Current transformers play an important role in the monitoring and protection of electrical power systems. CT's are instrument transformers used for converting primary current into a reduced secondary current for use with meters, relays, control equipment and other instruments.

The importance of instrument transformer tests is often underestimated. Current transformers for metering purposes must have a high degree of accuracy to ensure precise billing while those used for protection must react quickly and correctly in the event of a fault.

Risks such as confusing instrument transformers for metering and protection, or mixing up connections can be reduced significantly by testing before initial use. At the same time, electrical changes in a CT, caused for example by aging insulation, can be identified at an early stage.

For these reasons and more, it is essential to inspect and calibrate current transformers and their connected instruments at regular intervals. There are 6 electrical tests that should be performed on CT's to ensure accuracy and optimal service reliability:

1. Ratio Test

2. Polarity Test

3. Excitation (Saturation) Test

4. Insulation Resistance Test

5. Winding Resistance Test

6. Burden Test