GL.5
Date Code 20070131 Instruction Manual SEL-749M Relay
Glossary
Pickup Time—RTD
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 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.
PT
Abbreviation for potential transformer. Also referred to as a voltage trans-
former or VT.
PTC
Abbreviation for Positive Temperature Coefficient, a thermistor detector that
makes use of the change of resistivity of semiconductor with change in tem-
perature. The thermistor detectors are often embedded in the stator winding of
the motor.
RAM
Abbreviation for Random Access Memory. Volatile memory where the relay
stores intermediate calculation results, Relay Word bits, and other data that are
updated every processing interval.
Reactive Power Element
A motor protection element that can trip the protected motor if the measured
reactive power exceeds a settable threshold.
Relay Word
The collection of relay element and logic results. Each element or result is
represented by a unique identifier, known as a Relay Word bit.
Relay Word Bit
A single relay element or logic result that the relay updates once each process-
ing interval. A Relay Word bit can be equal to either logical 1 or logical 0.
Logical 1 represents a true logic condition, picked up element, or asserted
contact input or contact output. Logical 0 represents a false logic condition,
dropped out element, or deasserted contact input or contact output. You can
use Relay Word bits in SEL
OGIC control equations to control relay tripping,
event triggering, and output contacts, as well as other functions.
Remote Bit
A Relay Word bit for which state is controlled by serial port commands,
including the CONTROL command, binary Fast Operate command, or
Modbus
®
command.
Residual Current
The sum of the measured phase currents. In normal, balanced operation, this
current is very small or zero. When a motor ground fault occurs, this current
can be large.
RMS
Abbreviation for Root-Mean-Square. Refers to the effective value of the sinu-
soidal current and voltage measured by the relay, accounting for the funda-
mental frequency and higher order harmonics in the signal.
ROM
Abbreviation for Read-Only Memory. Nonvolatile memory where the relay
firmware is stored.
RTD
Abbreviation for Resistance Temperature Device. An RTD is made of a metal
having a precisely known resistance and temperature coefficient of resistance.
The SEL-749M and the SEL-2600 modules can measure the resistance of the
RTD, and thus, determine the temperature at the RTD location—typically
embedded in the motor windings or attached to the races of bearings.
Courtesy of NationalSwitchgear.com
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