Quick Answer
Current transformer calibration measures the ratio error (%) and phase angle error (minutes) at specified burden and at 5%, 20%, 100%, and 120% of rated primary current, compared against a NABL-calibrated reference CT using a current comparator bridge per IEC 61869-2.
Key Takeaways
- CT calibration measures ratio error (%) and phase angle (minutes) per IEC 61869-2 and IS 2705.
- Metering CTs (Class 0.1, 0.2, 0.5, 1) must meet tight ratio and phase angle limits at 5–120% rated current.
- Protection CTs (5P, 10P) must meet accuracy and ALF (accuracy limit factor) requirements.
- CT calibration requires a precision current comparator bridge and calibrated reference CT.
- Prism calibrates metering and protection CTs for Gujarat's power utilities and industrial plants.
CT Accuracy Classes and Limits
| Class | Max Ratio Error | Max Phase Angle | Application |
|---|---|---|---|
| Class 0.1 | ±0.1% (at 100% In) | ±5 min | Revenue metering, high precision |
| Class 0.2 | ±0.2% (at 100% In) | ±10 min | Revenue metering, sub-metering |
| Class 0.5 | ±0.5% (at 100% In) | ±30 min | Industrial energy metering |
| Class 1 | ±1.0% (at 100% In) | ±60 min | General measurement, indication |
| Class 5P | ±1% (at 100% In), ±5% at ALF | ±60 min | Protection relay operation |
| Class 10P | ±3% (at 100% In), ±10% at ALF | — | General protection |
CT Calibration Procedure at Prism
Burden Connection
Connect rated burden (VA) at standard power factor (0.8 lag for metering CTs). Burden affects CT accuracy.
Reference CT
Connect NABL-calibrated reference CT (class higher than test CT) in series as reference standard.
Current Comparator
Use precision current comparator bridge to measure ratio error and phase angle difference between reference and test CT.
5-Point Test
Test at 5%, 20%, 100%, and 120% of rated primary current (additional points per standard).
Certificate
Issue NABL certificate with ratio error, phase angle at each test point, accuracy class confirmation, and expanded uncertainty.
Frequently Asked Questions
Why does CT burden affect calibration accuracy?
CT secondary burden (the impedance of metering and relay circuits connected to the CT secondary) affects the CT's magnetising current and thus its accuracy. CTs must be calibrated at their rated burden to give results representative of their in-service performance.
How often should CTs be calibrated?
Metering-class CTs (for energy billing) should be calibrated every 1–5 years depending on accuracy class and tariff category. Protection CTs are verified during annual protection relay testing. Revenue metering CTs for utilities are subject to periodic re-testing by CEIG.
Does Prism calibrate split-core (clamp-on) CTs?
Yes. Prism calibrates split-core and Rogowski coil CTs used for non-intrusive current measurement in energy monitoring systems.
What is the ALF (Accuracy Limit Factor) in protection CT calibration?
ALF is the multiple of rated current up to which a protection CT must maintain its accuracy class error limit. 5P10 means Class 5P with ALF = 10 (the CT must have < 5% composite error up to 10× rated current — needed for protection relay operation during fault conditions).
Can Prism calibrate HV/EHV current transformers?
Prism calibrates metering and protection CTs up to 33 kV class with primary currents up to 3000 A. For EHV (66 kV and above) CTs, calibration is coordinated with specialised high-voltage test facilities.
Written by
Er. Parthiv Kinariwala
Managing Director · Prism Calibration Centre · NABL CC-2480 · Ahmedabad
Er. Parthiv Kinariwala founded Prism Calibration Centre in 2004 and has over 20 years of hands-on experience in calibration engineering, NABL accreditation, and industrial compliance. His team performs 10,000+ calibrations annually from the Vatva GIDC laboratory, serving 5000+ industries across Gujarat.
Prism Calibration Centre — Vatva GIDC, Ahmedabad
Prism Calibration Centre
F-101, Rudraksh Complex 2, Phase 3, GIDC Vatva, Near Jasoda Nagar Cross Road, Ahmedabad — 382445, Gujarat, India
Phone: +91 98245 26444
Email: info@prismcalibration.com
NABL: CC-2480 · ISO/IEC 17025:2017
Hours: Mon–Sat, 9:00 AM – 7:00 PM