Underreach A term used to describe how the relay behaves during a fault
condition. For example a distance relay is under-reaching when
the impedance presented to it is greater than the apparent
impedance to the fault applied to the balance point, i.e. the set
reach. The relay does not “see” the fault but perhaps it should
have seen it. See also Overreach.
U/I-PISA Process interface components that deliver measured voltage and
current values
UTC Coordinated universal time. A coordinated time scale,
maintained by the Bureau International des Poids et Mesures
(BIPM), which forms the basis of a coordinated dissemination of
standard frequencies and time signals. UTC is derived from
International Atomic Time (TAI) by the addition of a whole
number of "leap seconds" to synchronize it with Universal Time
1 (UT1), thus allowing for the eccentricity of the Earth"s orbit,
the rotational axis tilt (23.5 degrees), but still showing the
Earth"s irregular rotation, on which UT1 is based. The
Coordinated Universal Time is expressed using a 24-hour clock
and uses the Gregorian calendar. It is used for aeroplane and
ship navigation, where it also sometimes known by the military
name, "Zulu time". "Zulu" in the phonetic alphabet stands for
"Z" which stands for longitude zero.
UV Undervoltage
WEI Weak end infeed logic
VT Voltage transformer
X.21 A digital signalling interface primarily used for telecom equipment
3I
O
Three times zero-sequence current. Often referred to as the
residual or the -fault current
Three times the zero sequence voltage. Often referred to as the
residual voltage or the neutral point voltage
1MRK506276-UUS B Section 15
Glossary
111
Operator's manual
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