RESISTANCE MEASUREMENTS:

NO LOAD AND EXCITATION CURRENT TEST:
No load losses (excitation losses) are the losses of a transformer that is excited at a rated voltage and frequency, but which is not supplying load. No load losses include core loss, dielectric loss and loss in the windings due to exciting current. The no-load of a transformer consists principally of the iron loss in the transformer core.
a) For the calculation of the IR.
b) for the calculation of winding temperature test.
c) As a base for assessing possible damage in the field.

IMPEDANCE VOLTAGE AND LOAD LOSS TEST:
Load losses are those losses in a transformer that are incident to the carrying of the load. Load losses include I²R loss in the windings due to load current and stray loss due to stray fluxes.
The voltage required to circulate the rated current under short-circuit conditions when connected on the rated voltage tap is the impedance voltage, and the watt loss measured is the load loss.

TEMPERATURE RISE TEST: (When required)
Transformers are tested under a loading condition that will give losses as near as possible to those obtained when the transformer is operating at its nameplate rating.
Surface temperatures are measured by thermocouples. Average winding temperature and average winding temperature rise are measured by the hot-resistance method. Ambient temperature shall be taken as that of the surrounding air, which should be not less than 10°C or more than 40°C.

APPLIED POTENTIAL TEST
Determines the dielectric strength of the insulation between windings and between the windings and ground.

INDUCED POTENTIAL TEST
Measures the dielectric strength of the turn-to-turn and layer-to-layer insulation.