Understanding metering and protection accuracy classes for current transformers according to IEC 61869-2 and IEEE
C57.13 standards
Understanding CT accuracy class explained is essential for selecting the right current transformer
for your application. The accuracy classes define how precisely a CT reproduces the primary current
in its secondary circuit, affecting measurement accuracy and protection reliability.
This guide covers both metering and protection accuracy classes according to IEC 61869-2 and IEEE
C57.13 standards, helping you choose the appropriate CT class for your specific requirements.
For example, a Class 0.5 CT has a maximum ratio error of ±0.5% at 100% rated current.
What is CT Accuracy Class?
CT accuracy class is a standardized rating that defines the permissible limits of ratio error and
phase displacement for a current transformer. The class determines how accurately the secondary current reflects the
primary current.
Two Main Categories
| Category | Purpose | Accuracy Priority |
|---|---|---|
| Metering Classes | Revenue/energy metering | Highest accuracy at normal currents |
| Protection Classes | Relay operation during faults | Accuracy maintained during overcurrents |
Metering Accuracy Classes (IEC 61869-2)
Metering accuracy classes are designed for precise current measurement under normal operating
conditions.
Standard Metering Classes
| Class | Ratio Error at 100% In | Phase Displacement | Application |
|---|---|---|---|
| 0.1 | ±0.1% | ±5 minutes | Precision laboratory |
| 0.2 | ±0.2% | ±10 minutes | Revenue metering |
| 0.2S | ±0.2% (extended) | ±10 minutes | Low-load revenue metering |
| 0.5 | ±0.5% | ±30 minutes | Sub-metering, industrial |
| 0.5S | ±0.5% (extended) | ±30 minutes | Variable load metering |
| 1.0 | ±1.0% | ±60 minutes | General indication |
| 3.0 | ±3.0% | Not specified | Approximate indication |
Error Limits at Different Currents (IEC 61869-2)
| Class | 1% In | 5% In | 20% In | 100% In | 120% In |
|---|---|---|---|---|---|
| 0.2 | — | ±0.75% | ±0.35% | ±0.2% | ±0.2% |
| 0.2S | ±0.75% | ±0.35% | ±0.2% | ±0.2% | ±0.2% |
| 0.5 | — | ±1.5% | ±0.75% | ±0.5% | ±0.5% |
| 0.5S | ±1.5% | ±0.75% | ±0.5% | ±0.5% | ±0.5% |
| 1.0 | — | ±3.0% | ±1.5% | ±1.0% | ±1.0% |
What Does “S” Mean in 0.2S and 0.5S?
The “S” designation (Special) indicates extended accuracy at low currents:
- Standard classes (0.2, 0.5): Accuracy guaranteed only from 5% to 120% of rated current
- Special classes (0.2S, 0.5S): Accuracy guaranteed from 1% to 120% of rated current
This makes “S” classes ideal for variable load applications where accurate measurement at low currents is
critical.
Protection Accuracy Classes (IEC 61869-2)
Protection accuracy classes ensure the CT provides reliable output during fault conditions when
currents may reach many times the rated value.
Standard Protection Classes
| Class | Composite Error Limit | ALF Options | Application |
|---|---|---|---|
| 5P | ±5% | 5, 10, 15, 20, 30 | General overcurrent protection |
| 10P | ±10% | 5, 10, 15, 20, 30 | Standard protection applications |
| PX | Specified by user | — | Differential, high-impedance |
Understanding 5P10, 5P20, 10P10, 10P20
The designation combines accuracy and Accuracy Limit Factor (ALF):
| Class | Meaning |
|---|---|
| 5P10 | ±5% error up to 10× rated current |
| 5P20 | ±5% error up to 20× rated current |
| 10P10 | ±10% error up to 10× rated current |
| 10P20 | ±10% error up to 20× rated current |
Example: A 100:5 CT rated 5P20 will maintain ±5% accuracy at currents up to 2000A (20 × 100A rated
primary).
IEEE C57.13 Accuracy Classes
The IEEE standard uses different designations for accuracy classes:
Metering Classes (IEEE)
| Class | Ratio Error Limit | Equivalent IEC |
|---|---|---|
| 0.3 | ±0.3% at 100% In | ~Class 0.5 |
| 0.6 | ±0.6% at 100% In | ~Class 1.0 |
| 1.2 | ±1.2% at 100% In | ~Class 3.0 |
Relay Classes (IEEE C57.13)
| Class | Meaning |
|---|---|
| C100 | Ratio error ≤10% up to 20× rated at 1Ω burden (100V) |
| C200 | Ratio error ≤10% up to 20× rated at 2Ω burden (200V) |
| C400 | Ratio error ≤10% up to 20× rated at 4Ω burden (400V) |
| C800 | Ratio error ≤10% up to 20× rated at 8Ω burden (800V) |
How to Select the Right Accuracy Class
For Revenue Metering
| Load Characteristic | Recommended Class |
|---|---|
| Constant high load | 0.2 or 0.5 |
| Variable load (factories) | 0.2S or 0.5S |
| Low power factor loads | 0.2S (lower phase error) |
For Protection Applications
| Protection Type | Recommended Class |
|---|---|
| Overcurrent (50/51) | 5P10 or 10P10 |
| Directional (67) | 5P10 |
| Differential (87) | 5P20 or PX |
| High-impedance | PX or TPX |
🏭 Factory Experience:Based on our CT manufacturing experience:
- Don’t Over-Specify: Many customers request 0.2S class when 0.5 would suffice.
Over-specification increases cost without benefit if load never operates below 5% rated current. - Match Application to Class: Use metering classes for revenue/billing. Use protection
classes for relays. Never use protection-class CTs for revenue metering. - Consider Extended Range: For installations with highly variable loads (solar plants, EV
chargers), the “S” classes provide significant accuracy improvement at low power levels. - ALF Selection: Choose ALF based on actual fault current levels. Over-specifying ALF (e.g.,
5P30 when 5P10 is sufficient) makes the CT larger and more expensive.
Frequently Asked Questions
What is CT accuracy class?
CT accuracy class is a standardized rating defining the maximum allowable ratio error and phase displacement. For
example, Class 0.5 means the CT’s ratio error does not exceed ±0.5% at rated current.
What is the difference between 0.5 and 0.5S class?
Class 0.5 guarantees accuracy from 5% to 120% of rated current, while 0.5S (Special) guarantees accuracy from 1% to
120%. The “S” class is more accurate at low currents.
What does 5P20 mean?
5P20 means the CT maintains ±5% composite error (the “5P”) at currents up to 20 times rated current (the “20”). This
is suitable for protection applications with high fault currents.
Can I use a metering CT for protection?
Metering CTs can become saturated during high fault currents, potentially causing relay malfunction. Always use
protection-class CTs for protection applications.
Which accuracy class is best for revenue metering?
Class 0.2 or 0.2S is recommended for revenue metering where accuracy directly affects billing. Class 0.5S is
acceptable for most industrial sub-metering applications.
Related Articles
- Current Transformer (CT):
Complete Guide - CT Ratio Calculation &
Selection - Distribution
Transformer Sizing Calculator - Single Phase Distribution
Transformer Guide
References
- IEC 61869-2 – Instrument Transformers: Current
Transformers - IEEE C57.13 – Standard Requirements for
Instrument Transformers - CT Accuracy Standards
– CTL Systems
Disclaimer: This information is provided for educational purposes. Always verify accuracy class requirements with
applicable standards and consult qualified engineers for specific applications.