CT ratio is described as the ratio of primary current input to secondary current output at full load. For example, a CT with a ratio of 300:5 will produce 5 amps of secondary current when 300 amps flow through the primary.

If the primary current changes, the secondary current output will change accordingly. For example, if 150 amps flow through a300 amp rated primary the secondary current output will be 2.5 amps.

(300:5 = 60:1) (150:300 = 2.5:5)

Unlike the voltage or power transformer, the current transformer consists of only one or very few turns as its primary winding. This primary winding can be of either a single flat turn, a coil of heavy duty wire wrapped around the core or just a conductor or bus bar placed through a central hole.

The ratio test is conducted to prove that the ratio of the CT is as specified, and to verify the ratio is correct at different taps of a multi tap CT. The turn's ratio is equivalent to the voltage ratio of potential transformers and can be expressed as follows:

N2/N1=V2/V1

N2 and N1 are number of turns of secondary and primary windings

V2 and V1 are the secondary and primary side voltage readings

Ratio tests are performed by applying a suitable voltage (below saturation) to the secondary of the CT under test while the primary side voltage is measured to calculate the turns ratio from the expression above.

DANGER: Use caution when conducting a CT ratio test and DO NOT apply a voltage high enough that would cause the transformer to saturate. Applying a saturation voltage will result in readings that won't be accurate.