# Step down transformer



## Aydin2011 (Apr 2, 2013)

Aydin2011 said:


> I have step down transformer, control transformer 480 single phase to 110 vac. Transformer secondary x1 hot side 0.2 ohm with ground but transformer working. There is polarity issue? I know ground and natural connect with body.
> 
> Thx for input


The earth conductor is there to provide a return path for the fault current wile the fault exists, i.e. until the circuit protective device(s) operate(s) so as to disconnect the fault.


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## micromind (Aug 11, 2007)

Since the 120 side of the transformer is isolated from the 480 side, it doesn't matter which lead is grounded, either X1 or X2. 

Just don't ground both of them.......

Lol.


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## Aydin2011 (Apr 2, 2013)

Sorry maybe I couldn't explain what I ask? I have troubleshooting so I just measure the x1 secondary hot with ground .2 ohm direct short. But there is no trip breaker. X2 is natural with ground. 
Thanks


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## ScooterMcGavin (Jan 24, 2011)

The low resistance reading you are seeing between x1 and x2 is the winding resistance of the secondary. If you think about what the secondary of the transformer is, its nothing more than some copper wire wrapped around an iron core. From a DC perspective(which your ohm meter uses to measure resistance). It just see's the resistance of the wire. Since x2 is grounded it will show a short. Same thing if you checked the resistance of motor windings you would have a very low reading as well. Essentially by putting your ohm meter on x1 and x2 you're creating the "short". You could draw it out and that might help make more sense. Usually when a transformer has gone bad your nose will be the first thing to let you know its toast.


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## Aydin2011 (Apr 2, 2013)

I am reading low resistant hot and ground. 

Thanks


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## Big John (May 23, 2010)

Aydin2011 said:


> I am reading low resistant hot and ground....


 Normal. The winding resistance is likely going to be very low because the transformer depends on AC impedance to limit current. With one side of the coil grounded you will read through that coil to ground at some very low value.


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## Aydin2011 (Apr 2, 2013)

I may have different issue in the system. Short some where. Wired thinks when I turn on main 3 breaker trip. But when I turn on control breaker slowly they don't trip. It is old system with new breaker. AB load in load out din rail type maybe wrong type breaker. 

Thanks all input


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## DesignerMan (Jun 13, 2008)

Aydin2011 said:


> I may have different issue in the system. Short some where. Wired thinks when I turn on main 3 breaker trip. But when I turn on control breaker slowly they don't trip. It is old system with new breaker. AB load in load out din rail type maybe wrong type breaker.
> 
> Thanks all input


Your breaker may not be sized correctly or could be the wrong trip setting.
Refer to NEC table 450.3(B) to determine the correct size for your transformer.
I actually prefer fuses to CB's on transformers because I have had similar issues in the past. If you want to use a CB I'd suggest using one with a highly inductive trip curve (curve D on an Allen-Bradley DIN mounted CB).


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## Aydin2011 (Apr 2, 2013)

DesignerMan said:


> Your breaker may not be sized correctly or could be the wrong trip setting.
> Refer to NEC table 450.3(B) to determine the correct size for your transformer.
> I actually prefer fuses to CB's on transformers because I have had similar issues in the past. If you want to use a CB I'd suggest using one with a highly inductive trip curve (curve D on an Allen-Bradley DIN mounted CB).


That is problem I found breaker was wrong and I am replacing them
Thank you


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