The Basics of Current-Sensing Relays

A range of manufacturing and production processes rely on current-operated relays to provide a continuously adjustable trip-current setting. They are able to protect mechanical apparatus from jam-up or other overloading conditions that result in measurable increases in motor current. Functionally, they sense current levels and provide an output signal when a specified current level is reached. Current-sensing relays are used to:

• Signal high-current conditions, such as a clogged grinder.
• Identify low-current conditions, such as a pump that has encountered a low-water condition. Sense the current a motor is drawing to feed the current to a programmable logic controller (PLC).

To meet the unique requirements of a diverse set of applications, a wide range of devices and options is currently available to designers, installers, and maintenance professionals, including plug-in style, base-mounted, DIN-rail mount, and donut-style. These types of devices offer the following capabilities:

• Sensing both AC and DC current — from milliamperes to several amperes.
• Sensing thousands of AC amperes with a current transformer (CT).
• Current setpoints can be fixed or adjustable.
• Input AC or DC.
• Analog output — voltage or current — or contact closure.
• Self- or looped-power units.
• Fixed or adjustable internal time delays.