Monitoring Relays

Relays are switches that open and close circuits. They control one electrical circuit by opening or closing contacts in another circuit. When a relay contact is normally open (NO), there is an open contact when the relay is not energized. When a relay contact is normally closed (NC), there is a closed contact when the relay is not energized. Applying electrical current to the contacts will change their state. Many relays use an electromagnet to mechanically operate a switch, but other operating principles such as solid-state relays are also used. Relays are used where it is necessary to control a circuit by a low-power signal or where several circuits must be controlled by one signal. Depending on the structural features and operating principle, relays are of different types such as monitoring, regulating, auxiliary and protective. Classification of the types of relays depend on the function for which they are used.

Monitoring relays are protective control devices that receive input signals, monitor and determine them, and then output an alarm signal if a set value is reached. Monitoring relays can evaluate currents, levels, temperatures and voltages in machines and equipment, and either give an alarm signal or initiate a shutdown process if certain thresholds are met.

Current monitoring relays prevent issues due to overcurrent or undercurrent. If an overcurrent or undercurrent occurs within an application, the current monitoring relay will activate, signaling an alarm or switch off the flow of energy through the circuit. 

Level monitoring relays are used to monitor fill levels as well as mixing ratios. The relay monitors both a minimum and maximum value, allowing it to control pumps to protect against tank overflow and dry operation.

Voltage monitoring relays can be designed for use on single-phase AC or DC voltages to protect motors and connected equipment from over-voltage or under-voltage conditions. Three-phase voltage monitoring relays monitor over-voltage and under-voltage conditions, as well as phase loss, phase imbalances and phase reversal.