The term Normally Open (NO) applies to manual valves or dampers which the operator must leave open in normal operation. A control valve which should go to the open position on control signal failure should be labeled FO, i.e. Fail Open. The control signal may be pneumatic, hydraulic or electric.
The term Normally Closed (NC) applies to manual valves or dampers which the operator must leave closed in normal operation. A control valve which should go to closed position on control signal failure should be labeled FC, i.e. Fail Closed. The control signal may be pneumatic, hydraulic or electric.
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