The transformers must have in the high voltage winding two leads, in addition to the main one, for a branch strip that guarantees to obtain the nominal power. The main derivation is the one that corresponds to the highest voltage.
The manufacturer shall specify the short-circuit impedance, in percent, of the main leads of each pair of windings and other combinations of leads which he deems necessary at the reference temperature. BG11-1-4-4
The measured short-circuit impedance shall remain within the tolerance limit of ± 7,5% for 2-winding transformers in relation to the value stated by the manufacturer. In the case of transformers of the same design, the difference between the short-circuit impedances of any two transformers shall not exceed 7,5% for 2-winding transformers in relation to the value stated by the manufacturer.
The manufacturer and the reconditioner shall ensure the maximum no-load losses and the maximum total losses, at the reference temperature, with sinusoidal voltage at the rated frequency at the main branch. The maximum losses allowed for each power are those established in NBR 5440.
The losses obtained on the test of one or more single-phase or three-phase transformers, in a certain order of supply, shall not exceed the percentage guaranteed losses mentioned above.