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Current Transformer | Selecting and Using Transducers Series: For the Measurement of Electric, Power, Voltage, and Current

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May 24, 2016

For AC (alternating current) applications, most manufacturers’ transducers will not accept direct current input over 20 amperes. For higher amperages, current transformers are utilized.

What is a current transformer? These transformers are most often the ‘donut’ looking type. The current carrying conductor is passed through the opening or window of the ‘donut’. The secondary winding of the current transformer is wound by the manufacturer on the toroidal iron core which makes the ‘donut’ shape.

On most North American manufactured current transformers, the secondary is wound to produce 5 amperes when rated current is passed through the window. The turn’s ratio is expressed as 100:5 or 3,500:5. The first number represents the rated full-scale primary current. The primary windings consist of the single pass of the current carrying conductor through the window. The second number represents the full-scale secondary current in amperes. A 3,500:5 ratio current transformer steps the current from 3,500 amperes down to 5 amperes.

CTY Current TransformerCTI Current Transformer21279 Current Transformer

Important: As with potential transformers, only use instrument grade, or metering class, current transformers with power measuring transducers for the best accuracy.

Connect the loads on current transformers in series being careful not to exceed the burden rating. The phase angle shift introduced by current transformers is sensitive to the loading. Therefore, keep the burden to a minimum by using adequately sized secondary leads, and keeping the secondary leads as short as possible.

CAUTION: Current transformers can and will develop a lethal voltage and will possibly self destruct if the secondary is open when primary current is present. People have been hurt, and equipment has been damaged, when the secondary winding of a current transformer is open. Never disconnect the secondary or leave it open when there is the possibility of primary current.

It is essential that experienced persons install current transformers. If you must make a connection to the current transformer while it is in use, short the secondary winding before doing anything. Some current transformers have a shorting block for this purpose. Auxiliary shorting blocks are available as well.

Current transformers are rated for the voltage class for which they are to be used. These classes are: 600 volts, 5,000 volts, 8,700 volts, 15 kilovolts, 25 kilovolts, and 34.5 kilovolts. Make certain that the current transformers are rated for the voltage with which they are working or that the conductor is insulated for the class voltage. Current transformers being used on conductors with voltages greater than 600 volts must have the secondary grounded to an earth ground.

Precision AC Current Transformer`PC5 Watt Transducer


Interested in reading the entire Selecting and Using Transducers for the Measurement of Electric, Power, Voltage, and Current document? (Click here)


Need more help selecting the best and most accurate power measurement tool for your project? Call Ohio Semitronics, Inc. today, 1-800-537-6732
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