Understanding the Importance of Shorting Secondary Windings in Current Transformer Testing
- KPM Tech Team
- Sep 24
- 2 min read
Updated: Nov 10
When testing a current transformer (CT), it is crucial to short the secondary windings of the other CT cores not under test. This practice is essential for safety and accuracy reasons. Below is a comprehensive breakdown of this procedure.
1. Prevent Dangerous High Voltages (During Primary Current Injection Method)
A current transformer behaves like a step-up voltage transformer when its secondary is open. If a CT secondary is left open while current flows in the primary, the CT attempts to maintain magnetic balance by dramatically increasing the secondary voltage. This situation can lead to several serious consequences:
Very high voltages (hundreds or even thousands of volts)
Insulation failure
Electrical hazards for personnel
2. Protect the CT from Damage
Open-circuiting a CT secondary under load can cause core saturation and overheating. This condition may permanently damage the insulation or magnetize the core, ultimately ruining the CT's accuracy.
3. Ensure Accuracy During the Test (During Both Primary Current Injection Method & Secondary Voltage Ratio Method)
During CT testing (e.g., ratio, burden, excitation tests), it is vital to ensure that:
Only the CT under test is actively influencing the readings.
Other CT cores do not induce stray voltages or magnetic coupling effects.
By shorting their secondaries, one can eliminate unwanted interactions and minimize interference from other magnetic paths.
Example Scenario
Imagine you are testing the protection core of a multi-core CT, which may also have a metering core. If the metering core’s secondary is left open, the primary current will induce voltage in it. That induced voltage can become dangerously high and:
Affect your test of the protection core.
Risk damaging the CT.
To mitigate these risks, you should short the metering core's secondary while testing the protection core.
Summary Table
Reason | Why It Matters |
Prevent high secondary voltages | Avoid danger to people and equipment |
Protect CT insulation and core | Avoid overheating and saturation |
Ensure accurate test results | Remove stray flux and magnetic interference |
At KPM, we are committed to exploring innovative and cutting-edge technologies in CT/PT testing solutions. For inquiries, collaborations, or to learn more, feel free to reach out to us at consultant@kpmtek.com.
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