Designing criteria for selectivity and reliability of building power systems

In this paper several specific aspects of power system design are analysed, such as sizing, the selection of protective devices and coordination, by correlating them with a number of general power system features, so as to obtain in a “natural” form high levels of reliability and selectivity. A power system general optimization process methodology is suggested, especially usable in all the cases in which the system´s “architecture” is an actual project variable. It is shown how the maximum and minimum short circuit current values may be the main factors determining the selection of circuit ratings. During power system design the best procedure seems to be that which, from the short circuit level on the main switchboard, leads to a selection of circuit ratings also on the basis of the distances of estimated load sets. By iterating this procedure, the “architecture” of the system is adapted to the specific cases