Handbook of Electrical Engineering

374 HANDBOOK OF ELECTRICAL ENGINEERING

a) Single conductor above a flat plane.

Leakage capacitance.

Self-inductance.

b) Two conductors in parallel.

Coupling capacitance between the conductors.

Self-inductance of each conductor.

Mutual inductance between the conductors.

b) One screened conductor in a cylinder.

Coupling capacitance.

Self-inductance of the conductor.

Mutual inductance between the conductor and the cylinder.

Invariably the cable length is very much greater than the radius of a conductor and its separation
from other conductors in the cable or its screening. Therefore all the capacitances and inductances
are distributed along the length of the cable. The conductor resistance and the insulation leakage
resistance are also distributed. For practical calculations it is adequate to ‘lump’ these parameters
into single elements of inductance, capacitance and resistance.

13.5.2 Screening against external interference

Instrumentation cables frequently run in parallel along the same routes as heavy current power cables.
The routing is designed in such a manner that a prespecified spacing is used between power cables
and instrument cables. Table 13.1 gives typical minimum spacings between the cables that run in
the same trench or set of racks. There are situations where a power cable can radiate interference,
particularly in the form of mutually induced currents, for example:-

• Single-core cables run in groups.


• Cables carrying unbalanced currents.


• Cables carrying harmonic currents, e.g. drilling power systems.


• Cables carrying surge currents, e.g. starting large motors direct-on-line.


• Cables carrying fault currents of high magnitude, particularly if they flow in the armouring.

Table 13.1. Separation of electronic and power

cables

Power cables and

control cables

Minimum Separation

of cables (mm)

110 V or 10 A 300

240 V or 50 A 450

415 V or 500 A 600

3300 V to 33,000 V 1000

Currents above 200 A 1000