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).
What it looks like when it fails
Coils fail open (no click, no action) and contacts burn or weld — especially when they've been switching more than their rating, DC, or an unsuppressed inductive load. A welded contact means the output stays on with the coil dead.
Plug-in relays add their own failure layer: corroded or spread socket pins that make intermittently. The relay tests fine on the bench; the fault lives in the base.
How to test it
Unplug it (or lift a coil lead) and meter the coil across its pins — OL condemns it. Energise it and listen for the click, then meter the changeover both ways: C to NO closed when energised, C to NC closed at rest.
In circuit, a welded or burnt contact shows as volt-drop: measure across the closed contact under load — it should be millivolts. And check the base: wiggle the relay in its socket while watching the output for intermittents.
Gotchas
The flag or LED on a relay shows the coil is energised — not that the contacts actually switched. A clicked relay with a burnt contact looks perfectly healthy from the front.
Contact ratings are AC figures: the same contact on DC arcs longer and welds at a fraction of the printed amps. Suppress inductive loads (diode on DC coils, RC snubber on AC) or the new relay dies as fast as the old one — swapping the relay 'fixes' it until the real culprit kills the replacement.
Safety first
Volt-drop checks are live work on an energised control circuit — licensed work, due care. And isolate before swapping a relay: the machine can operate the instant a healthy one goes in.
Isolate, lock out / tag out, and prove dead before working unless a live test is specifically required, authorised, and carried out under proper supervision. Always follow local regulations, your site procedures, and the equipment manufacturer's documentation.
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.