Split-core AC/DC current sensing, explained without the maths
Why an ordinary CT cannot see DC residual current, and what that means for EV charger safety compliance.
A conventional current transformer works by induction. A changing magnetic field in the core induces a current in the secondary winding. That word — changing — is the whole problem.
What a plain CT misses
Direct current produces a steady field. A steady field induces nothing. So an ordinary AC CT is blind to a DC fault current, and will happily report zero while a DC residual current flows.
On an EV charging installation that matters, because a fault inside the vehicle can inject DC onto the installation's earth. Worse, a DC component can saturate the core of the AC-only residual current device that is supposed to be protecting the circuit, blinding that too.
What the AC/DC sensor does differently
An AC+DC capable sensor uses a Hall element or a fluxgate arrangement that responds to the field itself rather than to its rate of change. Our SWARNA split-core AC-DC units come in 50 A, 75 A and 200 A, with a 0–5 V analogue output that a charger controller or BMS can read directly. There is also a 200 A DC-only sensor for battery and solar DC bus measurement, and a conventional 100 A / 50 mA AC CT for metering and leakage duty.
Why split core matters on retrofit
A split-core sensor hinges open and clips over a live conductor. No busbar is disconnected, no cable is cut, and the site does not need a shutdown to add measurement to an existing installation.
Practical notes
- Mind the orientation arrow. A reversed sensor gives a valid-looking signal of the wrong sign.
- Keep the conductor centred in the window. Off-centre placement introduces position error, especially at the top of the range.
- Check the output scaling against the controller's input range before commissioning, not after.
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