What is a Current Transformer?

What is a Current Transformer?

Published by Hoyt Meter on 5th Apr 2018

What is a current transformer?

What is a Current Transformer / CT ?

A Current Transformer or C.T. is an Instrument transformer that transforming high current into a smaller, more manageable value that can be safely measured. Current transformers are used in electricity metering, protective relaying, and in other applications that require accurate measurement of electric current.

Current Transformer

A “CT” is a single-winding transformer with a copper wire wound through its center opening around the entire circumference of its toroidal-shaped laminated iron core.

The two ends of this winding connects the toroid coil to a pair of wires which travel through a shielded cable and connect to a high-precision instrument (ammeter) input which displays the electrical current through the primary in amps.

The primary winding of a current transformer consists of one or very few flat turns, a coil of heavy-duty wire wrapped around the core, or just a conductor or bus bar fed through the center hole. The current transformer is often referred to as a series transformer since the primary winding is in series with the current carrying conductor supplying a load.

Types of Current Transformers

On the basis of their applications in the field, CTs can be broadly classified into two types:Indoor current transformers and Outdoor current transformers.

Indoor Current Transformers

In terms of constructional aspects, Indoor CTs can be further classified into the following types:

  • Bar Type CT (having a bar of suitable size and material used as primary winding).
  • Toroidal (Window) Type CT (having an opening in the center to accommodate a primary conductor).
  • Wound Type CT (having a primary winding of more than one full turn wound on the core)

Outdoor Current Transformers

These CTs are designed for outdoor application and typically used in high-voltage power systems. In terms of insulation aspects, Outdoor CTs can be further classified into:

  • Gas-Type CTs (using a gas for insulation)
  • Oil-Filled CTs: (using an oil for insulation)

Based on the application they are further classified into: Measurement CTs and Protection CTs.

Current Transformer

Selection of Hoyt Current Transformers

Selecting the right current transformer requires careful consideration of several factors. Firstly, determine the specific application of the current transformer, whether it's for current measurement, protection, or metering. Next, establish the maximum current value that needs to be accurately measured, as this will help determine the appropriate current rating of the transformer.

Hoyt Meter is privileged to offer a wide selection of AC Current Transformers for energy management systems and instrumentation. All models are compatible with our analog panel


More information:

For additional information, please call our representatives: 800-2583-652.


  • Understanding Current Transformers. Electrician's Journal
  • Current Transformer: Learn its Construction, Working, Types & Uses (Testbook.com)