A single phase transmission line and a telephone line are

A single phase transmission line and a telephone line are both symmetrically strung one below the other, in horizontal configurations, on a common t ower. The shor t est and l ongest distances between the phase and telephone conductors are 2.5 m and 3 m respectively. The volt age (volt /km) induced in the telephone circuit, due to 50 Hz current of 100 amps in the power circuit is

Mutual inductance between lower line and telephone line,

M = 4 × 10^-7 In		D_P₁T₂		H/m
		D_P₁T₁

= 4 × 10^-7 In		3		H/m
		2.5

= 0.73 × 10^–7 H/m
Hence voltage induced by power line into the telephone line,
|V_T| = ωMI
= 314 × (0.73 × 10^–7) × 100
= 2.29 H/km