As compared to a 2-wire D.C. distributor, a 3-wire distributor with same maximum voltage to earth uses only

As compared to a 2-wire D.C. distributor, a 3-wire distributor with same maximum voltage to earth uses only

Right Answer is:

33.3 percent of copper

SOLUTION

As compared to a 2-wire D.C. distributor, a 3-wire distributor with same maximum voltage to earth uses only 33.3 percent of copper.

  • For the same conductor length, the same amount of power, same losses and same maximum voltage to earth, 3 wire DC system requires minimum conductor area
  • For transmitting the same amount of power at the same voltage, a three-phase transmission line requires less conductor material than a single-phase line; The three-phase transmission system is so cheaper
  • For a given amount of power transmitted through a system, the three-phase system requires conductors with a smaller cross-sectional area; This means a saving of copper and thus the original installation costs are less

Important Point:

Below is given the table which shows the ratio of conductor-material in any system compared with that in the corresponding 2-wire DC system. Cos φ is the power factor in an AC system.

System Same maximum voltage to earth Same maximum voltage between conductors
DC system: Two-wire 1 1
DC: Two-wire mid-point earthed 0.25 1
DC: 3 wire 0.3125 = 5 / 16 1.25
Single-phase: 2 wire 2/cos2ϕ 2/cos2ϕ
Single-phase: 2 wire mid-point earthed 0.5/cos2ϕ 2/cos2ϕ
Single-phase: 3 wire 0.625/cos2ϕ 2.5/cos2ϕ
2-phase: 4 wire 0.5/cos2ϕ 2/cos2ϕ
2-phase: 3 wire 1.457/cos2ϕ 2.914/cos2ϕ
3 phase, 3 wire 0.5/cos2ϕ 1.5/cos2ϕ
3 phase, 4 wire 0.583/cos2ϕ 1.75/cos2ϕ
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