Voltech ATi User Manual page 361

Primary Terminals 'High' and 'Low': The zero phase reference terminals, which
are often the same as the energized terminals.
Secondary Terminals 'High' and 'Low': The turns ratio and polarity
measurement will be made between the primary and secondary terminals.
Use the PC Editor's 'Measure' button (together with a sample part) if you are
unsure of how to choose the test conditions.
Notes: The signal is applied to the primary winding, or the winding that has
the largest number of turns.
However, if by doing this, the expected voltage on the winding with the
smallest number of turns falls below 1mV, then the test voltage should be
increased.
This may also require an increase in the test frequency so that the current
taken by the driven winding does not become too large, but in general this
frequency increase should be kept as small as possible to avoid problems
caused by stray capacitance at high frequencies.
Where Matching in Groups Is Important
In some transformer designs, the turns ratio between a primary winding and
a secondary winding is not as important as the ratio between different
primaries or different secondaries.
To make the most accurate measurements in such cases apply the test
signal to the primary winding and measure the turns-ratio from primary to
one of the secondaries.
Then, leaving the primary energized as above, measure the turns ratio
between the secondaries.
Next, energize a secondary winding (possibly at a different voltage and / or
frequency depending on its inductance) and measure the turns ratio between
the various primaries.
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