Fail-closed solenoid operated valves open when:

Fail-closed solenoid-operated valves are designed to remain closed under normal conditions, specifically when the solenoid coil is not energized. When the coil is electrically energized, it generates a magnetic field that moves the valve's plunger or armature. This movement opens the valve, allowing fluid to flow. Thus, the principle behind fail-closed solenoid valves is that their default position is closed, and they only open when power is supplied to the solenoid coil.

This operation is essential in systems where keeping the valve closed without power is critical for safety and function. The design helps prevent leakage or uncontrolled flow in case of power loss, which can be vital in many applications, such as refrigerant management in refrigeration systems. Understanding the operational states of such valves is crucial for effective system design and troubleshooting.