The turns ratio is the number of turns in the primary winding divided by the number in the secondary, and it sets the voltage the transformer delivers for a given input. The test that verifies it is known by its initials, TTR, for Transformer Turns Ratio.
It is measured by injecting a low voltage into one winding and reading what appears on the other, across every tap changer position. The deviation from the nameplate value must stay within the 0.5 % set by IEC 60076-1; a larger deviation points to shorted turns, a mispositioned tap changer or a damaged winding.
It is a mandatory acceptance test and one of the first performed on any suspicion, because it is fast, non-destructive and locates the problem by winding and by tap. Together with winding resistance and excitation current it forms the basic trio of electrical transformer diagnostics.