The low voltage open circuit test is used to confirm that windings have the correct ratio of turns between them, and that the phasing of the windings is correct.

This test is used for signal, pulse and switched mode power transformers where the normal operating conditions require only small excursions of the B-H curve, never extending beyond the linear region.

On the AT only, a high voltage open circuit test is available for testing line frequency transformers at their operating point (up to 270V, 1kHz and 2A). See also the test TR - turns ratio.

Measurement conditions

To measure low voltage open circuit, the test signal is applied to one winding called the energized winding.

The voltages across another winding (or the same, energized winding) is measured. The low voltage open circuit measurement is the rms value of the voltage measured and can be an ac or a dc voltage.

It is recommended that you choose the winding with the highest number of turns as the one to be energised.

Choosing test conditions

The recommended test conditions depend on the inductance of the energised winding; they are given in the table below assuming that the energised winding is the one with the highest number of turns:

Inductance of the Energised Winding	Preferred test signal Frequency Voltage
100nH → 1μH 1uH → 10μH 10uH → 100μH 100uH → 1mH 1mH → 10mH 10mH → 100mH 100mH → 1H 1H → 10H 10H → 100H 100H → 1kH 1kH → 10kH	300kHz 100kHz 30kHz 10kHz 1kHz 100Hz 100Hz 50Hz 50Hz 50Hz 20Hz	10mV 30mV 50mV 100mV 100mV 100mV 300mV 1V 5V 5V 5V

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