1. Failure of a primary-side phase. Here the star point can reach a higher potential uncontrolled. This can lead to the destruction of the loads and the transformer.
2. Network configuration: Centre tap grounded. This is a typical network in North America. For example, there are voltages of 3x 480 V or 2x 240 V to grounded centre point.
3. Network configuration: Corner grounded delta. Likewise typical network form in North America and Asia. Here, 3-wire cables are mostly used, whereby the green/yellow wire is used simultaneously for the outer conductor. This is by no means to be confused with the cables used in European countries for single-phase networks!
4. In many applications, the neutral conductor of the transformer is grounded. This is sometimes required in different standards and regulations. This earthed connection would then have a direct contact to the "centre tap grounded" or "corner grounded" point and thus a short circuit to the outer conductor!
5. Output-side consumers such as line filters, primary switching regulators, frequency converters (DC link) are usually equipped with Y capacitors. These cause the same error as described in point 4.