What Exactly is a Current Transformer?

The basic principals and uses of Current Transformers

A current transformer (CT) is a device which falls under the broad category of instrumental transformers.  These transformers are ordinarily used to measure the current flowing through a load supplying conductor.

The working of Current Transformers

Current transformers, just like normal transformers, have a primary and a secondary winding. The primary winding, supplying a load, is normally a current carrying conductor consisting of one or two turns either wound around or passing through a hollow core. The secondary winding of the transformer consists of many turns supplying a small load or meter. The current produced in the secondary is thus directly proportional to the current flowing through the primary winding. This makes the device ideally suited to measure currents within a conductor while not subjecting the device to the load current.

[A schematic illustration of a Current Transformer. Image create by Author]

The current flowing in the secondary winding of a current transformer is usually specified and thus has a rated value. The magnitude of the current is normally 1 A or 5 A. The current flowing in the primary winding of the transformer is also normally rated at a set value. The current rating of the current transformer is given as: Ip/Is. Thus for a current transformer which carries a current of 100 A in the primary winding and a current of 5 A in the secondary winding will have this current rating (or CT Ratio): 100/5.

The current transformer thus has a set primary and secondary current rating and thus should have a set turns-ratio (N). The turns-ratio of an ideal transformer is defined by the following definition:

From the given definition it is clear that if the primary winding only consists of one turn that the secondary current will be inversely proportional to the number of secondary winding.


If we have a 100/5 current transformer we have:

Thus if NP is one turn, then the secondary winding needs 20 turns when the we solve for NS .

The direction and orientation of the secondary winding with regard to the primary will affect the direction of flow of the secondary current. It is thus of great importance that the CT be installed in the correct orientation as to prevent unintended damage to the CT or metering device.

The transformer secondary winding should never be open-circuited as a very large voltage will be developed across the secondary terminals which could electrocute personnel or damage equipment.


The Burden of a Current Transformer

The current transformer normally has a rating called its burden. The burden of a CT is the maximum load the CT can operate under without being damaged or losing accuracy. The burden is normally given as a value in ohm or VA.

Accuracy of a Current Transformer

The real current transformer does contain non-idealities just like its normal counterpart. These non-idealities thus affect the accuracy of the CT. The accuracy of the CT is normally given as a percentage and CT’s are divided into accuracy classes. Both the Accuracy and class of the CT should be displayed on its nameplate.

The Types of Current Transformers

Current transformers can in all shapes in sizes. The three main types of current transformers and their uses are listed below.

  • Bar-type Current Transformer: The bar-type CT is a type of current transformer where the primary winding is a bus bar which is directly connected in series with the current conducting element which is supplying a load. The bar-type CT thus only has one turn on the primary side as there is only one bus bar positioned inside the core of the CT. These transformers have high levels of insulation as they are directly bolted onto the current carrying element. Bar-type CT’s thus have fairly high current ratings.
  • Window-type Current Transformer: The window-type CT mainly consists of the core and a secondary winding without a primary conductor. The primary conductor supplying a load thus needs to be disconnected when installing the window-type CT and placed through the CT. This CT also has a lower voltage rating as compared to the bar-type CT. The primary winding thus needs to be highly insulated to avoid damaging the device. The construction of the window-type CT thus also allows the addition of additional turns to the primary winding if needed.
  • Wound-type Current Transformers: The wound-type CT’s have a primary and secondary winding wound a toroidal structure, similar to the construction of a traditional transformer. These CT’s are really used as they operate at much lower currents and ratings when compared to normal CT’s.

Switchman-Products offers a full range of Current Transformers, visit the website here for more details.

Article by: Jannes Smit, 3rd  year Electrical Engineering student at the University of the Witwatersrand.