Current Transformer Frequently Asked Questions

What does a current transformer do?

A current transformer reduces high primary current to a lower, measurable secondary current. Protective relays, meters, and monitoring equipment use this information to evaluate transformer and power-system operation.

Can a transformer CT be replaced in the field?

Many current transformer replacements can be completed in the field. The required work depends on the CT’s location, transformer design, accessibility, and whether oil must be removed or lowered to reach the component.

How do you select a replacement CT?

The replacement must be selected according to the required ratio, current ratings, accuracy or protection class, burden, polarity, insulation level, dimensions, mounting configuration, and secondary circuit requirements.

Is testing required after CT replacement?

Yes. Testing is necessary to verify that the replacement CT has the correct electrical characteristics, orientation, tap selection, polarity, and secondary connections before the transformer returns to service.

How long does current transformer replacement take?

The schedule depends on the transformer design, CT location, replacement component availability, access requirements, and extent of testing. Reviewing drawings and CT specifications before the outage helps establish an accurate project schedule.