The Single-phase TTR (Transformer Turn Ratio) Test Set measures the turns ratio and exciting current of windings in power, potential and current transformers. Deviations in turns ratio readings indicate problems in one or both windings or the magnetic core circuit. The single-phase, hand-cranked model measures highly sensitive turns ratios of up to 129.99:1 with accuracy of ±0.1%. An optional auxiliary transformer extends the ratio range to 329.99:1. The TTR set operates on the principle that the voltage ratio of the transformer at no load is practically equal to the true turns ratio. The major source of error in a transformer is a primary impedance drop due to magnetizing current, which is kept to a minimum by excitation at a fraction of rated voltage. By employing a design that meets both of these conditions along with the use of a null balance system, the turns ratio of a transformer can be determined accurately. When measuring the turns ratio of distribution and power transformers, the accuracy is well within 0.1%.