Edited by Leland Teschler

Typical monitoring relay (Overcurrent sensing model shown)

Typical monitoring relay
(Overcurrent sensing model shown)

can generally be found in industrial wastewater plants and other settings that need a warning about outofbounds voltages or electrical currents. A typical application is to signal a warning when a pump motor is drawing too much current, as happens with a clogged water filter. In this case the monitoring relay senses the motor current (typically through a shunt connection) and actuates a relay if current remains above a dialed-in level longer than a delay time the user has also set. The relay switches off once current drops back to allowable levels.

In recent years, monitoring relays have also found roles where ac power quality is suspect. Here specific models may be used to signal over or under-voltage coming from the ac mains.

Typical specifications for these devices include settable delay times ranging from 0.2 to 10 sec, switching thresholds that can be set from 10 to 100% of the range, and capacities for electrical current that range from 100 mA to 10 A. Setting accuracy is generally ±5% and repeat accuracy <2%

Some devices also incorporate what is called a window function. This lets the user specify both a high threshold and a low threshold for relay operation. If the device notices line conditions outside the window, the relay activates. However, it is more typical for these devices to actuate on conditions that are over or under a set threshold, but not both.

Monitoring relays also come in three-phase as well as singlephase versions. Switching capacity of the relay is generally 3 to 5 A at 250 Vac. And monitoring relays are exclusively singlepole double-throw electromechanical devices; no manufacturer makes one incorporating a solid-state relay.

Finally, monitoring relays can be packaged either as panelmount devices or in a form that can attach to a DIN rail. The DINrail types tend to be more compact.

Phoenix Contact (phoenixcon.com) provided information for this article.