Current transformers are devices which converts alternating current that is passing through primary winding to secondary winding by the conversion ratio, so that high currents can be measured by electronic devices.

Although current devices may be classified by connection and core type as isolator, bus-bar, support etc., in this document they will be classified only by their electrical specifications.


On the current transformers there is label that defines almost every parameter of the device. For example, 400//5A, 5P20; 10VA.

A. Transformer Ratio

As expected the first parameter is the transformer conversion ratio. For example, at the given sample label the conversion ratio is 400/5A.This means that, according to IEC 60044-1 standard, only one ampere will flow through secondary windings when primary flow is 80A. Likewise 5A will flow from secondary when primary is 400A.

400/5A, 5P20; 10VA
400 - Primary Current
5A - Secondary Current

As required, secondary current of transformers could be either 5A or 1A.

B. Transformer Type

Current transformers are either used for measurement or protection purpose. While high accuracy is required in measurement transformers, a saturation limit is the main factor in protection transformers. When there is an overcurrent, protection transformer should saturate and protect the connected device to its secondary winding.

On the label, it is possible to understand whether it is a measurement or protection type with the “P” and “Fs” codes.

400/5A, 5P20; 10VA
5P - Transformer type
20 - Overcurrent factor

Transformer type continues with a number after “P” or “Fs”. This number is called the current saturation factor. Type and saturation factor together mentions the Security Factor of the transformer.

Saturation factor determines the limit current that the transformer may operate while providing the requested accuracy, where all other conditions are met. The maximum current of the sample transformer is 8kA because the factor is 20 and the nominal primary current is 400A (400 x 20).

C. Accuracy Class

Current transferred from primary winding to secondary windings will always have an error. The error occurs both in amplitude and phase angle. Accuracy classes defers by the transformer type (measurement or protection).

At measurement transformers 0.1, 0.2s, 0.2, 0.5, 0.5s, 1 and 3 classes are used according to IEC 61869-1. Those numbers are the error rate when the transformer operated at 100% power capacity. For example, a current transformer with 100A primary current, will flow a current between 4.95A and 5.05A from its secondary windings while 100A is flowing through the primary windings.

The protection type transformers are categorised two accuracy according to IEC 60044-1 which are 5P ve 10P. The accuracy details of these kind of transformers are defined in table below.

Accuracy Class Error in current amplitude (%) Error in phase angle Combined error value
minute crad
5P ±1 ±60 ±1.8 5
10P ±3 - - 10

Transformers with lower accuracy value class can be use as a replacement with higher accuracy value. This means that a Cl 0.5 transformer could be used instead of a Cl 1 transformer.

D. Power

Although it’s commonly discarded one of the most important characteristics of a transformer is its power.

All devices that are connected to a current transformer will consume some power while making the measurement. In addition to the devices there will be a loss at the cable regarding the length and type of the cable.

Selected transformer’s power should be the minimum available power that can meet the power requirements of connected devices and cable losses. Selecting a higher power transformer will reduce the accuracy.

Secondary Current5 A
Cable cross-section2.50 mm2
Cable unit resistance56.00 Ω/mm2
Total Cable Length15.00 m
Measurement Device Self-Consumption3.00 VA
Cable Resistance0.21 Ω
Loss at Cables5.36 VA
Total Losses8.36 VA
Selected Transformer Power10.00 VA


[1] Dr. Bora Alboyacı, “Enerji Dağıtımı Sunumu”

Emre Sami Süzer - Operations Director - Aktif Mühendislik