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 Critical motors are frequently equipped with phase-sequence or reverse-phase relay protection.
When connected to a suitable potential source, phase-sequence and reverse-phase relays close
their contacts on reversal of phase rotation. These relays also can be used to sense unbalanced
voltage or undervoltage conditions.
4.3.12 Frequency Relays. Frequency relays sense underfrequency or overfrequency
conditions during system disturbances. Most frequency relays have provision for adjustment of
operating frequency and voltage. The speed of operation depends on the deviation of the actual
frequency from the relay setting. Some frequency relays operate instantaneously if the frequency
deviates from the set value. Others are actuated by the rate at which the frequency is changing.
The usual application of this type of relay is to selectively drop system load based on the
frequency decrement in order to restore normal system stability.
4.3.13 Temperature Relays. Temperature relays usually operate in conjunction with
temperature detecting devices such as resistance temperature detectors or thermocouples located
in the equipment to be protected and are used for protection against overheating of large motors
(above 1500 hp), generator stator windings, and large transformer windings.
4.3.14 Pressure Relays. Pressure relays used in power systems respond either to the rate of
rise of gas pressure (sudden pressure relay) or to a slow accumulation of gas (gas-detector relay),
or a combination of both. Such relays are valuable supplements to differential or other forms of
relaying on power, regulating, and rectifier transformers.
4.3.15 Auxiliary Relays. Auxiliary relays are used in protection schemes whenever a single
protective device alone cannot provide all the functions necessary for satisfactory protection.
Auxiliary relays are available with a wide range of coil ratings, contact arrangements, and
tripping functions, each suited for a particular application. Some of the most common
applications of auxiliary relays are circuit breaker lockout, circuit breaker latching, targeting,
multiplication of contacts, timing, circuit supervision, and alarming.
4.3.16 Relay Device Numbers and Functions. The standard device numbers assigned to the
more commonly used relays and the associated functions are listed below. The numbers are
frequently used in connection diagrams, in instruction books, and in specifications.
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