UFC 3 -520-01
June 10, 2002
Point of Common Coupling (PCC)--The point of interface between two different parts
of the power system where the propagation and characteristics of a power quality
variation can be evaluated. With respect to evaluation of harmonic voltage and current
limits at the supply to an end user, this is the point on the system where another end
user can be supplied.
Power Quality-- The concept of powering and grounding sensitive equipment in a
manner that is suitable to the operation of that equipment.
Primary Protection--First-choice relay protection
in contrast with backup relay
Radial System--A system in which independ ent feeders branch out radially from a
common source of supply.
in RMS voltage
at the power frequency for
cycle to 1 minute.
Selective Opening (Tripping) --The application of switching devices in series such that
(of the devices carrying fault current) only the device nearest the fault will open and the
devices closer to the source will remain closed and carry the remaining load.
Service Voltage--Voltage at the facility service entrance location.
--A n abnormal condition (including
of relatively low impedance,
whether made accidentally or intentionally, between two points of different potential.
Surge Protector--A device composed of any combination of linear or nonlinear circuit
elements and intended for limiting surge voltages on equipment by diverting or limiting
surge current; it prevents continued flow of follow current and is capable of repeating
these functions as specified.
Swell--An increase in RMS voltage or current at the power frequenc y for a duration of
0.5 cycle to 1 minute.
Symmetrical Current--A current wave symmetrical about the zero axis.
Tactile Signs--Signs perceptible by touch that provide critical egress information to the
Thermal Trip Unit--A bimetal element connected to the breaker trip unit to provide
protection with an inverse time characteristic.
Time-Current Curve--A graphical representation of the expected time of a breaker to
open in response to overcurrent.