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Protective Relays for Backup Power Systems

Protective relays are sometimes used in backup power systems to protect circuits and load equipment from undesirable or potentially damaging electrical conditions and events. The following sections summarize their use in backup power systems and critical power equipment.

Protective Relay Overview

A protective relay compares inputs from an electrical sensing device against predetermined set points. When it detects an out-of-range condition, it signals another device to produce a responding action. For example, a protective relay may monitor current on a medium-voltage conductor using a current transformer; when the value of the current transformer output corresponds to an over-current condition, the protective relay will signal a circuit breaker to trip to protect downstream equipment. Such an arrangement is illustrated below.
When overcurrents occur, the arrangement above is often used to trip medium voltage breakers, which do not feature integral trip units. Nevertheless, changing the input devices and output destinations make protective relays useful for detecting other conditions. For instance, monitoring a signal from a potential transformer can be used to protect against undervoltages or overvoltages. Output from a thermal sensor can be used to protect equipment from overheating, and an optical sensor can be used to trigger arc flash protection. Output contacts can be used to operate a device, activate an alarm, or annunciate a condition.

Common Uses in Backup Power Systems

In practice, protective relay arrangements serve a very broad range of applications and can be the focus of an engineer’s entire career. While the scope of possibilities is impractical to describe in this article, three common applications for backup power systems include:

Medium Voltage Breaker Operation: Transfer switches and paralleling switchgear are available for medium voltage systems. Protective relays are used to trip these breakers when electrical anomalies such as overcurrents occur. Consequently, protective relays may be included in transfer switching or paralleling switchgear solutions to provide the necessary protection from overcurrent conditions.

Protection When Paralleling Onsite Power with Utility: Some backup power systems parallel on-site power sources with utility services. Closed Transition Transfer Switches do this for very short durations (typically <100 milliseconds) to avoid momentary power disruptions that could otherwise be associated with a transfer of load between power sources. Both soft-load transfer switches and paralleling switchgear can be used to parallel power sources for indefinite periods. For these applications, utility companies present varying requirements for responding to faults that may occur while paralleled. Some utility companies will require fast-acting protective relays to disconnect paralleled sources when faults occur. For more information, refer to the ASCO Power Technologies White Paper entitled Connecting Closed Transition Transfer Switches to Utility Services.

Protection from Specific Electrical Events and Conditions: Protective relays are used to sense a variety of conditions and protect systems from a wide range of electrical events. For example, the following list of ANSI Device Numbers indicates just some of the functions of protective relays that are used with backup power systems:
Device 15 - Automatic Synchronizer
Device 25 - Synch Check
Device 27 - Under Voltage
Device 32 - Reverse Power
Device 40 - Loss of Excitation
Device 46 - Current Balance
Device 47 - Voltage Negative Phase Sequence
Device 49 - Over Temperature Relay
Device 50 - Instantaneous Over Current
Device 51 - Time Delay Over Current
Device 59 - Over Voltage
Device 67 - Directional Over Current
Device 81O - Over Frequency Relay
Device 81U - Under Frequency Relay
Device 87 - Differential Relay
Additional information about ANSI Device numbers can be found in the IEEE Standard for Electrical Power System Device Function Numbers, Acronyms, and Contact Designations.

Protective Relay Availability

For backup power applications, protective relays can be obtained in a few different ways. Critical power equipment manufacturers may offer protective relays as orderable accessories for transfer switches and paralleling gear. Relays for specific applications may also be incorporated into engineered-to-order critical power equipment to match project specifications, such as custom equipment that integrates service entrance and power distribution features. Manufacturers can often provide guidance on protective relay solutions that can be used with their equipment.


Protective relays can be used to monitor and respond to a wide variety of electrical conditions and events. Used to trip medium voltage breakers, they also provide protection against other conditions in medium and low voltage systems, as indicated by the many ANSI Device Numbers assigned to protective relays by IEEE. Most critical power equipment manufacturers can provide guidance for common protective relay applications.

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