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Current Transformers

Technical Bulletin No. 15

​ By Tom Colella, Engineering Manager

Current Transformers (CT) are an instrument type of transformer which take high currents and reduce them to an extremely lower value providing an easy and safe method of monitoring circuits without breaking-out the wiring. Measurement is generally made using a standard digital or analog meter. The current transformer can be provided in single- or poly-phase design. Current transformers have a variety of applications -- from utility power management to precision measurements in medical, automotive, avionics, telecommunications industries and the military.

There are three basic configurations of a current transformer:

  • Toroidal Core: Measure currents from 50 to 5,000 amps, with core opening from 1 to 8 inches in diameter. This type does not contain primary windings. However, the line that carries the current is threaded through the center hole in the transformer.
  • Split Core: Measure currents from 100 to 5,000 amps with a core opening from 1 to 13 inches in diameter. The split core has one end removable so that the load conductor does not have to be disconnected to install the current transformer.
  • Wound Primary: Measure currents from 1 amp to 100 amps, since the load current passes through the primary windings in the CT.

A current transformer is the same as a power transformer except the primary is placed in series with a conductor that carries a high current AC. This type of transformer consists of only a very few turns as its primary winding. The primary winding can be a single turn of heavy-duty wire wrapped around the core. The secondary of the current transformer is generally a ratio with comparison to the primary. The secondary can consist of a large number of turns wound on a low-loss magnetic core, depending upon how much the current is stepped down and it is generally rated a 1 to 5 amps [see Figure 1].

Current transformers can step down current levels from thousands of amperes to a known ratio. The primary and secondary currents are expressed in a ratio such as 100:5. This means that for 100 amps flowing in the primary conductor, the secondary will indicate (flow) 5 amperes of current. Or, for a rating of 500:1, there would be 500 amperes flowing in the primary, and 1 ampere with flow in the secondary.

Figure 1: Basic Current Transformer Construction
Figure 1: Basic Current Transformer Construction

Three Phase Current Transformer

This type of transformer is, in essence, three interconnected single phase transformers in a single housing – accomplished by either using a single “3 phase core” or three individual toroidal cores. Figure 2 shows an example of the three phase current transformer.

Figure 2: Three Phase Current
Figure 2: Three Phase Current

Accuracy for current transformers – as well as measurement accuracy – are listed in IEC 61869-1, Classes 0.1, 0.2s, 0.2, 0.5, 0.5s, 1 and 3. The reason there is a class designation is to categorize the accuracy of the current transformer. For example, the error of the primary to secondary current for a Class 1 current transformer is + 1% at the full rated current, the error of a Class 0.5 current transformer is + 0.5%, etc. The “s” after the class designation indicates high accuracy and is generally used in tariff metering situations. The other parameter to consider is errors in phase, which are also described in each Class rating.

Other considerations in measurement accuracy are: loads, external electromagnetic fields, phase change, capacitive coupling between primary and secondary, resistance between primary and secondary, temperature, burden, and core magnetizing current.

Current transformers are intended to be proportional devices. Therefore, the secondary winding should never be in an open condition, which could damage the device.


A current transformer converts high primary currents into a low-current secondary value through the use of magnetic cores. Current transformers can be a non-invasive way to monitor high currents in the power industry, instrumentation in avionics, automotive, military and the telecommunications industries.

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