Protecting Industrial Control Circuits

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Addressing relay challenges with the modern technology

BY MARK STEPHENS & ALDEN WRIGHT, Electric Power Research Institute

In 2009, the Electric Power Research Institute (EPRI) began an effort to create improved alternating current (AC) “ice cube” relays that could withstand voltage sags without dropping out.

Their white paper entitled “AC Ice Cube Relays Applied for Improved Power Quality” outlined the basic design and vulnerability of these common devices that often appear in modern industrial control circuits. These are called “ice cube” because they are housed in clear plastic having the appearance of an ice cube (see photo).

Similar appearing relays with direct current (DC) coils are generally robust to common power quality problems as long as their corresponding DC power supplies are robust as well. However, “ice cube” relays with AC coils have shown extreme sensitivity to voltage sags.

With an average dropout at or near 70 percent of nominal for a cycle or less (as short as one-quarter of the cycle—four milliseconds), these devices can lead to as many as 13 equipment shutdowns per year in a distribution-fed commercial or industrial facility.

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