TM 5-684/NAVFAC MO-200/AFJMAN 32-1082
RELAYS AND CONTROLS
Section I-RELAYS, ELECTRIC POWER APPARATUS
11-1. Relay functions.
regulating, monitoring, programming, and auxiliary
control. Some relays may qualify for more than one
A relay is an electric device designed to interpret
classification, depending on their application.
input data in a prescribed manner. When specific
(2) Inputs. This classification has to do with
input conditions occur, the relay responds to cause
the input to which the relay responds such as cur-
contact operation or a similar sudden change in
rent or voltage.
associated electric control circuits.
(3) Operating principles. This classification
a. Electric power apparatus relays. This section
identifies the relay's operating principles or struc-
describes electric power apparatus relays and relay
tural features, such as electromechanical or solid-
systems which are designed to operate circuit
breakers and contactors, usually medium-voltage
units. Relays can be set more precisely than fuses.
(4) Performance characteristics. Relays are se-
Relays are adjustable with respect to both time and
lected to perform certain functions. To standardize
current, a feature that also applies to solid-state,
on reference use, they are given device function
direct-tripping, low-voltage circuit breakers.
numbers by IEEE C37.2. (Device function numbers
also describe other electrical power apparatus
b. Input data. Input data analyzed is usually
equipment in addition to relays.) Device function
electrical, but may be mechanical or thermal, or
numbers readily identify devices in drawings, dia-
evaluate other conditions or a combination of condi-
grams, instruction books, publications and specifi-
tions. Electrical conditions can be overcurrent,
cations. The use of "52" for circuit breakers, "51" for
overvoltage or under-voltage, a combination of cur-
an ac time overcurrent relay, "65" for a governor,
rent and voltage, current balance, direction of cur-
and "86" for a lockout relay provides a simple brief
method of designation of the device's operational
performance. The standard also covers suffix letters
c. Industrial control relays. Relays of the type
used for main devices (such as "N" for neutral),
designed primarily for industrial control, for switch-
actuating qualities (such as "A" for amperes), auxil-
ing of communication or other low-level signals, or
iary devices (such as "CS" for control service), oper-
for any other equipment not controlling electric
ating device components (such as "TC" for trip coil),
power apparatus are described in the CONTROLS
and auxiliary contact positions (such as "a" and "b",
section of this chapter.
"aa" and "bb"). Familiarity with this standard, in-
11-2. Relay fundamentals.
cluding typical elementary diagrams, will help in
understanding device operations.
Electric power apparatus relays operate to quickly
sense problems and speedily isolate power systems
11-3. Relay construction.
under fault conditions. Such an action limits the
All relays operate in response to one or more elec-
extent of electrical equipment damage and provides
trical or physical quantities to open or close contacts
a means to limit outage periods. Their definition,
or trigger power electronic devices, such as thyris-
classification, and functional use in electrical power
tors. Relays will generally be of the electromechani-
systems are defined by industry standards, pre-
cal or solid-state type.
pared and coordinated by IEEE. Maintenance per-
sonnel should be familiar with these standards.
a. Electromechanical relays. These relays have
IEEE also provides recommended selection and ap-
been used for years and provide simplicity, reliabil-
plication practices, which are used by engineers de-
ity, security, low-maintenance, and long life. Basic
signing military facilities.
units are constructed to respond instantaneously or
a. Classification: Relays can be classified by
with a time-delay to the actuating quantity.
functions, input operating principals, and perfor-
(1) Instantaneous units. Instantaneous units
mance characteristics. ANSI/IEEE C37.90 covers
act on an electromagnetic attraction operating prin-
classifications of relays and also standard service
ciple wherein a plunger, solenoid, hinged armature,
conditions, ratings, tests, and temperature rise.
or balance-beam is pulled into a coil or pole face of
(1) Functions. Relays are classified according
an electromagnet. They can be used in both ac and
to their primary purposes, which are protective,
dc power systems.