CT Xfmr
A current transformer produces a scaled secondary current from an alternating primary current. It is used for measurement and protection. Electrical isolation is useful, but safe operation depends on the rated insulation, burden and installation.
An alternating primary current creates magnetic flux that induces secondary current. Ideally, the current ratio is the inverse turns ratio; for a one-turn primary, Is ≈ Ip/Ns. The secondary burden, magnetizing current and core saturation affect accuracy. A conventional CT does not measure steady DC.
In plain terms
Like a window in a wall that lets you measure the wind speed outside without opening the door — the primary current's magnetic field bleeds through the donut, and the secondary winding tells the meter what's happening on the other side.
Why designers use it
- Scaling AC current for metering or protection.
- Separating the primary circuit from the measurement circuit within the rated insulation limits.
- Matching a specified secondary output to an appropriate burden and instrument.
Best for
- Power metering
- Protective relays
- Solar inverters
Key specifications
- Ratio: Primary-to-secondary rated current or turns ratio
- Burden: Allowed secondary load, including leads and instruments
- Accuracy: Check ratio and phase error under stated conditions
- Insulation: Match the installation voltage and required safety category
- Frequency: Use the rated AC range; not steady DC
When not to use it
- For steady DC measurement with a conventional transformer.
- With an open secondary while primary current flows.
- Outside the rated insulation, current, burden or frequency limits.
Common mistakes
- Opening the secondary of an energized current-output CT: dangerous voltage can develop. Follow the manufacturer's de-energizing and shorting procedures; do not disconnect it live.
- Exceeding the rated burden or assuming every CT has an internal burden resistor.
- Ignoring polarity when phase or directional measurements matter.
- Treating galvanic isolation as a guarantee that exposed connections are safe to touch.
Where you will find it
- An energy meter can use a CT to scale AC current for its measurement circuit. The burden and measurement circuit must match the transformer.
- Protection systems use CTs with characteristics chosen for fault-current behaviour, which need not match those of a metering CT.
A short history
A current transformer couples an alternating primary current to a secondary winding through a magnetic field. With the specified burden connected, its secondary current represents the primary current at a reduced scale. This lets measuring instruments and protective relays monitor larger currents while maintaining electrical isolation between the primary and secondary circuits.
Good to know
- The secondary load is called the burden.
- The open-circuit voltage is design- and condition-dependent; the safety rule is not permission to test it by unplugging the secondary.