How to prevent excessive secondary load on current transformers?

I. Preventive Measures During the Design Phase
‌1. Load Capacity Matching Calculation
Select the rated capacity based on 60% of the actual load (e.g., total impedance of a 5A system ≤ 0.5Ω).
In differential protection circuits, ensure impedance matching between the CTs on both sides (error ≤ 0.5%).
‌2. Optimizing Cable Selection
Shorten cable length or increase cross-sectional area (4mm² copper cable resistance ≤ 0.44Ω per 100 meters).
For harmonic environments, multiply the calculated impedance by a correction factor of 1.2-1.5.
‌II. Operational and Maintenance Measures
‌1. Regular Inspection
Quarterly Test the secondary circuit impedance (using the volt-ampere method to verify actual load).
Check the terminal contact resistance (should be ≤0.1Ω).
2. Equipment Management
Reduce the number of instruments connected in parallel (add 0.2Ω load for each additional 0.5-class meter).
Install an overvoltage protector to prevent open-circuit failures.
III. Special Scenarios
1. Harmonic Environment
Use a wide-range 0.2S-class CT (achieve 0.2-class accuracy at 10% load).
2. Differential Protection Circuit
Calculate the secondary impedance balance of each CT individually (difference ≤10%).