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

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• 26 Aug 2024

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

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 Pn = 45 VA

Its accuracy class = 5P

Its accuracy limit factor (ALF) = 10

For I = ALF, In, 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:

1. ### Number 5 (Composite Error):

2.  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.
3. ### Letter P (Protection Class):

4. 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.
5. ### Number 20 (Accuracy Limit Factor):

6.  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.

## Difference Between “5P20” and “5P10”

1. ### 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.”
2. ### 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.

## FAQ

### What is 5P in a current transformer?

The first figure of the accuracy class designation 5P.. or 10P.. defines the maximum value of the composite error in %, to which the transformer must satisfy in the overload area. The letter P means Protection. The rated accuracy limit factor of the transformer is stated directly after the P of the class designation

### What is the difference between 5P10 and PS class CT?

The protection class CTs 5P10,5P20,10P10 are used for overcurrent protection in feeders. Here, note that in differential protection, PS class CT is used. Never use a protection class CT for differential protection. Let us understand why protection class CT is unsuitable for differential protection

### What is 5P20 Class CT error?

A 15 VA-5P20 CT has a guaranteed error of less than 5 % when it is subjected to 20 times its nominal current and delivers into its nominal load (15 VA to In). Each of the characteristics b, c, d is a function of the two others

### What is 10P10 CT?

Example – CT with class 10P10 – Where first letter 10P indicates the maximum(10%) percentage error & last number 10 indicates the number of times the rated current

### What is 5P10 and 5P20?

In the context of current transformers, the terms 5P10 and 5P20 refer to accuracy classes that specify the instrument’s accuracy under certain conditions. 5P10: This accuracy class signifies that the current transformer has an accuracy of 5% for primary current values between 10% and 100% of its rated capacity

### What does 5P20 mean?

The 5P20 can be explained as below: P = Protection Class. 5 = Accuracy Number in percentage. It means the maximum permissible error will be 5% 20 = Accuracy limit factor indicates that the CT will sense the current with the specified accuracy even with 20 times of its secondary current flows in the secondary

### What is CT rating 5P10?

For example, 5P10 transformer accuracy limit factor is 10. This means than current transformer will measure with less than 5% error, when primary current is not bigger than 10 times the rated nominal current. However, 10 for this transformer is rated accuracy limit factor

### What is .2 and .2S class CT?

0.2 and 0.2S refers to the accuracy of the Current Transformer. A 0.2S CT has a much higher accuracy than 0.2. For Accuracy Class 0.2, the declared accuracy is guaranteed from 5% loading. However the accuracy deteriorates under lower load conditions and below this load, the error is not guaranteed

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