GL.2
SEL-749M Relay
Instruction Manual
Date Code 20080918
Glossary
Assert
—
Delta
tronic devices. The SEL-749M Relay uses ASCII text characters to communi-
cate using the relay front- and rear-panel EIA-232 serial ports.
Assert
To activate; to fulfill the logic or electrical requirements needed to operate a
device. To apply a short-circuit or closed contact to an SEL-749M input. To
set a logic condition to the true state (logical 1). To close a normally-open out-
put contact. To open a normally-closed output contact.
Breaker
Auxiliary Contact
A spare electrical contact associated with a circuit breaker that opens or closes
to indicate the breaker position. A form-a breaker auxiliary contact (ANSI
Standard Device Number 52A) closes when the breaker is closed, opens when
the breaker is open. A form-b breaker auxiliary contact (ANSI Standard
Device Number 52B) opens when the breaker is closed and closes when the
breaker is open.
Checksum
A numeric identifier of the firmware in the relay. Calculated by the result of a
mathematic sum of the relay code.
CID
Abbreviation for Checksum Identifier. The checksum of the specific firmware
installed in the relay.
Contiguous
Items in sequence; the second immediately following the first.
CR_RAM
Abbreviation for Critical RAM. Refers to the area of relay Random Access
Memory (RAM) where the relay stores mission critical data.
CRC-16
Abbreviation for Cyclical Redundancy Check-16. A mathematical algorithm
applied to a block of digital information to produce a unique, identifying num-
ber. Used to ensure that the information was received without data corruption.
CT
Abbreviation for current transformer.
Current
Unbalance Element
Protection element that calculates the magnitudes of the measured phase cur-
rents, calculates the average of those magnitudes, determines the magnitude
with the largest deviation from average. It then calculates the difference
between the magnitude average and magnitude of the phase with the largest
deviation from the average. Finally, the relay calculates the percent unbalance
current by dividing the difference value by the motor rated full load amps or
by the average magnitude, whichever is larger. Unbalance current causes heat-
ing in the rotor of the protected motor. The unbalance element can trip the
motor in the presence of heavy unbalance to prevent rotor damage resulting
from overheating. In the SEL-749M, this element works together with the
motor thermal element, which also provides unbalance current protection.
Deassert
To deactivate; to remove the logic or electrical requirements needed to operate
a device. To remove a short-circuit or closed contact from an SEL-749M
input. To clear a logic condition to the false state (logical 0). To open a nor-
mally-open output contact. To close a normally-closed output contact.
Delta
A phase-to-phase connection of voltage transformers for electrical measuring
purposes. Typically, two voltage transformers are used with one primary lead
of the first transformer connected to A-phase and the other lead connected to
B-phase. The second voltage transformer is connected to measure the voltage
from B-phase to C-phase. When two transformers are used, this connection is
frequently called "Open-Delta."
Also used for motor winding configuration during Star-Delta starting.
Содержание SEL-749M
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