- phase-to-phase: a three-phase insulation configuration where one phase terminal
is disregarded. In particular cases, the neutral and the earth terminals are also
disregarded.
- longitudinal, having two phase terminals and one earth terminal. The phase
terminals belong to the same phase of a three-phase system temporarily separated into
two independently energized parts (open switching devices). The four terminals
belonging to the other two phases are disregarded or earthed. In particular cases one
of the two phase terminals considered is earthed.
3.8 nominal voltage of a system: A suitable approximate value of voltage used to
designate or identify a system. [IEV 601-01-211
3.9 highest voltage of a system: The highest value of operating voltage which occurs
under normal operating conditions at any time and at any point in the system.
[IEV 601-01-231
3.10 highest voltage for equipment (U,,,): The highest r.m.s. value of phase-to-phase
voltage for which the equipment is designed in respect of its insulation as well as other
characteristics which relate to this voltage in the relevant equipment Standards.
[IEV 604-03-011
3.11 Isolated neutral system: A system where the neutral point is not intentionally
connected to earth, except for high impedance connections for protection or measurement
purposes. [IEV 601-02-241
3.12 solidly earthed neutral system: A system whose neutral point(s) is(are) earthed
directly. [IEV 601 -02-251
3.13 Impedance earthed (neutral) system: A system whose neutral point(s) is(are)
earthed through impedancest o limit earth fault currents. [IEV 601 -02-261
3.14 resonant earthed (neutral) system: A system in which one or more neutral points
are connected to earth through reactances which approximately compensate the
capacitive component of a single-phase-to-earth fault current. [IEV 601 -02-27)
NOTE - With resonant earthing of a system, the residual current in the fault is limited to such an extent
that an arcing fault in air is usually self-extinguishing.
3.15 earth fault factor: At a given location of a three-phase system, and for a given
system configuration, the ratio of the highest r.m.s. phase-to-earth power frequency
voltage on a healthy phase during a fault to earth affecting one or more phases at any
point on the system to the r.m.s. phase-to-earth power frequency voltage which would be
obtained at the given location in the absence of any such fault. [IEV 604-03-06]
3.16 overvoltage: Any voltage between one phase conductor and earth or between
phase conductors having a peak value exceeding the corresponding peak of the highest
voltage for equipment. [IEV 604-03-09, modified]