Current Transformer Accuracy Classes: 0.2S vs 0.5S vs 5P vs 10P

2026-03-02

Current Transformer Accuracy Classes: 0.2S vs 0.5S vs 5P vs 10P – Complete Engineering Guide

Executive Summary

Current Transformer (CT) accuracy classes define the precision with which a CT reproduces primary current in its secondary winding. Understanding the distinction between metering classes (0.2S, 0.5S) and protection classes (5P, 10P) is critical for proper CT selection in electrical power systems. This document provides comprehensive technical analysis of accuracy class specifications, ratio error limits, phase displacement requirements, and application selection criteria based on IEC 61869-2 and IEEE C57.13 standards.

Key Takeaways:

Metering classes (0.2S, 0.5S, 1.0) prioritize accuracy at normal load currents for revenue and energy metering applications
Protection classes (5P, 10P) maintain accuracy during fault conditions with specified Accuracy Limit Factor (ALF)
The “S” designation indicates special accuracy requirements at low currents (down to 1% In)
Class 0.2S provides ±0.2% ratio error for revenue metering; Class 5P provides ±1% error at rated accuracy limit current
Improper CT class selection can result in billing errors, protection misoperation, or equipment damage

Accuracy Class Definitions

Metering Classes (IEC 61869-2)

Metering CTs are designed for accurate measurement of current under normal operating conditions. The class designation indicates the maximum permissible percentage ratio error at rated current.

Class	Ratio Error at In (%)	Typical Application
0.1	±0.1	Laboratory standards, precision measurement
0.2	±0.2	Revenue metering, high-accuracy energy measurement
0.2S	±0.2	Revenue metering with extended low-current accuracy
0.5	±0.5	General metering, industrial energy monitoring
0.5S	±0.5	General metering with extended low-current accuracy
1.0	±1.0	Panel meters, non-critical monitoring

Protection Classes (IEC 61869-2)

Protection CTs are designed to maintain accuracy during fault conditions when primary currents may exceed rated current by factors of 10-30. The designation includes the Accuracy Limit Factor (ALF).

Class	Composite Error (%)	Application
5P	±1	General protection, overcurrent relays
10P	±3	Less critical protection applications
5PR	±1	Protection with remanence limitation
10PR	±3	Protection with remanence limitation

The “P” designation indicates protection class, and the preceding number (5 or 10) indicates the maximum composite error percentage at rated accuracy limit current.

IEEE C57.13 Class Designations

IEEE standards use a different naming convention:

Metering: 0.3, 0.6, 1.2 (corresponding to IEC 0.2, 0.5, 1.0)
Protection: C100, C200, C400, C800 (C-class) or T-class for wound/donut types

The IEEE C-class rating indicates the secondary terminal voltage the CT can deliver at 20 times rated current without exceeding 10% ratio error. For example, C400 means the CT can deliver 400V at 20In with ≤10% error.

Ratio Error and Phase Displacement Limits

Metering Class Accuracy Requirements (IEC 61869-2)

Ratio error (current error) is defined as:

Ratio Error (%) = [(Kn × Is – Ip) / Ip] × 100

Where: Kn = rated transformation ratio, Is = secondary current, Ip = primary current

Class 0.2 and 0.2S Requirements

Current %	0.2 Ratio Error (%)	0.2S Ratio Error (%)	Phase Displacement (minutes)
5%	±0.75	±0.75	±30
20%	±0.35	±0.35	±15
100%	±0.2	±0.2	±10
120%	±0.2	±0.2	±10

Critical difference: Class 0.2S maintains specified accuracy down to 1% of rated current, while Class 0.2 is only specified from 5% to 120%. This makes 0.2S essential for revenue metering where low-load accuracy affects billing.

Class 0.5 and 0.5S Requirements

Current %	0.5 Ratio Error (%)	0.5S Ratio Error (%)	Phase Displacement (minutes)
5%	±1.5	±1.5	±90
20%	±0.75	±0.75	±45
100%	±0.5	±0.5	±30
120%	±0.5	±0.5	±30

Protection Class Accuracy Requirements

Protection CTs are characterized by composite error rather than separate ratio and phase errors. Composite error accounts for both magnitude and phase displacement under transient conditions.

Class 5P Requirements

Parameter	Limit
Ratio error at In	±1%
Phase displacement at In	±60 minutes
Composite error at rated accuracy limit current	5%
Typical ALF values	5, 10, 15, 20, 30

Class 10P Requirements

Parameter	Limit
Ratio error at In	±3%
Phase displacement at In	Not specified
Composite error at rated accuracy limit current	10%
Typical ALF values	5, 10, 15, 20

Accuracy Limit Factor (ALF)

ALF defines the multiple of rated current up to which the protection CT maintains its specified accuracy:

ALF = Rated Accuracy Limit Current / Rated Primary Current

Example: A 5P20 CT with 1000A primary rating maintains ±1% composite error up to 20,000A (20 × 1000A). This ensures reliable relay operation during fault conditions.

Standard ALF values: 5, 10, 15, 20, 30 (IEC 61869-2)

Burden Impact on Accuracy

CT accuracy is specified at rated burden. Actual burden affects performance:

Rated burden: Typically 2.5, 5, 10, 15, 20, or 30 VA
Actual burden: Sum of relay impedance + lead resistance + connection resistance
Lead resistance: R = ρ × (2L) / A (accounting for go-and-return path)

Critical consideration: If actual burden exceeds rated burden, the effective ALF decreases:

Effective ALF = Rated ALF × (Rated Burden / Actual Burden)

For protection applications, always verify that the CT can deliver required accuracy at the actual connected burden.

Application Selection Guidelines

Revenue Metering Applications

Recommended Class: 0.2S or 0.5S

Revenue metering requires the highest accuracy because billing errors directly impact financial transactions. Key requirements:

Class 0.2S for high-value transactions (>100kW demand)
Class 0.5S for general commercial/industrial metering
Extended low-current accuracy (1% to 120% In) captures all energy consumption
Phase displacement accuracy critical for power factor and reactive energy measurement
Typical burden: 2.5-5 VA (modern electronic meters have low burden)

Why “S” class matters: At 1% load, a Class 0.5 CT may have ±3% error, while 0.5S maintains ±1.5%. Over a year, this difference can represent significant billing discrepancy for facilities with variable load profiles.

General Metering and Monitoring

Recommended Class: 0.5 or 1.0

For non-revenue applications such as energy monitoring, load profiling, or display metering:

Class 0.5 for energy management systems
Class 1.0 for panel meters and local indication
Standard accuracy range (5% to 120% In) is typically sufficient
Cost optimization possible without compromising functionality

Protection Applications

Recommended Class: 5P10, 5P15, 5P20, or 10P depending on relay type

Overcurrent Protection (50/51)

Class 5P10 or 5P15 typically sufficient
ALF selected based on maximum fault current
Ensure CT does not saturate before relay operates

Differential Protection (87)

Class 5P20 or 5P30 recommended
Matched CTs on all branches (same class, ratio, burden)
Higher ALF prevents false tripping during external faults
Consider TPX/TPY/TPZ classes for transformer differential

Distance Protection (21)

Class 5P20 minimum
Phase displacement accuracy affects impedance measurement
Consider transient performance (TP classes) for critical applications

Earth Fault Protection

Class 5P10 typically adequate
Core balance CTs (CBCT) for sensitive earth fault
Consider 10P for non-critical applications

Selection Calculation Example

Scenario: 11kV feeder with 630A load current, 12.5kA fault current, overcurrent relay

CT Ratio: 800/5A (next standard above 630A)
Fault current multiple: 12,500A / 800A = 15.6 × In
Required ALF: Minimum 20 (next standard above 15.6)
Class selection: 5P20
Burden calculation:
- Relay burden: 0.5 VA
- Lead resistance (50m, 2.5mm²): 0.7 Ω × 5A² = 17.5 VA
- Total: 18 VA → Select 20 VA rated burden
Final specification: 800/5A, 5P20, 20 VA

Engineering Checklist

CT Specification Checklist

Before finalizing CT selection, verify:

☐ Rated primary current ≥ maximum continuous load current
☐ Rated secondary current matches relay/meter requirements (typically 5A or 1A)
☐ Accuracy class appropriate for application (metering vs protection)
☐ ALF sufficient for maximum fault current (protection CTs)
☐ Rated burden ≥ calculated actual burden
☐ Short-time thermal current (Ith) ≥ system fault level
☐ Dynamic current (Idyn) ≥ peak fault current (2.5 × RMS fault)
☐ Insulation level appropriate for system voltage
☐ Physical dimensions fit available space
☐ Terminal arrangement accessible for wiring

Installation Verification

☐ Polarity markings correct (P1/S1 orientation)
☐ Secondary grounding at one point only (prevent circulating currents)
☐ Lead size adequate for burden (typically ≥2.5mm² for 5A CTs)
☐ No open-circuit condition possible (shorting terminals during maintenance)
☐ Burden measurement matches design calculation
☐ Insulation resistance test passed (>100 MΩ)
☐ Ratio test confirms nameplate ratio
☐ Magnetization curve available for protection CTs

Common Mistakes to Avoid

❌ Using metering CT (0.2S) for protection – will saturate during faults
❌ Using protection CT (5P) for revenue metering – poor low-load accuracy
❌ Ignoring burden calculation – leads to accuracy degradation
❌ Multiple grounding points on CT secondary – causes measurement errors
❌ Open-circuiting energized CT secondary – dangerous high voltage
❌ Mismatched CTs in differential schemes – causes false tripping
❌ Insufficient ALF for fault current – relay may not operate
❌ Ignoring remanence effects in protection CTs – consider PR or TP classes

Standards Reference

International Standards

Standard	Title	Scope
IEC 61869-1	Instrument transformers – Part 1: General requirements	General definitions, testing, marking
IEC 61869-2	Instrument transformers – Part 2: Current transformers	CT-specific requirements, accuracy classes
IEC 61869-6	Instrument transformers – Part 6: Additional requirements	Low-power instrument transformers
IEC 60044-1	Instrument transformers – Part 1: Current transformers	Superseded by IEC 61869-2 (legacy reference)

IEEE/ANSI Standards

Standard	Title	Scope
IEEE C57.13	Standard Requirements for Instrument Transformers	US standard for CT/PT ratings and testing
IEEE C57.13.1	Guide for Field Testing of Relaying Current Transformers	Field testing procedures
IEEE C57.13.3	Guide for Grounding of Instrument Transformer Secondary Circuits	Grounding requirements

National Standards

GB 1208 (China): Current transformers – Aligned with IEC 61869-2
BS EN 61869-2 (UK): Adoption of IEC standard
IS 2705 (India): Current transformers – Similar to IEC with regional adaptations

Accuracy Class Cross-Reference

Application	IEC 61869-2	IEEE C57.13	GB 1208
Precision revenue metering	0.2S	0.3	0.2S
General revenue metering	0.5S	0.6	0.5S
Panel metering	1.0	1.2	1.0
General protection	5P10, 5P20	C100, C200	5P10, 5P20
Less critical protection	10P10, 10P20	C50, C100	10P10, 10P20

Testing Requirements

Type Tests (IEC 61869-2):

Temperature rise test
Impulse voltage test
Short-time current test
Accuracy verification at rated burden
Lightning impulse test (for outdoor CTs)

Routine Tests:

Ratio verification
Polarity check
Insulation resistance
Power frequency withstand voltage
Inter-turn insulation test

Special Tests (when specified):

Magnetization curve
Transient performance (for TP classes)
Remanence factor measurement
Frequency response

Additional Resources

IEC 61869 series available from IEC Webstore
IEEE C57.13 available from IEEE Standards Association
CT application guides from major manufacturers (ABB, Siemens, Schneider Electric)
NETA ATS standards for acceptance testing specifications

Document Version: 1.0 | Technical Review: Engineering Standards Team | References: IEC 61869-2:2012, IEEE C57.13-2016

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