A relay refers to an electrically operated switch or component used to break or interrupt a circuit. Voltage relays run based on a preset level of voltage, or the force that drives an electrical current between two points.

Currents flow through the relay coil, creating a magnetic field. The field attracts the switch lever and enables it to change position. Voltage switches typically offer two positions based on whether the coil's current is on or off. Voltage relays can be over-voltage relays, under-voltage relays or a combination.

An over-voltage relay operates when the current produced by a load, or device connected to the output of a circuit, exceeds a predetermined value.The over-voltage relay connects to a transformer, or device that transfers electrical energy from one circuit to another. The relay is calibrated to operate at or over a preset voltage level. When the relay is switched on, one or multiple contacts trip, or open a circuit breaker.

An under-voltage relay operates when the voltage that flows through a relay’s coil falls below a predetermined value. Under-voltage relays protect loads against voltage drops that can lead to power shortages and brown-outs. The relay contacts trip the circuit breaker when the voltage decreases to a certain level.