Basic Training Course
Voltage Gradient Across High Voltage Cable Insulation
Core insulation may be coloured red, yellow, blue and black to identify the
three phases and neutral. Twin cores are coloured red and black. Singlecore
cables are identified by coloured PVC tape applied to the outer
sheath.
2.4 Cable Stress Relief
The copper screen is often terminated in a 'stress cone', which may be
seen in Figure 7. This is to spread the electric stress which would
otherwise tend to concentrate where the screen is cut off at a cable end
and could lead to breakdown. This is further discussed in para. 6.4
2.5 Bedding
The bedding consists of a layer of PVC extruded over the core insulation
as a base for the armouring.
2.6 Armouring
Mechanical protection of the cable is provided by a single layer of wire
strands laid over the bedding. Steel wire is used for 3-core or 4-core
cables, but single-core cables have aluminium wire armouring. With 3-core
or 4-core cables the vector sum of the currents in the conductors is
zero, and there is virtually no resultant magnetic flux. This is not so
however for a single-core cable, where eddy-current heating would occur
if a magnetic material were used for the armouring. Armouring is
described as Steel Wire Armoured (SWA) or Aluminium Wire Armoured
(AWA).
Outer Sheath
The outer sheath of extruded PVC protects the armouring and the cable
against moisture and generally provides an overall protective covering.
High-voltage cables are identified by outer sheaths coloured red; a black
sheath indicates a low-voltage cable (see also para. 7)
2.8 Selection of Power Cables
The following considerations are taken into account when selecting a
power cable for a particular application:
(a) The System Voltage and Method of Earthing
A low-voltage system usually has a solidly earthed neutral so
that the line-to-earth voltage cannot rise higher than (line
volts) ÷ ? 3. However, cables for low-voltage use are
insulated for 600V rms score to earth and 1000V rms core
to core.
High-voltage cables used in Shell installations are rated
1900/3300V or 3810/6600V or 6600/11000V, phase/line. In
selecting the voltage grade of cable, the highest voltage to
earth must be allowed for. For example, on a normal 6.6kV
unearthed system, a line conductor can achieve almost 6.6kV
to earth under earth-fault conditions, To withstand this, a
cable insulated for 6600/11000V must therefore be used.
(b) The Normal Current of the Cable
The conductors within a cable have resistance, and
therefore /2 R heating occurs when currents pass through
them, The maximum permissible temperature of the cable
depends upon the material of the insulation, and a conductor
size must be chosen so that this temperature is not
exceeded. Tables giving the continuous current-carrying
capacities of different cables are given in manufacturers'
literature and in the Regulations for the Electrical
Equipment of Buildings published by the Institution of
Electrical Engineers.
The temperature of a cable depends not only on the rate of
heat input due to the passage of load current but also on the
rate at which the heat can be carried away. When using the
tables of current ratings it is important to note whether
they refer to cables laid in the ground laid in ducts or laid in
air. De-rating may be necessary if a number of cables are
run in close proximity to each other.
Another consideration in selecting a cable is the voltage (IR)
drop from the source of supply to the load. A drop of 1V in a
440V circuit is of little consequence, but it is a significant
percentage when the circuit operates at 24V,
(c) Abnormal Currents in the Cable
One abnormal condition is a sustained overload; a cable must
be protected so that an overload cannot persist long enough
to cause damage to the insulation by overheating. For
example, for PVC cables laid in air, the overload must not be
greater than 1.5 times the continuous maximum rated
current and must not persist for longer than four hours.
Another abnormal condition is when a cable has to carry a
through short-circuit current. In this case the temperature
of the conductor may be allowed to rise to a higher value,
say 150°C, for the short interval between the onset of the
fault and its disconnection. The short-circuit current that a
given cable can withstand depends upon the speed with which
the protection operates. For example, a PVC cable having
conductors of 185mm2 has the following short-circuit
ratings:
46kA for 0.2s
20.3kA for 1.0s
11.7kA for 3.0s
The 0.2s rating would be suitable for use with fuse
protection, but where relay-operated circuit breakers are
concerned, a longer time rating would be necessary. Again,
tables of short-circuit ratings are available in
manufacturers' literature.
3 Control Cables
Control cables usually have conductors either 1.50mm2 or 2.50mm2 in crosssection.
The insulation, bedding and outer sheath are of PVC, and they are steel
wire armoured. Multicore cables are available having 2, 3, 4, 7, 12, 19 and 27
cores, each core being identified by a number on the insulation. The outer
sheath of control cables is coloured green.
4 Mineral Insulated Cables
Mineral-insulated (MI) cables are used where the integrity of a circuit is of
great importance. They are particularly resistant to fire and are used in
circuits, such as communications or emergency lighting, which must continue
operational as long as possible after fire has broken out. They are also very
robust and resistant to mechanical damage
