When designing or analyzing an electrical measurement and protection system, one of the most important aspects to understand is the accuracy class of Current Transformers (CTs) and Potential Transformers (PTs). These instrument transformers are essential for stepping down high voltages and currents to safe and measurable levels. But how accurately they perform that job depends on their accuracy class.
In this article, we’ll decode the meaning of CT PT accuracy class, dive into applicable instrument transformer accuracy standards, compare Class 1 vs Class 0.5, and provide real-world examples including burden calculations. We’ll also guide you on selecting the right CT/PT for metering vs protection.
What is Accuracy Class?
The accuracy class of an instrument transformer indicates how closely the secondary output (current or voltage) matches the scaled-down version of the primary quantity under specified conditions of burden and frequency.
In simple terms, it defines how much error is permitted in the transformer’s output:
- For Current Transformers (CTs): It is the maximum allowed composite error in percentage under rated conditions.
- For Potential Transformers (PTs): It is the ratio error and phase displacement under specified burden and voltage.
Example: A CT of Class 1 means that the secondary current may deviate by up to ±1% from its ideal value at rated current and burden.
Accuracy Standards for CTs and PTs
Instrument transformer accuracy is governed by national and international standards. The most widely followed standards are:
- IEC 61869 series (replaces IEC 60044): International standard for both CTs and PTs.
- IS 16227 (India): Mirrors IEC 61869, applicable to instrument transformers.
- IS 2705: Indian standard for CTs.
- IS 3156: Indian standard for PTs.
These standards define accuracy classes, burden ranges, accuracy limit factors, and testing conditions.
Understanding Accuracy Classes of CTs and PTs
Let’s break down what different accuracy classes mean for Current Transformers and Potential Transformers.
Current Transformers (CTs)
| Class | Application | Accuracy |
|---|---|---|
| 0.1 | Precision metering | ±0.1% |
| 0.2 | High-precision metering | ±0.2% |
| 0.5 | Standard metering | ±0.5% |
| 1 | General metering | ±1% |
| 3 | Low-accuracy metering | ±3% |
| 5P | Protection class | ±1% up to ALF |
| 10P | Protection class | ±3% up to ALF |
Key Definitions:
- Burden: The load (in VA) connected to the CT secondary, including meters, wiring, and relay input impedance.
- Accuracy Limit Factor (ALF): For protection CTs, it’s the multiple of rated current up to which the CT maintains accuracy. A 5P10 CT means 5% error at 10 times rated current.
Potential Transformers (PTs)
| Class | Application | Ratio Error | Phase Displacement |
|---|---|---|---|
| 0.1 | Precision metering | ±0.1% | 10 minutes max |
| 0.2 | High-accuracy metering | ±0.2% | 15 minutes |
| 0.5 | Standard metering | ±0.5% | 30 minutes |
| 1 | General metering | ±1% | 60 minutes |
| 3P | Protection PT | ±3% | 120 minutes |
| 6P | Protection PT | ±6% | 240 minutes |
Note: 1 minute of angle = 1/60 of a degree.
Class 1 vs Class 0.5: What’s the Difference?
The difference between Class 1 and Class 0.5 is in permissible error:
- Class 1 CT allows ±1% error.
- Class 0.5 CT allows ±0.5% error.
When to use Class 0.5?
- In billing and energy accounting systems.
- Where revenue-grade metering is required.
When to use Class 1?
- For general monitoring and system analysis.
- When high accuracy is not critical.
Protection CT vs Metering CT
These are fundamentally different in function and design.
| Feature | Metering CT | Protection CT |
|---|---|---|
| Primary Purpose | Accuracy at normal load | Accuracy at high faults |
| Accuracy Class | 0.1, 0.2, 0.5, 1 | 5P, 10P, PX, PR, PS |
| Burden Sensitivity | High | Moderate |
| Saturation Point | Low | High (delays saturation) |
| Accuracy Limit Factor | Not specified | Specified (e.g., 5P10) |
| Application | Energy meters, displays | Overcurrent relays, trip units |
Example with Burden Calculation
Problem:
You are using a 5P10 CT rated at 100/5 A, with a burden of 15 VA. What is the maximum current it can handle without exceeding 5% error?
Solution:
- Rated secondary current = 5 A
- Accuracy Limit Factor (ALF) = 10
- Max primary current = 10 × 100 = 1000 A
- The CT must maintain ±5% accuracy up to 1000 A with a 15 VA burden.
So, if your relay and wiring present a combined burden ≤15 VA, your protection system will function accurately up to 10× rated current.
Selection Tips Based on Application
✅ For Metering:
- Choose Class 0.2 or 0.5 for energy billing.
- Ensure burden is within CT/PT rated burden.
- Use Class 0.1 PTs for precision lab-grade metering.
✅ For Protection:
- Use 5P10 or 10P10 CTs for overcurrent relays.
- Check that CT saturation point exceeds expected fault level.
- Use Class 3P or 6P PTs for voltage-based protection schemes.
✅ For Mixed Applications:
- In dual-core CTs, use one core for metering (e.g., 0.5 Class) and one for protection (e.g., 5P20).
- Use separate metering and protection PTs where accuracy is critical.
Conclusion
Understanding CT PT accuracy class is vital for ensuring the reliability and safety of metering and protection systems. Whether you’re comparing Class 1 vs Class 0.5, calculating burden, or choosing between protection CT vs metering CT, accuracy class plays a pivotal role in performance and compliance.
Selecting the correct instrument transformer accuracy isn’t just about numbers—it’s about ensuring accurate measurement under the worst-case conditions. With evolving smart grid systems, this attention to detail matters more than ever.
Before diving deeper into CT PT accuracy class, it’s helpful to understand the working principle and construction of Potential Transformers (PTs). You can explore that in our detailed article: Potential Transformers (PTs): Working Principle, Types, and Applications
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