GL.4
SEL-751A Relay Instruction Manual Date Code 20100129
Glossary
IG—Power, Q
IG
Residual current, calculated from the sum of the phase currents. In normal,
balanced operation, this current is very small or zero. When a ground fault
occurs, this current can be large.
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 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.
MIRRORED BITS
Protocol for direct relay-to-relay communications.
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 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 output contact, the protected equipment 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 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 degrees, and the C-phase voltage lags B-phase voltage by 120
degrees. 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 degrees.
Pickup Time
The time measured from the application of an input signal until the output sig-
nal asserts. The time can be settable, as in the case of a logic variable timer, or
can be a result of the characteristics of an element algorithm, as in the case of
an overcurrent element pickup time.
Pinout
The definition or assignment of each electrical connection at an interface.
Typically refers to a cable, connector, or jumper.
Power, P
Real part of the complex power (S) expressed in units of Watts (W), kilovolt-
watts (kW), or megawatts (MW).
Power Factor
The cosine of the angle by which phase current lags phase voltage in an ac
electrical circuit. Power factor equals 1.0 for power flowing to a resistive load.
Power, Q
Reactive part of the complex power (S) expressed in units of Vars (W), kilo-
vars (kVar), or megavars (MVar).
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