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 is 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 earth-fault current
3U
O
Three times the zero sequence voltage. Often referred to as
the residual voltage or the neutral point voltage
Section 15 1MRK504114-UEN E
Glossary
120
Operator's manual
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