GL.4
SEL-749M Relay Instruction Manual Date Code 20080918
Glossary
IN—Phase Rotation
IN
Neutral current measured by the relay IN input. The IN input is typically con-
nected to the secondary winding of a window-CT for motor ground fault
detection on resistance-grounded systems.
LCD
Abbreviation for Liquid Crystal Display. Used as the relay front-panel alpha-
numeric display.
LED
Abbreviation for Light-Emitting Diode. Used as indicator lamps on the relay
front panel.
Load Jam Element
A motor protection element that, when enabled, can trip the protected motor if
the rotor stops turning due to a sudden increase in load torque or decrease in
bus voltage. When the rotor stops, the motor phase current increases. The
relay detects the stopped rotor using a settable overcurrent element and trips
after a settable time-delay.
Load Loss Element
A motor protection element that, when enabled, can trip the protected motor if
the motor shaft is suddenly decoupled from the mechanical load. The relay
detects the sudden decrease in mechanical load through use of an undercurrent
or underpower element.
Motor Thermal Element
A motor protection element that measures motor current, calculates a repre-
sentation of the energy dissipated in the motor, and compares the present
energy estimate to trip thresholds defined by the relay settings. The output of
the motor thermal element is represented as a % Thermal Capacity. When the
% Thermal Capacity reaches 100, the relay trips to protect the motor. The
Motor Thermal Element provides motor protection for the following condi-
tions that cause motor overheating: locked rotor, overload operation, and cur-
rent unbalance.
NEMA
Abbreviation for National Electrical Manufacturers Association.
Neutral
Overcurrent Element
A protection element that causes the relay to trip when the neutral current
magnitude (measured by the IN input) exceeds a user settable value. Used to
detect and trip in response to motor or cable ground faults.
Nominal Frequency
Normal electrical system frequency, usually 50 or 60 Hz.
Nonfail-Safe
Refers to an output contact that is not energized during normal relay opera-
tion. When referred to a trip or stop output contact, the protected motor
remains in operation unprotected when relay power is removed or if the relay
fails.
Nonvolatile Memory
Relay memory that is able to correctly maintain data it is storing even when
the relay is de-energized.
Overfrequency Element
A protection element that causes the relay to trip when the measured electrical
system frequency exceeds a settable frequency.
Phase Reversal Element
A protection element that detects the phase rotation of the voltage or current
signals applied to the protected motor, and trips if that phase rotation is the
opposite of the desired phase rotation.
Phase Rotation
The sequence of voltage or current phasors in a multi-phase electrical system.
In an ABC phase rotation system, the B-phase voltage lags the A-phase volt-
age by 120°, and the C-phase voltage lags B-phase voltage by 120°. In an
ACB phase rotation system, the C-phase voltage lags the A-phase voltage by
120°, and the B-phase voltage lags the C-phase voltage by 120°.
Courtesy of NationalSwitchgear.com
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