What Is a Current Transformer?
A current transformer (CT) is a special transformer that converts a high AC current into a much smaller, proportional current that can be measured safely.
How Does It Work?
The conductor carrying the current to be measured passes through the core as the transformer's primary (usually a single turn). The many turns on the secondary scale the current down by the transformation ratio. For example, a 1000:1 CT reduces 100 A to 0.1 A.
Why Is It Used?
Measuring a high current directly is dangerous and difficult. A CT galvanically separates the measurement circuit from the main line and reduces the large current to a small, safe value. In this respect it's a member of the current sensor family.
| Property | Explanation |
|---|---|
| Transformation ratio | Primary:secondary (e.g. 1000:1) |
| Isolation | Separation from the main line |
| Class | Accuracy grade |
Important Safety Rule
A CT's secondary must never be left open-circuit. If the secondary is open while there's current in the primary, a dangerously high voltage forms at its terminals; so the secondary is always closed through a load (burden resistor).
Where Is It Used?
Energy meters, panel ammeters, overcurrent protection relays and power-monitoring systems.
Common Mistakes
- Leaving the secondary open: a dangerous high voltage and core damage occur.
- Trying to measure DC: a CT measures only AC; for DC a Hall-effect sensor is needed.
- Out-of-range current: if the rated current is exceeded, accuracy is degraded.
Frequently Asked Questions
Does a CT measure DC current?+
What is a burden resistor?+
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