--- title: "Accuracy classes of CT" subject: "Switchgear and Protection" chapter: 2 key: "semester_6/switchgear_and_protection:ct-accuracy-class" source: https://www.bctengineer.com/q/semester_6/switchgear_and_protection/ct-accuracy-class site: BCT Engineering --- # Accuracy classes of CT Switchgear and Protection · Chapter 2: Current and Potential Transformers - Asked in: 2071 Magh, 4 marks - Video: [Accuracy class of Current Transformer | Explained | TheElectricalGuy](https://www.youtube.com/watch?v=28sWUdVGqb8) — Gaurav J - TheElectricalGuy - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_6/switchgear_and_protection/ct-accuracy-class - Whole chapter: https://www.bctengineer.com/md/semester_6/switchgear_and_protection/chapter-2 ## Question What are the various accuracy class of CT? List out current error, phase displacement error and composite error for each class. ## 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. ## Answer 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. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Which are the measuring CT accuracy classes? A. 0.1, 0.2, 0.5, 1, 3 and 5 B. 5P and 10P C. PS and PX D. 0.5P and 1P Answer: A. Measuring CTs are classes 0.1, 0.2, 0.5, 1 (at 100-120 % In) and 3, 5 (at 50-120 %). (Key point: Measuring classes) 2. What are the limits of a 5P protective CT at rated current? A. Current error +-3 %, no phase limit, composite 10 % B. Current error +-1 %, phase displacement +-60 minutes, composite error 5 % at the accuracy limit current C. Current error +-5 %, phase 60 minutes, composite 1 % D. Current error +-0.5 %, phase 30 minutes Answer: B. 5P has +-1 % current error, +-60 min phase and 5 % composite error at the rated accuracy limit current. (Key point: Class 5P) 3. What does 5P20 mean? A. 5 % ratio error up to 20 A B. 20 % composite error C. Class 5P accurate up to 20 times rated current D. Burden of 20 VA at 5 A Answer: C. The number after P is the accuracy limit factor: accuracy is maintained up to 20 x rated current. (Key point: ALF meaning) 4. Which statement is correct? A. Measuring CTs must not saturate B. Class PS is specified by composite error only C. ISF applies to protection CTs D. Measuring CTs have an instrument security factor so they saturate at fault current, protecting meters; class PS is specified by knee point voltage and turns ratio error Answer: D. Measuring CTs have an ISF (e.g. 5 or 10) to protect meters; PS/PX CTs for differential schemes are specified by knee point voltage, exciting current and resistance. 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