Classification of Lines(electrically or physically)

Lines may be classified either electrically or physically.

Electrical Classification

1.Direct Current (DC) systems

(a) DC 2-wire ,1-wire earthed

(b) DC 1-wire ,earth return

(c) DC 2-wire, mid-point earthed

(d) DC 3-wire.

2.Alternating Current (AC) system 

(a) Single-phase,2-wire,1-wire earthed.

(b) Single-phase,1-wire,earth return.

(c) Single-phase,2-wire,mid-point earthed.

(d) Single-phase,3-wire

(e) Two-phase ,3-wire

(f) Two-phase,4-wire

(g) Three-phase,3-wire

(h) Three-phase,4-wire

All these system are used in practice. Every system has got its own field of application.The line economics varies with the individual  system.Direct current 2-wire,mid-point earthed system is used for transmitting large amounts of power over load distances.

Single-phase a.c.system is rarely used for transmitting purposes. Three-phase 3-wire a.c.system finds its greatest use for transmission and primary distribution.For secondary distribution work three-phase,4-wire system is common.The industrial and other big loads are supplied by three-phase,4-wire system.The domestic and other small load consumers are supplied with single-phase power obtained from the individual phases of a three-phase supply.

Physical Classification

The other criterion of classification  of lines is their physical configuration.The lines may be overhead,underground or underwater.Again,overhead lines may either have bare conductors or insulated conductors.

Most of the high voltage and extra high voltage lines have bare conductors since these are most economical.The conductors on overhead lines are sometimes insulated for safety or other considerations.The insulated aerial cabled although costlier than the bare conductors are far cheaper than the underground cables. They  are used in densely populated urban areas at relatively low voltages or for rural distribution work at medium voltages.

For long river or sea crossing,it is not possible to erect overhead lines due to their longer spans.In such cases the need of underwater cables becomes inevitable.such cables are very costly to manufacture and install.