intermediate switch
A light switch with two pairs of two terminals, which can be switched between straight-through or swapped wiring. Typically used as part of a circuit to control a light or other device from three or more locations.
intermediate switch: three-way control from the middle
An intermediate switch is a four-terminal switch used in lighting circuits where control from three or more locations is needed. Unlike a simple on-off switch with two terminals, the intermediate switch has two pairs of terminals and internally connects them in one of two configurations: straight-through or crossed. This switching action allows it to sit between two other switches in a circuit and reverse the signal path without cutting power or breaking the loop.
The intermediate switch is always positioned between two intermediate or two-way switches; it cannot operate alone. In a three-location lighting setup, you wire the first location with a two-way switch, the middle location with an intermediate switch, and the third location with another two-way switch. Current can flow through any permutation of switch positions because the intermediate switch can flip the polarity of the connection it receives, effectively toggling whether the two-way switches agree or disagree with each other.
Where and why it matters
Long hallways, stairwells with landings, and corridors in factories or warehouses often need switching from multiple points. Adding an intermediate switch in the middle of the run is cheaper and cleaner than running extra conductors all the way back to the first switch. Each additional control point beyond two requires one more intermediate switch in series; a four-location circuit uses two intermediate switches between two two-way switches.
The intermediate switch must be rated for the circuit voltage and current, typically 10 amp at 250V in domestic UK wiring. The terminals are usually labeled 1, 2, 3, 4 or marked with arrows showing the straight-through and crossed positions. Confusion about wiring or terminal connection is a common fault; the switch itself will function but the circuit logic will be inverted, leaving the light on when it should be off or vice versa at some switch positions.
In industrial settings, this logic is now often replaced by relay-based or programmable switching systems, but intermediate switches remain standard in building electrical work and are mandated by wiring regulations for multi-point control. They are durable, require no power supply, and have no moving parts other than a spring-loaded rocker or lever mechanism.