closed

A closed switch or circuit is one in which the conductive path is complete, allowing electrical current to flow uninterrupted from source to load. The contacts are touching, the gate is open (in the case of a gate valve on a gas line), or the element is positioned to bridge a gap. This is the default operating state for most circuits in normal service.

The term origin is purely mechanical: the circuit itself is "closed" as a loop when current can flow through it, versus "open" where the path is broken. In practice, a closed switch does not mean the circuit is at rest, only that the switch component itself is not the limiting factor preventing flow. A closed breaker can still carry no current if a load is disconnected elsewhere in the circuit.

Closed is contrasted with open, the state where contacts are separated, the switch lever is up, or a valve is shut. In safety systems, an open position is often preferred as a fail-safe condition because it prevents unintended current flow. Conversely, many monitoring circuits use normally closed switches as sentries: they carry a small supervisory current until opened by a fault condition, signaling that something has stopped working.

Electrical engineers distinguish between "closed" as a static state and "closing" as the dynamic action of moving contacts together. The closing time of a relay or breaker is the interval between command and electrical continuity. Very fast closing (under 10 milliseconds for small relays) is needed in switching and protection circuits; slower closing can allow inrush current to spike dangerously in inductive loads.

In three-phase industrial circuits, individual poles of a switch or breaker are described independently as closed or open. A single open pole in a three-phase disconnect is sufficient to de-energize the circuit, but an incomplete closure (where one pole lags behind) creates a dangerous single-phase condition. This is why maintenance teams verify all poles are fully closed before energizing a machine.

The physical state of a closed switch changes with wear and contamination. Oxide buildup on contacts, dirt in the mechanism, and mechanical friction all raise the force required to achieve true closure and lower the reliability of the connection. Regular inspection confirms that "closed" actually means closed: resistance testing across closed contacts is standard preventive maintenance on critical switchgear.