Executive Summary
Secondary burden is a critical but often overlooked parameter affecting current transformer accuracy. Excessive burden causes ratio error and phase displacement to exceed specified limits, leading to metering inaccuracies and protection misoperation. This technical note examines burden effects on accuracy classes and provides calculation and calibration strategies.
Understanding Secondary Burden
Definition
Secondary burden is the total impedance connected to the CT secondary terminals, expressed in ohms (Ω) or volt-amperes (VA) at rated secondary current.
Burden Components
- Relay/meter input impedance: Typically 0.01-0.1Ω for modern digital devices
- Secondary cable resistance: Depends on length, cross-section, and temperature
- Terminal and connection resistance: Typically 0.01-0.05Ω per connection
- Test switch resistance: 0.005-0.02Ω when closed
Burden Calculation
Total burden (Ω): Z_b = Z_relay + Z_cable + Z_connections
Cable resistance: R_cable = ρ × L / A
- ρ = copper resistivity (0.0175 Ω·mm²/m at 20°C)
- L = cable length (round-trip distance, meters)
- A = conductor cross-section (mm²)
Example: 50m of 4mm² cable (round-trip: 100m)
- R_cable = 0.0175 × 100 / 4 = 0.4375Ω at 20°C
- At 75°C (operating temperature): R = 0.4375 × 1.22 = 0.534Ω
Impact on Accuracy Classes
Metering CTs (Class 0.2, 0.5, 1.0)
| Burden Condition | Ratio Error Limit | Phase Displacement Limit |
|---|---|---|
| At rated burden | Per class (e.g., ±0.2% for 0.2S) | Per class (e.g., ±10 min for 0.2S) |
| At 25-100% rated burden | Must remain within class limits | Must remain within class limits |
| Exceeding rated burden | Errors increase proportionally | Errors increase proportionally |
Key point: Metering CTs are typically specified at 25-100% of rated burden. Operating outside this range voids accuracy guarantees.
Protection CTs (Class 5P, 10P)
Protection CTs are specified at rated accuracy limit current (e.g., 5P20 = 20× rated current):
- At rated burden: Ratio error ≤ 5%, phase displacement ≤ 60 min (for 5P)
- Exceeding rated burden: CT may saturate before reaching accuracy limit current
- Consequence: Relay may not operate correctly for fault currents
Burden Error Mechanisms
1. Ratio Error Increase
Higher burden requires higher secondary voltage to drive the same current. This increases magnetizing current, which does not contribute to secondary output, causing negative ratio error (secondary current lower than expected).
Formula: ε_ratio ≈ (I_mag / I_secondary) × 100%
2. Phase Displacement Increase
Magnetizing current lags the secondary current by approximately 90°. As burden increases and magnetizing current increases, the phase angle between primary and secondary current shifts.
Impact: Critical for power metering (affects kWh measurement) and directional protection (affects torque angle).
3. Saturation Risk
Higher burden reduces the margin to saturation. For protection CTs, this can cause premature saturation during fault conditions.
Burden Specification Guidelines
Standard Burden Values (IEC 61869-2)
| Rated Output | Standard Burden (5A secondary) | Typical Application |
|---|---|---|
| 2.5 VA | 0.1Ω | Modern digital meters/relays, short cables |
| 5 VA | 0.2Ω | General metering, moderate cable length |
| 10 VA | 0.4Ω | Protection CTs, longer cables |
| 15 VA | 0.6Ω | Protection CTs, long cables or multiple devices |
| 20 VA | 0.8Ω | High-burden applications, analog instruments |
| 30 VA | 1.2Ω | Special applications, very long cables |
Selecting Rated Output
- Calculate actual burden: Sum all components (relay + cable + connections)
- Apply safety margin: Select CT with rated output ≥ 1.5 × actual burden
- Verify accuracy: Ensure accuracy class is maintained at actual burden
Example:
- Relay burden: 0.05Ω
- Cable (100m, 4mm²): 0.534Ω at 75°C
- Connections: 0.05Ω
- Total actual burden: 0.634Ω
- Required rated output: ≥ 1.5 × 0.634 × 5² = 23.8 VA → Select 30 VA CT
Cable Sizing Optimization
Cable Cross-Section Selection
| Cable Length (round-trip) | Minimum Cross-Section | Resistance at 75°C |
|---|---|---|
| ≤ 50m | 2.5 mm² | 0.42Ω |
| 50-100m | 4 mm² | 0.53Ω |
| 100-200m | 6 mm² | 0.53Ω |
| 200-300m | 10 mm² | 0.53Ω |
| >300m | Consider 1A secondary | – |
1A vs 5A Secondary
For long cable runs, 1A secondary reduces burden by 25× (I²R relationship):
- 5A secondary: Burden = I²R = 25 × R
- 1A secondary: Burden = I²R = 1 × R
- Benefit: Same cable resistance produces 25× less VA burden
Trade-offs:
- 1A CTs typically cost more
- Some legacy relays require 5A input
- 1A circuits more susceptible to noise (lower signal level)
Calibration and Testing
Factory Calibration
CTs are calibrated at specific burden points per IEC 61869-2:
- Metering CTs: Typically calibrated at 25%, 50%, and 100% of rated burden
- Protection CTs: Calibrated at rated burden and accuracy limit current
Field Verification
- Insulation resistance: > 100 MΩ at 500V DC (secondary to ground)
- Winding resistance: Measure R_ct to verify within specification
- Burden verification: Measure actual burden with impedance meter or calculate from design
- Ratio test: Primary injection at rated current, verify ratio error within class limits
Accuracy Test Setup
- Connect CT secondary to calibrated burden box set to actual burden
- Apply primary current at 5%, 20%, 100%, and 120% of rated (for metering CTs)
- Measure secondary current magnitude and phase angle
- Calculate ratio error and phase displacement
- Verify within specified accuracy class limits
Common Mistakes and Corrections
Mistake 1: Ignoring Temperature Effect
Problem: Cable resistance increases ~22% from 20°C to 75°C.
Correction: Always calculate burden at maximum operating temperature, not room temperature.
Mistake 2: Underestimating Connection Resistance
Problem: Multiple terminals, test switches, and connections add up.
Correction: Allow 0.05-0.1Ω total for all connections in burden calculation.
Mistake 3: Mixing Devices on Same CT
Problem: Connecting metering and protection devices in series increases burden and compromises both functions.
Correction: Use separate CT cores for metering and protection, or use tapped CTs with separate terminals.
Engineering Checklist
Design Phase
- [ ] Calculate total burden (relay + cable + connections) at operating temperature
- [ ] Select CT rated output ≥ 1.5 × actual burden
- [ ] Verify accuracy class at actual burden (not just at rated burden)
- [ ] For cable runs > 100m, consider 1A secondary
- [ ] Specify cable cross-section to limit resistance
Installation Phase
- [ ] Use specified cable type and cross-section
- [ ] Minimize cable length (direct routing)
- [ ] Torque terminals to specification (avoid loose connections)
- [ ] Verify polarity before energization
Commissioning Phase
- [ ] Measure actual burden with impedance meter
- [ ] Verify burden ≤ CT rated output
- [ ] Perform ratio test at rated current
- [ ] Document as-installed burden for future reference
Conclusion
Secondary burden directly impacts CT accuracy and must be carefully calculated and controlled. Engineers who properly size CTs for actual burden, optimize cable selection, and verify burden during commissioning will achieve accurate metering and reliable protection performance.
Critical recommendation: Always calculate burden at maximum operating temperature (typically 75°C), not room temperature. A 20% margin is not sufficient—use 50% margin between rated output and actual burden.
Technical Reference: IEC 61869-1, IEC 61869-2, IEEE C57.13, ANSI C12