Protecting the upstream supply of an isolation transformer requires more than selecting a circuit breaker from the transformer rated current. The protection must tolerate transformer energisation while still clearing the minimum prospective fault current within the required disconnection conditions.
Two constraints can conflict
Transformer inrush current
When a transformer is energised, magnetising inrush can be several times its rated current for a short period. The actual magnitude and duration depend on transformer characteristics and the switching conditions. An unsuitable instantaneous trip setting can therefore cause nuisance tripping.
Minimum fault current
At the same time, cable impedance and source impedance reduce the prospective short-circuit current at the end of the circuit. The selected protection must still operate correctly for the minimum fault current relevant to the protection objective.
Why the letter on the breaker is not enough
It is too simplistic to conclude that a C curve, D curve or any other generic characteristic is always suitable or unsuitable upstream of a transformer. The correct selection depends on the transformer inrush data, rated current, cable and source impedances, prospective short-circuit currents, required disconnection times, breaking capacity and coordination with upstream and downstream protection.
A practical engineering workflow
- Obtain the transformer rated current and manufacturer inrush data.
- Calculate or model maximum and minimum prospective short-circuit currents.
- Check cable overload and short-circuit protection requirements.
- Compare the protection time-current characteristic with transformer energisation and fault conditions.
- Check breaking capacity and discrimination/coordination with adjacent devices.
- Where appropriate, evaluate an adjustable moulded-case circuit breaker or other protection solution rather than relying only on a fixed modular trip curve.
Engineering note: the applicable disconnection and protection requirements depend on the country, earthing arrangement, installation type and governing standards. Verify the applicable IEC/national rules and manufacturer data for the project.