UFC 3 -520-01
June 10, 2002
Ensure protective relays comply with ANSI/IEEE C37.90, Relays and Relay
Systems Associated with Electric Power Apparatus .
The most common condition requiring protection is a short circuit or overload.
Protective relays will be used to provide overcurrent protection for medium voltage
applications and possibly for larger low voltage load centers. Lower voltages usually
have overcurrent protection provided by direct-trip breakers or fuses. There are other
abnormal conditions that also require protection, including undervoltage, overvoltage,
open-phase, overcurrent, unbalanced phase currents, reverse power flow,
underfrequency, overfrequency, and overtemperature. Larger power systems require
even more types of protection. These types of protection require the use of protective
relays. Refer to IEEE 242 for guidance regarding electrical protection of specific
The functions performed by protective relays can be accomplished using
electromechanical or electronic multifunction devices. Originally, all protective relays
were electromechanical devices and it is not uncommon for a 50 year old
electromechanical relay to still be in service. Solid-state designs have been available
for many years and are preferentially used for new installations. Solid-state relays are
also referred to as multi -function relays because a single relay can be configured to
provide different types of protection simultaneously. This relay type is preferred
because it minimizes the variability of relay types throughout the power system.
Select overcurrent relays to maximize the level of selective coordination with
other devices. By selecting a relay with inverse, ver y inverse, or extremely inverse
characteristics, coordination can be improved for a specific situation. Solid-state relays
can be programmed for a specific overcurrent response. Refer to IEEE 242 for
Relays should be flush-mounted or semi -flush-mounted, back-connected, and
dustproof for switchgear or a switchboard panel.
Connect to external circuits via permanent wiring to the relay case. Ensure
the chassis is designed to slip in and out without disturbing the case or external
connections, thus allowing easy removal for testing and maintenance.
Ensure necessary test devices are incorporated within each relay and provide
a means for testing either from an external source of electric power or from associated
Ensure each relay is provided with an operation indicator and reset device.
and connections provided.
In areas of limited panel mounting space, multifunction relays should be used
because they offer the advantage of including several relay protective functions in a