The accuracy class of a current transformer defines how accurately it reproduces the primary current in the secondary circuit. It is a key selection factor because different applications require different levels of precision. Choosing an accuracy class that is too low can lead to measurement errors, billing disputes, or unreliable protection. Choosing a class that is unnecessarily high increases cost and may not provide practical benefits.
For general measurement, class 1.0 or 0.5 is often used. Class 0.5 means the CT error is within ±0.5% at rated conditions. This is suitable for many industrial monitoring and power measurement applications. Class 1.0 is acceptable for less critical measurements where a small error is tolerable. These classes are common in panel meters, controllers, and general-purpose monitoring systems.
For energy metering, higher accuracy is usually required. Class 0.5S and 0.2S are common in billing and utility metering. The "S" indicates that the CT maintains accuracy over a wider current range, including low currents. This is important because energy meters must accurately measure light loads as well as heavy loads. Class 0.2S is more accurate than 0.5S and is often used for high-value billing or precision energy management.
For protection applications, accuracy class is expressed differently. Classes such as 5P10, 5P20, and 10P10 are common. The first number indicates the composite error at the accuracy limit factor. For example, 5P means a 5% composite error. The second number, such as 10 or 20, is the accuracy limit factor. A 5P20 CT can maintain its specified error up to 20 times rated current. Protection CTs are not usually chosen for low phase error at normal load; they are chosen for reliable performance during faults.
Phase displacement is another important consideration. Even if the current magnitude is accurate, a phase shift between primary and secondary current can cause errors in power and energy measurement. For power measurement, low phase displacement is essential. Metering CTs are designed to have small phase errors, while protection CTs may have larger phase errors because they are mainly concerned with fault detection.
Burden affects accuracy. A CT is calibrated at a specific burden, often 25% or 100% of rated burden. If the actual burden is higher than the rated burden, the CT may saturate and lose accuracy. If the burden is much lower, accuracy may improve, but the CT may not operate at its ideal point. The connected meter, relay, terminals, and cables must all be included in the burden calculation.
Environmental and installation conditions can also affect accuracy. Temperature, vibration, and magnetic interference may influence performance. The CT should be installed with correct polarity and grounding. The secondary circuit must never be opened under load. After installation, calibration and testing may be required to confirm accuracy.
In conclusion, choose the accuracy class according to the application. Use 0.5 or 1.0 for general measurement, 0.2S or 0.5S for energy metering, and 5P or 10P classes for protection. Consider phase error, burden, and operating range. The right accuracy class provides the needed precision without unnecessary cost.