What Is a Current Transformer Accuracy Class?
A current transformer (CT) steps down high line currents to a measurable, safe level for meters, relays and instruments. Its accuracy class defines the maximum error the CT is allowed to introduce under specified conditions. Choosing the right accuracy class is essential: a metering application needs a small ratio error for billing accuracy, while a protection application needs predictable saturation behavior so that relays operate correctly under fault currents. Standards such as IEC 61869-2 and IEEE C57.13 define the class designations used worldwide.
Metering Accuracy Classes
Metering CTs are classified by the ratio error at rated current, expressed as a percentage:
| Class | Ratio Error at Rated Current | Typical Use |
|---|---|---|
| 0.1 / 0.1S | ±0.1% | Precision laboratory and calibration |
| 0.2 / 0.2S | ±0.2% | Revenue metering, high-accuracy measurement |
| 0.5 / 0.5S | ±0.5% | Commercial and industrial energy metering |
| 1 | ±1% | Indication and general measurement |
| 3 | ±3% | Rough indication, ammeters |
The suffix S (e.g., 0.2S) guarantees accuracy from 1% to 120% of rated current, which matters for meters that must stay accurate at light load.
Protection Accuracy Classes
Protection CTs are specified by the composite error and the accuracy limit factor (ALF). A class 5P10 means the composite error is no more than 5% at 10× the rated current. Common designations:
- 5P10 / 5P20 – composite error ≤5% at 10×/20× rated current; standard for overcurrent protection.
- 10P10 / 10P20 – composite error ≤10%; used where less precision is acceptable.
- PX and TP classes – for differential and distance protection where transient saturation behavior must be tightly controlled.
For protection duty, the knee point voltage and secondary excitation curve are as important as the class marking, because the CT must reproduce the fault current waveform long enough for the relay to operate without saturating prematurely.
Key Differences: Metering vs Protection CT
| Aspect | Metering CT | Protection CT |
|---|---|---|
| Priority | Accuracy at rated current | Accuracy and linearity at fault current |
| Class marking | 0.1/0.2/0.5/1/3 | 5P10, 5P20, 10P, PX, TP |
| Burden | Low, constant burden | Relay burden, may vary |
| Saturation | Saturates early by design to protect the meter | Must delay saturation to the ALF multiple |
| Cost | Optimized for small error at nominal current | Larger core, higher VA, transient capability |
How to Choose the Right Accuracy Class
- Identify the application: revenue metering, indication, or protection relay.
- For metering, select the class from the billing accuracy requirement (0.2S/0.5S for billing, 1 for indication).
- For protection, define the relay burden (VA) and the required ALF from the maximum expected fault current.
- Verify the CT’s rated secondary (1 A or 5 A) matches the meter/relay input.
- Check the knee point and excitation curve for differential applications.
Applications
- Revenue and sub-metering in commercial and industrial installations
- Overcurrent, earth-fault and differential protection in switchgear
- Energy management and power quality monitoring
- EV charging and renewable energy monitoring
- Miniature PCB and split-core CTs for retrofit metering
Current Transformers from Shaanxi SiRui Technology
Shaanxi SiRui Technology Co., Ltd supplies a wide range of current transformers – including mini PCB CTs, split-core CTs, zero-sequence CTs and custom CTs – with metering and protection accuracy classes tailored to your specification. Every CT is produced under an ISO9001:2015 quality system and complies with RoHS and REACH, verified by SGS and CTI. Share your rated current, burden and accuracy class requirements for a free sample and quotation.
FAQ
Q: Can a metering CT be used for protection?
A: Generally no. Metering CTs saturate at low multiples of rated current to protect instruments; protection CTs are designed to reproduce fault currents up to the ALF.
Q: What does 5P20 mean?
A: The CT has a composite error of no more than 5% up to 20 times its rated current. Higher ALF suits larger expected fault currents.
Q: Does a higher accuracy class always cost more?
A: Yes. Tight classes need more core material and tighter manufacturing control. Select the class that just meets your requirement to control cost.
Related Articles
- Current Transformer: Working Principle, Types & Selection Guide
- Split-Core Current Transformer: How It Works & Installation Guide
- Zero-Sequence Current Transformer: Working Principle & Specs
- Mini PCB Current Transformer: How It Works & Selection Guide
Need a CT with a specific accuracy class for your meter or relay? Contact Shaanxi SiRui Technology Co., Ltd for samples and custom designs.