Substation1
Dictionary of Electrical Engineering

Commonly used terms in the Electrical industry.

subtractive polarity
polarity designation of a transformer in which terminals of the same polarity on the low-and high-voltage coils are physically opposite each other on the transformer casing. With subtractive polarity, a short between two adjacent terminals results in the difference of the two coil voltages appearing between the remaining terminals.
Subtractive polarity is generally used on transformers larger than 500 kVA and higher than 34.5 kV. Smaller units use additive polarity.
See additive polarity