Current Transformer Fundamentals

Current Transformers (CTs) are commonly utilized for current monitoring purposes or for the conversion of primary current to reduced secondary current, which can be utilized by meters, relays, control equipment, and other instruments. CTs, which function as current transformers, reducing the magnitude of the measured current to a standardized value that is safely compatible with the instrument being used. The classification of current transformers is based on several important characteristics, which must be taken into consideration to determine the most suitable CT for a specific application.

CT Ratio

The primary current input and secondary current output at full load are compared using the CT ratio. When fully loaded, a CT with a ratio of 400:5 can handle 400 main amps and produce 5 amps of secondary current flow. The secondary current production will fluctuate according to any variations in primary current. For instance, the secondary current output will be 2.5 amps (200:400 = 2.5:5) if the primary current flow is lowered to 200 amps.


A CT’s polarity is determined by the way its coils are wound around its core (clockwise or counterclockwise), as well as how any leads are removed from the transformer box. To ease appropriate installation, all current transformers have the following designations:

  • (P1/H1) primary current, line-facing
  • direction(P2/H2) primary current, load-facing
  • direction(X1) secondary current

All current transformers also have subtractive polarity. While installing and connecting current transformers to power metering and safety relays, it is essential to maintain proper polarity.

Accuracy Class

The metric known as Accuracy Class describes how well a CT performs and establishes the maximum load that can be placed on its secondary. According to their accuracy class, CTs can be categorized as

  • Metering Accuracy CTs
  • Protection Accuracy CTs (Relaying CTs)

Occasionally a CT may have ratings for both groups.

Metering Accuracy In order to deliver measurements that are as accurate as possible, CTs are rated for certain standard burdens that range from very low to the maximum current rating of the CT. Utility companies frequently use these CTs to measure usage for billing purposes due to their high level of precision.

The five accuracy classifications for metering CTs specified by the IEC standard are 0.1, 0.2, 0.2S, 0.5,0.5S 1, 3 and 5.The accuracy class of a CT indicates the maximum permitted inaccuracy of the CT at the rated primary current and is stated as a percentage of the rated primary current.

What it represent by the 100/5 class 0.5 5VA metering CT?

  • At the rated primary current of 100A the secondary current would be 5A.
  • CT would have a maximum permitted error of 0.5% which is 0.5A at the rated primary current
  • Rated burden of the CT is 5VA, which indicates the maximum burden that can be connected to the secondary winding of the CT without exceeding the specified accuracy class.

Does it require to consider the current measuring range of CT?

According to standards, the declared accuracy of a CT can only be guaranteed between 100% to 120% of the rated primary current. If the actual primary current is less than 100%, the error increases significantly, and at 20% of primary current, the error doubles.

Even with special class 0.2S and Class 0.5S CTs, the declared accuracy of the CT can only be guaranteed between 20% to 120% of the rated primary current. If the primary current is less than 20%, the error increases significantly.

Accordingly when choosing the rated primary current of a current transformer (CT), it is important to consider the above mentioned fact that CT’s primary rated current should be selected as closely as possible to the anticipated actual load current at the installation location. Failure to choose the proper CT primary current may result in a very low percentage of the actual primary current relative to the rated primary current, leading to increased CT error.

Rating Factor of the CT

The rating factor of a Current Transformer (CT) refers to the multiples of the rated current that the CT can handle while still maintaining its accuracy. Typical rating factors for CTs are 1, 1.5, 2, 3, and 4. For example, a 1000/5A CT with a rating factor of 2 can accurately measure up to 2000A.

Rating factor is the maximum multiple of the rated current that the CT can measure accurately. It is an important specification to consider when selecting a CT for a particular application, especially when the system experiences occasional overloads or short-duration fault currents.

Protection Accuracy CTs, in comparison, are not as precise as Metering Accuracy CTs, but they are made to function quite accurately over a wider range of current. These CTs are commonly used to provide current to protective relays, and because of their wider current range, the protective relay can operate at various fault levels.

For protection CTs, which are used for protective relaying, the IEC 61869-2 standard also specifies accuracy classes. The accuracy limit factor (ALF), which is the proportion of the rated burden to the rated secondary current, is commonly used to grade protection CTs as 5,10,15,20 and 30. The limiting value of the ALF is used to express the accuracy class for protection CTs.

What it represent by a 100/5, 5P20 15VA protection CT?

  • At the rated primary current of 100A the secondary current would be 5A.

  • The letter P represents the protection class, and the numbers 5 and 20 represent the maximum percentage error allowed by the CT under different conditions.
  • Accuracy Limit Factor ALF of 20 can sense the current with its rated accuracy when 20 times the primary rated current flows through the CT.
  • The number “5” indicates the accuracy class of the CT which is the maximum allowable composite error of 5% when the current flowing through the CT is 20 times the rated primary current. The composite error takes into account both the ratio and phase angle errors of the CT, and is expressed as a percentage of the rated primary current.
  • 15VA indicates the maximum burden that can be connected to the secondary of the CT without exceeding the specified accuracy.

What is Class X/Class PS / Class PX CT?

Class X Current Transformer, or just Class X, is another name for PS Class CT. Current transformers (CTs) of classes X and PX are frequently employed with high impedance relays. Class X CTs are suitable for applications where accuracy is crucial, such as protective relaying, metering, and revenue metering, because to their high accuracy rating and low phase angle error. Because to their high saturation tolerance and minimal phase angle error, Class PX CTs are the best choice for high fault current applications, such as high-voltage power systems.

A protective relay that operates with a high input impedance—typically in the range of several hundred kilo- to mega-ohms—is referred to as a high impedance relay. The relay can accurately and dependably detect low-level fault currents thanks to its high impedance, even if the current is below the system’s nominal current rating.

In addition to the CT class, the selection of a CT for use with a high impedance relay is also based on the CT ratio, burden, and accuracy class. The CT ratio is chosen to match the nominal current rating of the system, and the burden is selected to match the impedance of the relay input circuit. The accuracy class of the CT is selected to ensure that the CT accurately measures the current over the entire range of current levels expected in the system.

CT Saturation

When the primary current running through a current transformer (CT) is so high that the magnetic core of the CT becomes magnetically saturated, this state is known as CT saturation. When this occurs, the magnetic field in the core can no longer grow, and the output current of the CT deviates from being proportionate to the primary current and becomes distorted. Protective relays and other devices that depend on CT output for precise current detection and measurement may experience mistakes due to CT saturation. To guarantee accurate measurement and safety, it is crucial to choose CTs with the proper saturation properties for the anticipated levels of primary current.

What is CT Burden?

The impedance that a current transformer’s secondary winding is subjected to is referred to as the CT burden. It is the total load that can be placed on the CT’s secondary circuit without causing the accuracy of the CT to degrade beyond its specified limits. CT load can be stated as a volt-ampere (VA) rating or as an impedance in ohms.

These steps must be taken in order to determine a device’s burden when it is connected to a CT:

  • Determine the burden of the device linked to the CT in VA or ohms impedance. This information is often given on the data sheet for the device.
  • Add the impedance of the secondary wire run. Measure the length of the wire between the current transformer and the weight (i.e. meter, relay, etc.).
  • Use a table to calculate the resistance of the wires connecting the secondary of the current transformer to the object, expressed in ohms or VA.
  • Make sure the total burden does not exceed the specified limits for the CT. The CT data sheet will offer the maximum permissible burden for the given CT. The accuracy of the CT could be impacted if the overall burden is greater than this amount.

The two elements that make up a CT’s overall impedance are as follows:

  • Lead wires of the CT’s resistance
  • Amount of resistance in the CT-connected meter, relay, or other device

Let’s consider a CT installation and measure the actual burden that occurs,

  • Digital Meters: Power Analyzers, Energy Meters, Over Current Relays < 0.3VA
  • Analog Ammeters < 0.5VA

Sample Calculation:

Let’s analyze what would be the total burden if a 1000/5 metering CT is connected with an Analog Meter,

Consider that 14 AWG wire which is having 2.5mm2 is used to connect the CT and the Analog Meter where the total length of the wire is 10m,

Resistance of 2.5mm2 wire = 0.0074Ω/m

Consider that CT primary connected Bus bar is loaded with 800A, where 4A are flowing through the CT secondary,

Total Burden = Burden at Cable + Burden at Analog Meter

                          = 0.0074*10*4² VA + 0.5VA = 1.684VA

It is apparent that 5VA-rated Current Transformers (CTs) can be utilized for any panel metering applications. However, it is crucial to ensure that all connections are properly fastened and that there are no loose connections. The secure connection of these components is essential to prevent increased resistance and impedance, which may cause measurement inaccuracies and lead to output current errors.


The following table indicates the burden, resistance and approximate wire length that can have for the secondary wiring. The secondary current is considered as 5A. Lead length referred to the total length of the wire (CT in and out).

Sample Calculation:

Burden = 8VA

Secondary Current = 5A

P = I²R

R = P/I²

R = 8/5² = 0.32

Factors to be considered in CT installation

  • Before installing the CT, check its mechanical and physical state.
  • Before attaching the CT, verify the connection requirements for the transformer for the instrument or the system.
  • Check that there is sufficient clearance between the primary and secondary circuitry wiring between the CT phases, the ground, and the secondary conductor.
  • Confirm that the shorting device on the CT is connected correctly until the CT is ready to be installed. When not in use, the secondary of the CT must always be attached to a burden (load). NOTE: An open-circuited secondary can result in a dangerously high secondary voltage.

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