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What Is a Current Transformer?

Revised: 12/13/21
Grainger Editorial Staff

There are more to power systems than meets the eye. In fact, we cannot actually see electricity, but we can see how it works (or doesn’t work). So many elements come together like a puzzle to form electrical power, one of those being a current transformer. Here’s what you need to know about this vital piece of equipment:

The Down Low on CTs

A current transformer is a device used to produce an alternating current in its secondary, which is proportional to the AC current in its primary. This is primarily used when a current or voltage is too high to measure directly. The induced secondary current is then suitable for measuring instruments or processing in electronic equipment, which typically needs isolation between the primary and secondary circuit.

This reduction of high-voltage currents allows for a convenient way of safely monitoring the actual electrical current flowing in an AC transmission line using a standard ammeter.

An electrical CT differs from a voltage or power transformer because it consists of only one or very few turns as its primary winding. What also sets it apart from a voltage transformer is that the primary current is not dependent on the secondary load current but instead is controlled by an external load. The CT ratio is equal to the number of secondary turns. This ratio is based on the primary conductor passing once through the transformer window.

Classifications and Types

Current transformers can be classified into two separate groups. The first, a measuring current transformer, is used along with measuring devices for the magnitude of current, energy and power. The other, a protective current transformer, is used along with protection equipment, including trip coils, relays, etc.

There are three basic types of current transformers:

  1. Wound type: consisting of two separate windings (primary and secondary) on a magnetic steel core with different turns, depending on the design.
  2. Bar type: consisting of a bar of suitable size and material used as the primary winding, which is equivalent to a single turn.
  3. Window (toroidal) type: has no primary winding but instead has an opening in the core through which the conductor carrying the primary load current is passed.

Make sure your facility has all of the equipment it needs, including electrical transformers to transmit, distribute and utilize alternating current electrical energy safely and effectively.

The information contained in this article is intended for general information purposes only and is based on information available as of the initial date of publication. No representation is made that the information or references are complete or remain current. This article is not a substitute for review of current applicable government regulations, industry standards, or other standards specific to your business and/or activities and should not be construed as legal advice or opinion. Readers with specific questions should refer to the applicable standards or consult with an attorney.

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