What is a Current Transformer Used For?

A current transformer is used to reduce electrical current in an AC circuit to allow safe measurement and monitoring of power. It produces a lower current in the secondary coil that is in direct proportion to the current in the primary coil. This allows voltage to be measured with minimal risk. This guide will look at the principles and applications of current transformers.

What is a transformer?

A transformer is the simplest possible piece of electrical machinery, having no moving parts. Discovered in the early 19th century by Michael Faraday, in its simplest form it consists of two coils of insulated wire, known as primary and secondary windings, magnetically linked by means of a core. This core usually consists of iron or steel although other materials are also used depending on the function of the transformer. The primary winding is supplied with alternating current, this, in turn, sets up an alternating magnetic flux in the core, producing an alternating current in the secondary winding. The secondary voltage is proportional to the number of turns in each winding, for example, a transformer with 1000 turns on the primary winding and 10000 turns on the secondary winding will give 1000 volts on the secondary when 100 volts are applied to the primary. The transformer is the simplest method of converting one voltage to another for any purpose.

The Current Transformer

For high voltage work it is not convenient or safe to connect electronic equipment used for metering directly to the supply, therefore a current transformer is used to reduce the current to a more manageable level. This current is then safely read, usually in amperes. Current transformers can be broadly divided into three groups.

Toroidal Current Transformer

These are ring-shaped transformers with an insulated circular core around which are the wire windings. The voltage is measured by passing the cable carrying the current through the hole in the centre of the ring.

Wound Transformer

This is, in essence, the simplest type transformer described earlier, where the output of the secondary winding is proportional to the ratio of the two windings.

Bar Type Current Transformers

Similar to the toroidal transformer, instead of passing a cable through the centre of the ring, a bus bar is used as the primary conductor. This means the conductor is a permanent part of the circuit.

Purpose and Applications

Without the aid of a current transformer, modern power systems would be impossible. From overhead power cables with a colossal 400,000 volts to the tiniest electrical circuits, the humble current transformer is found nearly everywhere.
At the larger end of the scale, there are the current transformers found in power stations and electrical substations throughout the National Grid. These are used for recording and monitoring current and are part of the space age looking apparatus you see when passing an electrical substation.
On a smaller scale, you will also find a current transformer in your electricity meter at home. In this case, it will be measuring and recording usage of electricity to enable the supplier to produce an accurate bill.
Handheld current transformers are also now very common. These have a large clamp that closes around the cable giving an accurate reading of current without the need to dismantle whatever system you wish to measure.

Protective Current Transformers

In addition to their role as current reducers, current transformers can also be used as inputs for protective relays which will isolate the affected circuit if an abnormal or fluctuating current is detected.


As transformers are often dealing with very high voltages, safety is of high importance. They should only be installed by a qualified electrician and the manufacturer’s instructions should be adhered to.

Terms related to Current Transformers

Ratio-Refers to the turns ratio on the primary and secondary windings.
Rated Primary Current-The value of the current to be transformed.
Rated Secondary Current-The value of the current in the secondary winding that is to be read.

Burden-The burden is the maximum load that can be applied to the secondary winding. Accuracy Class-This defines the most suitable transformer for the task. Class 0.1 to 0.2 are for precision measurements, class 0.5 for kilowatt hour meters, class 3 for industrial measurements and class 5 for approximate measurements.

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