What Is a Current Transformer?
A current transformer converts high primary current into a safe, standard secondary current for meters, analysers and protection relays.
Operating Principle
The primary current creates magnetic flux and induces a proportional current in the secondary winding, allowing instruments to measure high current safely.
Primary and Secondary Current
Primary current is the actual line current. Secondary current is usually a standard 5 A or 1 A signal.
Meaning of 100/5A and 200/5A Ratios
A 100/5A ratio means 100 A primary current produces 5 A secondary current. The meter uses this ratio to calculate the real current.
How to Select the Right CT
CT selection depends on load current, busbar/cable size, accuracy class, VA burden and application.
| Application | Suggested property | Note |
|---|---|---|
| Energy metering | Class 0.5 or 0.2 | Billing needs better accuracy |
| Panel analyser | Class 1 or 0.5 | Cable distance adds burden |
| Protection relay | 5P10 / 10P | Saturation under fault current matters |
| Power factor correction | Correct ratio and direction | Check P1-P2 and S1-S2 direction |
Metering CT and Protection CT Differences
Metering CTs are selected for accuracy in normal operation; protection CTs are selected for relay performance during fault currents.
| Criterion | Metering CT | Protection CT |
|---|---|---|
| Purpose | Meter and analyser | Protection relay |
| Class | 0.2, 0.5, 1 | 5P10, 10P |
| Behaviour | Accuracy at normal current | Avoid saturation at fault current |
Accuracy Class
0.2
High accuracy metering and billing.
0.5
Energy monitoring and panel analysers.
1
General measurement.
5P10
Protection relays with defined composite error.
Why Burden (VA) Matters
Burden is the total load connected to the CT secondary, including devices and wiring. If the VA rating is too low, measurement error can increase.
Installation Notes
Direction
P1-P2 and S1-S2 must be correct.
Wiring
Secondary cables should be short and properly sized.
Earthing
Secondary circuit should be earthed at the proper point.
Shorting terminal
Use shorting terminals for maintenance.
Why Secondary Terminals Must Not Be Left Open
If the CT secondary is left open while primary current flows, dangerous high voltage may occur. This is a serious risk for personnel and insulation.
Conclusion
Correct CT selection is critical for measurement accuracy, protection reliability and site safety.
FAQ
What does 100/5A mean?
100 A primary current produces 5 A secondary current.
What happens if the secondary is open?
Dangerous high voltage can occur.
What does class 0.5 mean?
It defines measurement accuracy under specified conditions.
Why does burden matter?
Incorrect VA burden can reduce accuracy.