It consists of e 115 VAC source, a 10 A lised switch, a variable autotransformer(Variac), an ammeter and three voltmeters. A ratio measurement accuracy of lee would require 0.5 accuracy voltmetcrs or 4 digit digital voltmeters. Measurement accurac can be improved by having a stable sinusoidal voltage regulator ahead of the variable autotransformer. Test voltages used to detemine high-voltage power transformer ratios are usually 1/1000 of the winding's rated voltage or less to reduce testing hazards. The simplest test values to choose are those for which a decimal point reading would read the ame as high- and low-winding nameplate kVvalues. test voltages read are in volts Ubut read directly as if they were For example. it the nameplate says the voltage on a particular lap should be 112,750 volts to 13,800 volts, energize the high-voltage winding at 112.75 volts and expect to read 13,80 volts on the low side, Digital meters with four- or five-place accuracy are needed to make readings this exact. Measurenents should be recorded for every tap-changer position Test evaluations are aided by pre-calculating the expected test voltages and filling out the data sheets ahead of time. In making voltage measurements, separate voltmeter leads connected directly to bushing teminals must be used to avoid errors due to voltage drop in the source leads. The ammeter should be included in the test setup to monitor input current to the transformer in case the current is high due to a short circuit, shorted turn, or other connection problems Because induced voltage from nearby current-carrying bus or lines could modify measurements, make low-resistance connections directly to transformer bushing