1. The unit has not been properly regulated, so the boiler is over pressure.
2. The steam turbine trips or the steam control valve is closed suddenly.
3. The electric generator trips or load shedding happens.
4. High-pressure heater is suddenly disconnected.
5. When the coordinating control system is operating, it has failure and cause safety valve to act.
6. The safety valve acts by mistake.
7. The high-pressure branch system suddenly opens, thus the reheater is over pressure, the safety valve acts.