Current Transformer (TC).

Do you know what a current transformer is and what is it for? Here you will learn what a current transformer is, what it is for, what are the most used types, their respective characteristics, and symbols that facilitate when using the equipment. Come on, guys!

What is the current transformer?

The current transformer is a device that assists the measuring and protection instruments so that they can function properly and safely, without the need for the nominal current to be the same as that required for the load to function.

Characteristics of a current transformer.

The current transformers (CTs) most used are those of nominal current, where their primary windings have a smaller number of turns, while on the secondary they have a greater number of windings, but with thinner wires.

There are several symbologies used to identify the current relationships, where the primary current is always the load current of the circuit and the secondary current can always change according to some cases and applications of the transformer, facilitating the construction of the measuring equipment, being scaled in reduced sizes and compatible with the secondary current ratio.

Current transformer, bar type.

The initials P1, P2, S1 and S2 are used to identify the inputs and outputs of their connection terminals according to the construction characteristics of the equipment, these nomenclatures are generally used in measurement transformers, power relays, etc.

Most CTs are of subtractive polarity which is where the current travels from the primary to the secondary, and the wave of the secondary current has the same path from S1 to S2, if it occurs differently it is considered as additive polarity, but in this case only occurs when the transformer is made to measure, as the NBR-6856 standard itself indicates that current transformers with subtractive polarity are used.

As there are several types of jobs that require the current transformer, it was necessary to build several models to meet the needs of each activity where they would be employed, only 3 CT models most used will be cited, explaining their constructive characteristics and where are employed.

Current transformers

Tc coiled type

This transformer is used in measurements where the current is less than the 200A to 5A ratio. Its internal constitution is very simple, composed of a primary winding with few turns, the amount of wires needed to support the load current is a significantly larger amount in the winding of the secondary and with the finer turns, sufficient to support the measuring equipment.

Tc type window.

The construction of this transformer is very similar to the CT type bushing, the difference is that its isolation medium between the primary and secondary coils is made through air, the interesting thing about this transformer is that it does not have a fixed primary coil, as this The model consists of an opening in the core, where the conductor to be measured is passed, thus the same conductor becomes the primary circuit.

CT bar type.

The current bar type transformer has the main characteristic of making low voltage measurements, with the highlight of reducing relatively high currents in a low secondary current that will be transmitted in such a way that, in addition to isolating the circuit from the measurement network, it can offer conditions to monitor the circuit through an instrument.


To facilitate when defining the current relationships, some symbologies were adopted, a method stipulated by the NBR-6856 standard.

Colon (:) – Used to express nominal relationships Ex: 300: 5.

The hyphen (-) – Used to separate currents where they have rated currents in different windings, Ex .: 300-5A, 300-300-5A (two primary windings), 300-5-5 (two secondary windings).

X: It is assigned when it is necessary to separate primary and nominal currents 300 × 600-5A where the windings can be connected in series or parallel.

Bar (/) – Used to separate rated primary currents that have been obtained by tapping the primary or secondary winding 300 / 400-5A

The great advantage of using current transformers is being able to use compact measuring devices, in addition to ensuring safety, being able to carry out this work at any time and at a distance, using only programs that can make the necessary conversions, knowing the current that passes in each phase, thus knowing if the circuit has an imbalance problem.