientation.


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## Bad Electrician (May 20, 2014)

power said:


> Provided I am understanding your explanation correctly, it would read infinite. There is no connection between a transformer's windings and ground until it's wired in an orientation that grounds the system. Wye (star) systems are grounded at the starpoint, but NOT until the electrician performs that operation. Center-tapped grounded Delta are the same situation. There will be no reference to ground until the electrician wires the three (3) individual windings in the necessary orientation.


Not to over confuse the issue but infinity does not exist, except maybe in a vacuum (?) It is full scale of the meter for a megger.


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## di11igaf (Jan 1, 2012)

NC Plc said:


> Fair enough man. Under what parameters would you want X1 grounded?


Assuming he means a non center tapped delta- one option is corner grounding it.


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## di11igaf (Jan 1, 2012)

AcidTrip said:


> Ditto with H2.


I've admittedly never seen this config.
Is this saying the primary can be hooked up phase-neutral at 7200 OR phase-phase at 12.4?
Or is it saying you can ONLY use this with a 12.4 single phase with neutral aka 7200-neutral(assuming its this one)
Or am I missing something here?
Edit- more I think about it it has the be the latter. Former wouldn't make sense without changing secondary


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## Meadow (Jan 14, 2011)

di11igaf said:


> I've admittedly never seen this config.
> Is this saying the primary can be hooked up phase-neutral at 7200 OR phase-phase at 12.4?
> Or is it saying you can ONLY use this with a 12.4 single phase with neutral aka 7200-neutral(assuming its this one)
> Or am I missing something here?
> Edit- more I think about it it has the be the latter. Former wouldn't make sense without changing secondary



Chances are you see it every day when driving on the east coast, 90% of all North American pole pigs have this configuration.

The voltage is 7,200 volts only (L-N), but listed as 7.2/12.47 as the 3 phase system its normally applied on. GRDY means the system must have a grounded neutral since this will connect to the can acting as a return.


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