What Do the 5P10 and 5P20 Mean in a Current Transformer?

Electrical Technology

What Do the 5P20 and 5P10 Accuracy Classes Mean in Current Transformers?

For a current transformer (CT), different accuracy classes, such as 5P10, 5P20, or 10P10, are chosen based on the specific requirements and accuracy needs of the protection scheme in use. This accuracy class is suitable for many protective applications in electrical systems such as metering, protective, and general monitoring or control systems.

Marking of Accuracy Class in CT

Generally, the accuracy class notation is printed on a transformer nameplate right after the power rating of the transformer e.g. 15VA 5P10, 45VA 5P20 etc.

For example, a 45VA, 5P10 protection current transformer (CT) with

Rated Primary Current = 400 A

Rated Secondary Current = 5 A

45VA ➡️ Accuracy Power rating in volta-ampere
5P ➡️ Accuracy Class
10 ➡️ Accuracy limit factor

Its accuracy load P_n = 45 VA

Its accuracy class = 5P

Its accuracy limit factor (ALF) = 10

For I = ALF, I_n, is accuracy is 5% (5P).

What Does ‘5P20’ Mean in CT?

In current transformers (CTs), the notation “5P20” refers to the accuracy class and the knee point voltage of the CT. Let’s break down what each part of this notation means:

Number 5 (Composite Error): This number represents the accuracy class of the current transformer. In the case of “5,” it corresponds to an accuracy class of 5. Current transformers are classified into various accuracy classes based on their ability to accurately reproduce the primary current in the secondary circuit. A lower number indicates higher accuracy. Accuracy class 5 is generally used for applications where high accuracy is not critical, such as in monitoring or protection where a slight error in measurement is acceptable.
Letter P (Protection Class): The letter “P” in the notation represents the “Protection class.” It signifies that this particular current transformer is primarily designed for protective applications, like overcurrent or short-circuit protection.
Number 20 (Accuracy Limit Factor): This number represents the knee point voltage of the CT. The knee point voltage is the voltage level at which the CT begins to saturate, meaning it can no longer accurately represent the primary current. In the case of “20,” the knee point voltage is 20 times the rated secondary voltage. So, if the secondary voltage of the CT is, for example, 5 volts, then the knee point voltage would be 100 volts.

The 5P20 marking on a current transformer indicates that if the primary fault current is 20 times the rated current, the error will be 5%.

For example, suppose a CT has a current ratio of 1000/10. In the case of a fault:

The fault current in primary will be

1000 x 20 = 2kA.

fault current in secondary:

10 x 20 = 200 A ±5%

It clearly shows that the knee point is very high in the case of 20,000 amperes. Even during high fault currents, accuracy is not as important as ensuring that the CT operates for protection without saturating the circuit.

“5P20” represent that the current transformer has an accuracy class of 5, is primarily designed for protective applications, and has a knee point voltage 20 times the rated secondary voltage. This information is important for selecting the appropriate current transformer for specific applications. It ensures that it can provide accurate and reliable current measurements under specified conditions.

What is Meant by 5P10 in CT?

When the fault current does not exceed 10 times the rated nominal current on the primary side, the CT will exhibit an error of ±5%, provided that the rated accuracy limit factor is 10.

5: This number indicates the accuracy of the CT. When the current flowing through the CT is 20 times the rated primary current, the CT reads within a composite error of 5%.
P: This letter indicates that it is a protection class CT. Protection class CTs are designed to carry fault currents and maintain their accuracy during fault conditions.
10: This number, also known as the Accuracy Limit Factor (ALF), indicates the magnitude of fault current that can safely flow through the primary of the CT without saturating it. For instance, if a CT has a ratio of 800/5 amperes, it can handle a current up to 10 x 800 = 8 kilo-Amperes.

Let’s see another example, a current transformer with a 5P10 accuracy class and a primary-to-secondary ratio of 200/5 will operate correctly and provide circuit protection without saturating as long as neither the primary nor secondary current exceeds 2000 amps and 50 amps, respectively.

So, a 5P10 CT is a protection class current transformer that will maintain its accuracy within 5% when up to 10 times its rated primary current flows through it. If the current exceeds this, the CT will saturate.

Accuracy Limit Factor:
- 5P10: This means that the CT has an accuracy limit factor of 5, indicating it is designed to be accurate within 5% of the rated current under specified conditions. This accuracy class is suitable for applications where a high degree of accuracy is required.
- 5P20: In contrast, the “5P20” CT has an accuracy limit factor of 5 as well, but it is designed to be accurate within 20% of the rated current under specified conditions. This accuracy class offers less precision compared to “5P10.”
Intended Applications:
- 5P10: A “5P10” CT is often used for applications where precise current measurement is crucial, especially in protection and metering systems where a high level of accuracy is required. It is suitable for situations where slight variations in current measurement could have significant consequences.
- 5P20: A “5P20” CT is typically used in applications where a lower degree of precision is acceptable, such as in some general monitoring or control systems. It may not be suitable for applications that require highly accurate current measurements.