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• The rapid development of new technologies in telecommunications, electric power transmission and distribution, and electric railway systems has made the earlier issue of the Directives (1960 edition) out-of-date and this former issue is therefore being replaced completely.

• The first edition of the Directives was approved by the 2nd Plenary Assembly of CCITT in New Delhi 1960. The Directives were amended in 1965, 1974, 1978, 1982 and 1988. They provided a comprehensive description of electromagnetic effects due to the close proximity of telecommunication lines to power and electrified railway systems, containing regularly updated findings in this field and reflecting the current state of the art.

• In a very general sense the capacitive coupling expresses the relation between the potential (as a source) of the inducing electric line and the induced charging current, per unit length, (as a result) occurring on the telecommunication circuit. In the case of induced line with conductors insulated from earth in a parallel exposure the coupling is given as a relation between the inducing voltage and the wire-to-earth voltage caused by capacitive induction.

• This study of inductive coupling concerns the self-impedance of a conductor with earth return and the mutual impedance between two single conductors both with earth return. The theoretical background for evaluation of these impedances is presented in Volume III. In the present section, formulas, curves and tables are given which can be used to calculate the results of inductive coupling in practical cases. In each case the value of inductance is used and given instead of, or together with, the value of impedance. The relationship between impedance Z and inductance M, in the case of alternating currents of frequency f (angular frequency ω = 2 πf) is: Z = jωM. The expressions of impedances include real and imaginary components, the magnitudes of which vary greatly with the distance between the lines. In the neighbourhood of the inducing line the imaginary component of Z (the real component of M) predominates. The electric field induced along a parallel conductor in this area and the current density at ground level are almost in 90° phase shift to the inducing current. At large distances from the inducing line the real component of Z (imaginary component of M) is predominant: the electric field induced along a parallel conductor and the current flow at ground level are almost in phase opposition to the inducing current. The variation with distance of the phase difference between the magnitudes of both currents is continuous and fairly uniform. The imaginary components of M have negative values throughout the whole range, consequently when multiplying them by jω the calculated real component of Z will be positive.

• When a power system fault to earth occurs, some of the fault current returns via the earth, through the earth electrodes (e.g. that of the tower where the fault occurs and ground grid of a high voltage power station). The current through an earth electrode produces a potential difference between the electrode and remote earth. This potential difference is referred to as electrode (power station) ground potential rise (GPR). The area surrounding a high voltage earth electrode that is raised in potential is referred to as zone of GPR.