As known, the guard wires of overhead high voltage
are usually grounded through the grounding systems of support and
of the terminal stations. They do affect the zero sequence impedance
value of the line, Z0, which is generally, calculated assuming that the
wires guard are at ground potential. In this way it is not considered
the effect of the resistances of earth of supports and stations. In this
work is formed a formula for the calculation of Z0 which takes
account of said resistances. Is also proposed a method of calculating
the impedance zero sequence overhead lines in which, in various
sections or spans, the guard wires are connected to the supports, or
isolated from them, or are absent. Parametric analysis is given for
lines 220 kV and 400 kV, which shows the extent of the errors made
with traditional methods of calculation.
 A. Paolucci: Lessons of electrical, first Part, CLEUP Publisher, March
 P.Nefzger, J.F. Nolasco, U. Kaintzyk, F.Kiessling: Overhead Power
Lines, Springer, 2003.
 F. Iliceto, FM Gatta, S. Lauria, GO-Dokyi "Three-Phase and Single-
Phase Electrification in Developing Countries Using the Insulated Shield
Wires of HV lines Energized at MV "- CIRED art. 5/10, Session of
 FM Gatta, F. Iliceto-"Calculation of Current Flow in Grounding
Systems of substations and HV Line Towers, in Line Shield Wires and
Cable Sheaths during Earth Faults "- ETEP, Vol.8, No.2, March / April
 R. Benato, L. Caciolli: "Sequence impedances of insulated cables:
Measurements versus computations ", IEEE PES T & D Conference
2012, Orlando, Florida,.
 IEC 60909-2: Short-circuit in three-phase ac systems. Part 2: Data of
electrical equipment for short circuit current calculations. Edition 2008.
 R.Benato, L. Caciolli, and E. Zaccone: Sequence impedance
computation by means of Multiconductor cell analysis. 8th International
Conférence on Insulated Power Cables, 2011.
 L.Fellin, R. Benato: Electrical Systems, UTET Technical Sciences,
2010, ISBN 978-88-980618-9.