When we energise a transformer or autotransformer from a standstill, a transient current known as a connection spike, magnetising spike or inrush current is produced, which can exceed the rated current (In) several times over. This current, although momentary, can be sufficient to trip fuses or circuit breakers if it has not been taken into account in the protection design.
This spike is generated in only one of the windings, depending on which side the transformer is energised from:
For example: in an installation with solar panels, the transformer is energised from the mains side, not from the solar generation side. Similarly, if we install a transformer in a home to raise the voltage from 230V to 400V three-phase, the energised side will be the connection side (street). This phenomenon is analogous to the starting peak observed in electric motors, which is a result of the transient behaviour of the winding.
Why is it important to size protections according to the magnetising tip?
The connection tip determines the characteristics that the electrical protections installed in the transformer must have. If the protection is not correctly sized, it may trip unnecessarily when the equipment is connected or, worse still, fail to act in the event of a real overcurrent.
Therefore, correctly sizing the protections according to the magnetising peak is crucial in order to:
- Avoid unnecessary tripping.
- Protect the transformer against real overcurrents.
- Ensure continuity of supply in critical installations.
At POLYLUX, we manufacture transformers with different current peak limitations, classified according to how many times they exceed the nominal current (In) of the winding: ≤12In, ≤8In or ≤5In.
Primary and secondary protections when installing a transformer
- On the primary side (primary meaning the energised side), poorly selected protection can cause unwanted tripping of the circuit breaker at the moment of energisation, which can be misinterpreted as a short circuit, both in the transformer itself and in the downstream loads.
- On the secondary side, it is essential to limit overload correctly. This is because the primary protection, which is oversized to withstand the magnetising peak, is not effective against prolonged overloads. If adequate protection is not installed on the secondary side, there is a risk of continuous overheating, which could burn out the transformer due to a permanent overcurrent that is not detected or interrupted in time.
What protections should a transformer have? Table of recommended protections according to the inrush current
Well-dimensioned protection is essential to ensure the safety and durability of the electrical installation. It protects against both internal transformer faults and external network anomalies.
| Inrush current | PRIMARY | SECONDARY |
|---|---|---|
| ≤12In | 1.5÷2xIn D curve or gL fuse | ≤In C curve or gR fast fuse |
| ≤8In | In D curve or gL fuse | ≤In C curve or gR fast fuse |
| ≤5In | In C curve or gL fuse | ≤In C curve or gR fast fuse |
*In = Nominal current (A)
Protection criteria:
- Primary side: Protection against short circuits, sized according to the magnetising peak.
- Secondary side: Protection against overloads, sized according to the nominal current of the transformer (In).
The nominal value of the protection rating must not exceed the calculated value of the rated current:
- Single-phase: In = Nominal rating / Nominal voltage
- Three-phase: In = Nominal rating / (√3 × Nominal voltage)
*Important aspects to consider:
In the case of transformers (not autotransformers), the galvanic isolation between the primary and secondary prevents a residual current device (RCD) placed on the primary from detecting leaks or earth faults occurring in the secondary circuit.
This means that, even if the residual current device on the primary side operates correctly in the event of a fault on that side, it will not offer any protection against a leakage current that may occur on the secondary side. Therefore, if complete residual current protection is desired, it will be essential to also install a residual current device on the secondary side.
How to determinate the protection I need to install a transformer:
At POLYLUX, we know that choosing the right protection is essential to ensure the safety and proper functioning of a transformer. That is why our catalogue clearly lists the protections we recommend for each of our single-phase, three-phase and autotransformers.
POLYLUX single-phase and three-phase electrical transformers
All transformers manufactured by POLYLUX have a magnetising peak ≤12In, thanks to the use of high-quality magnetic cores and a high degree of compaction. Some examples of standard transformers with a peak ≤12In are the TTU, TT and TK models.
In addition, we offer specific solutions with lower peaks:
- Transformers with a peak ≤8In, for environments with medium protection sensitivity requirements, such as transformers for clinical use and eco transformers.
- Transformers such as the TK5IN, with a standard peak ≤5In, specially designed for applications where start-up peaks need to be minimised and coordination with highly sensitive protections optimised.
These options allow for greater flexibility in electrical design, facilitating more accurate and reliable protection in every scenario of use.
For more information, contact us or consult our related articles:
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