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ii) For cables where the above tests are not made, an rms value equal to 60% of the
lowest dc voltage to 85% of the lowest ac voltage used in factory tests to ensure the
breakdown strength between conductors and sheath or screen unless there is reason to fear
that the laying and jointing operations have caused any appreciable reduction in the
breakdown point. In such cases, special studies should be made to select the method for
determining the permissible limit.
Where only some of the cable pairs are terminated satisfying the above conditions, the
voltage limit indicated in (i) or (ii) is permitted on such cable pairs provided that the dielectric
strength from other parts in the cable is sufficient to avoid breakdown.
The isolating transformers and other line apparatus should have a dielectric strength
equivalent to or greater than available on the cable conductors, unless lightning protectors are
used.
Experience shows that dangerous levels of induced voltage are unlikely on cable
conductors where the above conditions are met and where faults on medium voltage inducing
lines are involved and where protective devices are used. Where the permissible induced
voltage on cable conductors is increased above the permissible levels for open wire lines, it is
desirable to consider safety precautions when work is carried out on these cables and to
ensure that equipment connected to the line can withstand the resultant common mode
voltages and currents.
Allowance may need to be made when considering permitted induced voltages induced into
Cables carrying significant telecommunications voltages (e.g. power feeding systems).
8.1.3 Permissible capacitively – coupled current :
In cases of capacitive coupling, a resulting current through a contact between a
conductor and earth of other metallic structure, upto 10 mA is permissible.
8.2 COUPLING BETWEEN CIRCUITS:
The coupling between two circuits may be conductive or alternately due to electric or
the magnetic field. These are distinguished as conduction, electrostatic induction and
electromagnetic induction. Even when all the three kinds of couplings occur simultaneously,
usually one of them will usually prevail.
8.3 CONDUCTIVE COUPLING:
Conductive coupling is present when two circuits I and II have a common branch (See
Fig.8.1). If the common branch is sufficiently well defined the distribution of the current and the
effect produced in II may be calculated. If they are coupled by a common resistance “r”, the
parasitic current I in circuit II is given by
I = Er_____
RW + Rr + Wr
Generally, Wr >> R >> r because I is a power circuit and II a telecommunication circuit.
It is approximately equal to I = Fr/RW. The effect in II is same as if a parasitic voltage
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