Relay
A small electrically-operated switch — like a miniature contactor — used to switch or route control signals.
What it is
A relay is a small switch operated by a coil. It's essentially a low-power contactor used in control circuits to switch signals, isolate one circuit from another, or multiply a single output into several.
How it works
Energising the coil pulls a contact over from its normally-closed (NC) position to its normally-open (NO) position. The shared terminal is the common (C).
A relay's contacts are rated for modest currents — switching a heavy load directly on a relay contact arcs and welds it, which is why an interposing relay drives a contactor instead.
Where it's used
Interfacing PLC outputs to field devices, latching circuits, interlocks, and converting one signal into several. Plug-in relays sit in a base so they can be swapped quickly.
Special types include latching (impulse) relays that hold state without power, and monitoring relays (phase failure, over/under voltage).
Related faults
Control relay coil not energising
A plug-in or interface relay isn't picking up — its indicator stays off and its contacts don't change, so whatever it controls never operates.
Control relay contacts sticking or welded
A control/interface relay's contacts stick closed (or won't make cleanly) — the controlled circuit stays on when it should be off, or switches unreliably.
Latching / impulse relay stuck in one state
A latching (impulse) relay won't change state on a pulse — lighting or a load stays on or off regardless of the switch, because the relay isn't toggling.
Related definitions
Contactor
An electrically-operated switch that uses a coil to make or break a load circuit, usually three-phase power.
PLC (Programmable Logic Controller)
An industrial computer that reads inputs, runs a program, and drives outputs to control machinery.
Control vs power circuits
Low-power control logic decides what happens; the power circuit carries the load — kept separate for safety and clarity.
Phase failure / monitoring relay
Watches a three-phase supply and disconnects the load if a phase is lost, unbalanced, or in the wrong sequence.