Relays and control

Every relay is two independent halves: a coil you give the order to, and contacts that carry the order out. The coil has its own voltage (12, 24, 230 V); the contacts have their own current and load ratings. This is what gets confused most often — 'coil 230 V' describes the control side, not a limit of 230 V on what the relay may switch.

A contact is either normally open (NO) or normally closed (NC). 'Normally' means 'while the coil is unpowered'. NO closes when the coil is energised; NC does the opposite and opens. Alarm circuits are built on NC, so that a broken wire counts as an alarm by itself.

Separating coil from contacts buys the real convenience: the control circuit can be low-voltage and safe while the power side stays at 230 or 400 V. A 24 V button on the wall, the power contact in the board — and touching the button's wiring is no longer dangerous.

A latching (bistable) relay changes state on every short pulse and then holds it on its own. That solves an old problem: light in a long corridor with four or five control points. Two-way switches need a heavy scheme of cross-wired cables; with a latching relay you fit plain push-buttons and bring them into one coil circuit — as many buttons as you like, and another can be added later.

A time relay counts an interval: switch on after a delay, off after a delay, pulse once per period. A staircase timer is that idea narrowed to one job: press the button, the light burns for the set time and goes out by itself. A good staircase timer also blinks a warning half a minute before the end, so nobody is left in the dark halfway up.

A photocell switches a load when it gets dark. Its sensor must go where the controlled fixture's own light cannot reach it — otherwise the relay switches the light on, sees that light, switches off, and does it all night.

A thermal overload relay protects a motor from what a breaker cannot see. The breaker catches short circuits and gross overload; what kills a motor is different — running for a long time at 15–20% above its rating. The current rises a little, the breaker stays quiet, the winding heats for weeks and one day breaks down. A thermal relay has the same thermal inertia as the winding and trips on exactly that condition. Set it from the current on the motor's nameplate, never by eye.

'Monitoring relay' is the general name for devices that watch a quantity and issue a command: voltage, phases, current, level, temperature. In a board they do not switch power themselves; they drive a contactor. The relay thinks, the contactor works.

A practical example on a pump: a breaker for short circuits, a thermal relay for overload, a phase-monitoring relay for a lost phase, a contactor to start and stop it, and START/STOP buttons in a low-voltage circuit. Each device answers for one kind of trouble — which is what makes the scheme readable and repairable.