Low-voltage network

Most modern secondary networks are operated at AC rated voltage of 100–127 or 220–240 volts, at the frequency of 50 or 60 hertz (see mains electricity by country).

From the distribution substation, feeders carry the power to the end customers, forming the medium-voltage or primary network, operated at a medium voltage level, typically 5–35 kV.

[3] Typically, a rural primary feeder supplies up to 50 distribution transformers, spread over a wide region,[4] but the figure significantly varies depending on configuration.

[6] North American LV networks feature much shorter secondary connections, up to 250 feet (80 m), while in European design they can reach up to 1 mile (1,600 m).

[7] In American (120 V) systems, the customers are commonly supplied directly from the distribution transformers via relatively short service drop lines, in star-like topology.

Power-system protection in radial networks is simple to design and implement, since short-circuit currents have only one possible path that needs to be interrupted.

Only in cases when the short circuit is located at the paralleling bus, or a total loss of primary supply occurs, the customer will remain out of service.

Faults on the low-voltage network are handled by fuses or local circuit breakers, resulting in loss of service only for the affected loads.

Grid networks are typically featured in downtowns of large cities, with connecting cables laid out in underground conduits along the streets.

[11] Primary feeder outages, as well as limiters and network protectors cleared because of previous faults, cause changes in load flow that are not readily detected, so their statuses may require a periodic inspection.