- 2071 Magh · 4 marks
Accuracy classes of CT
Short answer
Measuring CT classes 0.1, 0.2, 0.5, 1 specify current error (±0.1–1% at 100–120% In) and phase displacement (5–60 min); classes 3, 5 only current error (3–5% at 50–120%). Protective CTs 5P and 10P give current error (±1%, ±3%), phase displacement (±60 min for 5P), and composite error (5%, 10%) at rated accuracy-limit factor (e.g., 5P20). Class PS/PX for differential protection defined by turns ratio error, knee voltage, exciting current.
What are the various accuracy class of CT? List out current error, phase displacement error and composite error for each class.
The accuracy class of a CT states the maximum permissible errors at specified currents when the CT supplies its rated burden. The values below are as per IS 2705 / IEC 60044-1 (now IEC 61869-2).
Measuring CTs
Classes 0.1, 0.2, 0.5 and 1 (the class number is the current-error limit at 100–120 % of rated current; limits are also set at 5 % and 20 %), and classes 3 and 5 (limits at 50–120 %).
| Class | ±Current error at 5 % / 20 % / 100–120 % In | ±Phase displacement (minutes) at 5 % / 20 % / 100–120 % |
|---|---|---|
| 0.1 | 0.4 / 0.2 / 0.1 % | 15 / 8 / 5 |
| 0.2 | 0.75 / 0.35 / 0.2 % | 30 / 15 / 10 |
| 0.5 | 1.5 / 0.75 / 0.5 % | 90 / 45 / 30 |
| 1 | 3.0 / 1.5 / 1.0 % | 180 / 90 / 60 |
| 3 | 3 % (at 50–120 %) | Not specified |
| 5 | 5 % (at 50–120 %) | Not specified |
Composite error is not specified for measuring CTs; instead, an instrument security factor (ISF, e.g. ≤ 5 or ≤ 10) ensures that they saturate and protect the meters at fault current.
Protective CTs
Designated 5P and 10P, followed by the accuracy limit factor (ALF), e.g. 5P20 = class 5P, accurate up to 20 × rated current.
| Class | Current error at rated current | Phase displacement at rated current | Composite error at rated accuracy-limit current |
|---|---|---|---|
| 5P | ±1 % | ±60 minutes | 5 % |
| 10P | ±3 % | Not specified | 10 % |
- Standard ALFs: 5, 10, 15, 20, 30.
- Class PS (IS) / PX (IEC): special protection CTs for differential and restricted earth-fault schemes, specified by turns ratio error (≤ ±0.25 %), minimum knee point voltage, maximum exciting current at Vk/2 and secondary winding resistance, instead of error limits.
Typical use: 0.2/0.5 for revenue metering, 1 for indicating instruments, 5P for overcurrent and distance relays, PS for differential protection.
✓ Answer checked by Claude Opus 5.5 (high effort)
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