## Current Transformer Basics

**Current Transformer Basics**

*By Jon Rennie*

Current Transformers (CTs) are used extensively throughout electrical distribution systems for measuring, metering, and protection applications. They are a simple devices designed to produce an alternating current in the secondary winding which is directly proportional to the current in the primary conductor.

The most common type of CT is a Toroidal CT. Toroidal CTs are characterized by having the primary current conductor passing directly through the center core. Toroidal CTs are always connected in series which is why they are often referred to as “series transformers.”

Construction of a CT is simple. Secondary windings of copper magnet wire are wrapped around a hollow core of electrical steel and the primary conductor is run through the center of the core. The magnetic flux of the primary conductor will be picked up by the core and induce a current in the secondary windings that is proportional to the number of secondary windings. CTs come in many configurations but all share this basic construction.

CTs typically have a standard secondary output rating of 1 or 5 amps. A CT ratio is simply the primary and secondary currents being expressed as a ratio where the secondary is either 1 or 5 amps. A 100/5 ratio CT means that the primary current is 20 times greater than the secondary current. When 100 amps is flowing in the primary conductor, it will result in 5 amps flowing in the secondary winding.

By increasing the number of secondary windings, the secondary current can be made much smaller than the current in the primary circuit. As the number of turns goes up, the secondary current is reduced by a proportional amount. In a CT, the number of turns of the secondary winding and the current in the secondary winding are inversely proportional. A 100/5 ratio CT, for example, has 20 turns whereas a 100/1 ratio CT has 100 turns. Increasing the turns reduces the secondary current output.

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