what is shield in transformer ?
why it is apply & where it is on primary or secondary?

 

Usually between primary and secondary and connected to earth ground, in order to isolate any HF on the primary, usually seen in audio amps etc.
Also reduces capacitive coupling between pri & sec.
Max.

 

Also reduces capacitive coupling between pri & sec.
Max.

Sometimes a transformer with a shield is used in medical instruments to prevent dangerous leakage currents. When a patient is connected by electrodes even a small leakage current from the secondary to the primary can be fatal.