Choosing a current transformer

Choice of a CT depends on 2 criteria:

• the Ip/5 A ratio
• the installation type.

The Ip/5 A ratio
We recommend that you choose the ratio immediately higher than the maximum measured current (In).

Example: In = 1103 A; ratio chosen = 1250/5.

For small ratings from 40/5 to 75/5 and for an application with digital devices, we recommend that you choose a higher rating, for example 100/5.
This is because small ratings are less accurate and the 40 A measurement, for example, will be more accurate with a 100/5 CT than with a 40/5 CT.


The installation type

Choice of a CT model depends on the installation type:

• insulated cables
• mounting on bars.

Released for: Schneider Electric Thailand