Definition
A description of an electrical switch, relay, or valve whose contacts or passage are closed when the device is in its resting (de-energized or unactuated) state, and which open only when the device is actuated.
Plain English
When nothing is making the device do anything, it is closed and lets current or fluid pass through. It only opens when something switches it on.
Context Anchor
Seen in aircraft wiring diagrams, maintenance descriptions, and switch or warning-system discussions.
Derivation
‘Normally’ here means ‘in the normal resting state’ — not ‘usually.’ It refers to how the device sits when no one is pressing, energizing, or actuating it. Knowing this prevents the common misread of ‘normally’ as ‘most of the time.’
Why Pilots Care
Mechanics and pilots rely on this to know how a circuit will behave if power is lost or during initial power-up.
Intuition Check
Do not read closed as “physically shut.” In an electrical context, closed means the path is complete. Do not read normally as “usually during flight”; it means the device’s resting condition.
Example Sentence 1
The fire extinguisher discharge valve is normally closed, opening only when the pilot presses the discharge button.
Example Sentence 2
A normally closed pressure switch completes the low-oil warning circuit when engine oil pressure drops to zero.