
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.
Products
Voltage matters. Most relays are identified using the voltage the coil is rated for. The contact side of the relay will be rated for an entirely different voltage. It is very common for the contact side of the relay to be rated up to 250V AC (or DC). It is also important to note that AC and DC coils are different, and are NOT interchangeable.
Know the difference between Normally Open (NO) and Normally Closed (NC). Most relays have either a NO contact or a NC contact. It is most common to use the NO contact on a relay, however most circuits can function using one or the other depending on how it has been set up.
A relay is not a contactor. Although relays and contactors function in much the same way, they are NOT the same. Contactors are generally used in scenarios where large amounts of voltage and current need to be switched, whereas relays don’t typically switch more than 15 amps and 120V.
A little power goes a long way. Relays switch higher voltage circuits with lower voltage control signals. This allows isolation of more sensitive control equipment from higher voltage circuits. Relays are a cheap and easy way to remotely turn and off these types of circuits.
Relays can be used both ways. Although it is most common to see relays being used to switch higher voltage circuits, this is not the only way they can be used. Another use for relays is to monitor the “status” of certain devices, such as a motor. To do this, the relay would be wired so that the coil becomes energized when the motor is running. By doing this, a low voltage control signal is sent back to a PLC to signal that the motor is running.