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Various types of system circuit faults are possible. The most common is the phase-earth fault. During this fault, currents flow through earthing grids as well as through the earth. These currents are the consequence of electromagnetic coupling of conductors and earthing grid potentials and can he presented by current reduction factor. When earhing conductors are laid together with cables, the current reduction factor is very difficult to define. In this paper a mathematical determination model of current reduction factor is presented for the cable and earthing conductors laid in the same trench. Influence of both electromagnetic characteristics and earthing conductor geometrical characteristics as well as influence of geophysical features on the current reduction factor is analyzed in this paper.

Root causes of the blackout that happened on January 12, 2003 in southern part of Croatia (HR) and Bosnia Herzegovina (BH) are shown in this paper with respect to the role of the system operators. Action of the system operators during chronological sequence of the blackout events represents main topic of this paper. Root causes and consequences of the blackout are recognized and a set of countermeasures is proposed to the utilities in order to prevent its reoccurrence.