How it Works: Voltage Relay

Voltage relays manage preset amounts of voltage transmitted through electrical circuits.
••• Power plant image by MAXFX from

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.

Over-voltage Relay

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.

Under-voltage Relay

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.

Related Articles

What Is Pull in Voltage?
Types of Electrical Transformers
How to Read Amps on an Analog Multimeter
How to Calculate Millivolts to Amps
How to Convert 12 Volt to 6 Volt
What Is Pull in Voltage?
What Is the Function of a Voltage Regulator?
Types of Electrical Loads
How to Diagnose a Circuit Board With a Bad Transistor
How to Calculate Transformer VA Rating
How to Use a Variac
How to Design an RC Snubber
Types of Electrical Transformers
How to Calculate Transformer Load
How to Convert 240 Single Phase to 480 3 Phase
How to Wire a High & Low Voltage Three-Phase Motor
How to Convert mW to mA
How to Use Analog Multimeters
How to Calculate Voltages in Transistors
How to Build a 120V AC to 12V DC Power Converter