Open circuit test :-

This test is perform to find out rated core loss or shunt branch parameters. (R and X ).

This test is connect at rated voltage and rated frequency.

All measuring instruments connected across low voltage side. While high voltage is kept open circuited.

                           Open circuit test 

If this test is connected on low voltage side then low range of voltmeter is required.
To take the measurable reading of ammeter this test is connected on low voltage side.
During open circuit test no load current flows which is 2 to 5% of full load current.
Therefore the primary winding copper loss and primary leakage reactance drop can be neglected.
    
No load power factor of transformer is very low so we need to use a low power factor wattmeter for power measurement.

Short circuit test :-
 
This test is perform to find out rated copper loss and series branch parameter.
This test is connected at rated value of current.
All measuring instruments connected across high voltage side while low voltage side is keep short circuit.
If this test is conducted on high voltage side then low range of ammeter is required.
To take the measurable reading of voltmeter this test is connected on high voltage side.
                           Short circuit test 

During short circuit test the voltage applied is 5 to 10% of rated to circulate the full load current.
Therefore core loss at this reduced voltage can be neglected.
Since we have applied a voltage much less then rated voltage the flux is highly reduced by which core loss reduced and hence no load current reduces so no load current can be ignared as compare to rated current in the winding.
By ignoring no load current we are ignoring the shunt branch and only considering series branch.

Polarity test :- 

Polarity test is used to determine the polarity of secondary winding while we known the polarity of primary winding.
After determining the secondary polarity we can place the dot in both the winding at the terminal having same polarity.
                           Wrong connection
     
                          Right connection 

Polarity rest is required for parallel operation of transformer because in parallel connection terminal of same polarity must be connected to avoid any flow of circulating current.

Load test or Back to back test :- 
Open circuit test and short circuit test only talk about either core loss or copper loss but not both losses simultaneously.
The temperature rise of transformer in actual working condition will depends on both the losses so open and short circuit test can not predict temperature rise.
In small transformer we can operate the transformer at full load to predict the temperature rise but in large transformer it will means a large consumption Of energy which will be uneconomical.
So we need to have an economic heat run test where energy consumption will be minimize and at the same time we can predict temperature rise.
In sumpner test we have two identical transformer with their primary winding connected in parallel and secondary winding connected in series opposition.
                               Back to back test