2.1 GENERAL
Cables form an important
part of any installation but, because they are static, and in normal service
are very reliable, they do not always receive the attention that they deserve.
There are three
categories of cables associated with industrial installations - power cables,
control cables, and special cables for, for example, communications and data
transmission circuits. It is the first
two categories which are described in this chapter. A power cable contains one, two, three or
four cores each consisting of a copper conductor surrounded by insulating
material; a control cable usually has many cores and is known as a ‘multicore’
cable. Aluminium is sometimes used as a
conductor material; although its conductivity is less than that of copper, it
is somewhat cheaper. Corrosion problems, however, preclude its use on some
installations, particularly offshore.
2.2 POWER CABLES
Cables are designed
for both high-voltage and low-voltage transmission of power. Though the general construction is similar in
both cases, high-voltage cables have thicker insulation and usually have
smaller conductors, since low-voltage cables carrying bulk power handle the
heavier currents.
A power cable is made
up of one, two, three or four insulated conductors enclosed in a bedding. For mechanical protection, wire armouring is
wrapped around the bedding, and a coloured outer protective sheath, usually of
PVC, is extruded over the armouring, as shown in Figure 2.1. Each insulated conductor is known as a
‘core’.
FIGURE 2.1
THREE CORE POWER CABLE
2.2.2 Conductors
The size of the copper
conductor forming one of the cores of a cable is expressed in square
millimetres (mm2), and the current rating of the cable is dependent
upon the cross-sectional area of each core.
The very smallest cables have conductors consisting of only one strand
of copper; larger cables however have stranded conductors consisting of many
individual strands or wires laid up together; this gives flexibility, allowing
the cable to be bent more readily during installation. To achieve a circular conductor, the number
of strands follows a particular progression: 3, 7, 19, 37, 61, 127 etc, the
diameter of each strand being chosen to achieve the desired cross-sectional
area of the whole conductor.
As seen in Figure 2.2,
3-core and 4-core cables in the larger sizes have conductors with the strands
laid up in a segmental formation; this achieves a better space factor and
reduces the overall diameter of the cable.
It also reduces the inductance of the cable due to decreased spacing
between phases.
FIGURE 2.2
SEGMENTAL CORES
Standard conductor
sizes range from 1.5mm2 to 400mm2 for 2-core,
3-core and 4-core cables, and from 50mm2 to 1 000mm2 for
single-core cables.
Natural rubber or
oil-impregnated paper is no longer used for the insulation of cables up to
3 810/6 600V; synthetic materials are now used. For high-voltage cables the insulation is
ethylene propylene rubber (EPR) and for low-voltage cables it is polyvinyl
chloride (PVC). EPR has good electrical
properties and is resistant to heat and chemicals; it is suitable for a
conductor temperature up to 85oC.
PVC is a thermoplastic material, therefore care must be taken not to
overheat it; it is suitable for conductor temperatures up to 70oC. PVC insulated cables should not be laid when
the temperature is less than 0oC because it becomes brittle and is
liable to crack.
High-voltage cables
have an earthed metallic screen over the insulation of each core. This screen
consists of a lapped copper tape or metallic foil, and its purpose is to
control the electric field within the insulation and thus the voltage gradient
across it, as shown in Figure 2.3. Also, it avoids any interaction of the
electric stresses due to the voltages on different phase conductors within the
same cable.
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FIGURE 2.3
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. Single-core cables are identified
by coloured PVC tape applied to the outer sheath.
The copper screen is
often terminated in a ‘stress cone’, which may be seen in Figure 2.7. 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. 2.6.4.
The bedding consists
of a layer of PVC extruded over the core insulation as a base for the
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).
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. 2.7).
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 core to
earth and 1 000V rms core to core
High-voltage cables used in some installations are
rated 1 900/3 300V or 3 810/6 600V or
6 600/11 000V, phase/line. In
selecting the voltage grade of cable, the highest voltage to earth must be
allowed for. For example, on a nominal
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 6 600/11 000V must be used.
(b) The
Normal Current of the Cable
The conductors within a cable have resistance, and therefore I2R 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 150oC,
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.
2.3 CONTROL CABLES
Control cables usually
have conductors either 1.50mm2 or 2.50mm2 in
cross-section. 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.
2.4 MINERAL INSULATED CABLES
Mineral-insulated (Ml)
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
FIGURE 2.4
MINERAL INSULATED TWO CORE CABLE
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.
Ml cables are
constructed by assembling the single-strand conductor or conductors inside a
seamless copper tube. After threading a
number of ‘tablets’ of magnesium oxide insulating material onto the conductors,
the whole assembly - conductors, insulation and copper tube - is
drawn down through a series of dies until the magnesium oxide is crushed to a
powder and the whole cable is solid. The
final appearance is as in Figure 2.4.
After annealing to
make the cable more flexible, an outer sheath of PVC is applied.
Ml cables are
available in single-core from 1mm2 to 150mm2, in 2-core,
3-core and 4-core from 1mm2 to 25mm2, and in 7-core from
1mm2 to 4mm2.
Special jointing
techniques and materials must be used for terminating MI cables, and great care
must be taken to seal the cable ends against the entry of moisture.
2.5 METHOD OF SPECIFYING CABLES
There is a ‘shorthand’
method used to describe the construction of any cable, using abbreviations to
indicate the nature of the various materials. For example, a low-voltage cable
might be described as:
(Reference)
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(1)
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(2)
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(3)
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(4)
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(5)
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(6)
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(7)
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(8)
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(9)
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(10)
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Abbreviation
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0.6/1kV
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STR
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CU/
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PVC/
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PVC/
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SWA/
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PVC/
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HO2/
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HCL
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3-core, 150mm2
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Interpreted this
means:
1 0.6kV line to earth
2 1 kV line to line
3 Stranded conductor
4 Copper conductor
5 PVC conductor insulation
6 PVC bedding
7 Steel wire armoured
8 PVC outer sheath
9 see below
10 see below
Another example is:
6.6/11kV
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STR
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CU/ EPR/ SCR/ PVC/
AWA/ PVC/ HO2/ HCL
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1-core, 630mm2
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where EPR indicates ethylene propylene
conductor insulation
SCR indicates screened
AWA indicates aluminium
wire armoured.
The last two items (9
and 10) indicate the flammability and the toxicity of the synthetic materials
used in the cable. HO2 indicates
that a high level of oxygen is required to sustain combustion: in the case of
the specification this means more than 30% oxygen in the atmosphere. HCL denotes ‘Hydrochloric Level’ showing
that, when the synthetic materials burn, they produce hydrochloric acid gas
(HCl) which is highly poisonous and very corrosive.
In particular PVC,
when burnt, releases large quantities of HCl and also produces dense black
smoke; for example, a 1m length of cable containing, say, 6kg of PVC can
completely black out a room 1 000m3 in size within five minutes
of the fire starting.
2.6 INSTALLATION
Cables may be laid
discretely in the ground, run in ducts or clamped to cable trays; the third
method is the most common in offshore installations. Each cable must be identified at each end,
using a marker bearing the cable number.
There is a practical
limit to the conductor size which can be run as a 3-core or 4-core cable it
becomes too stiff and heavy to handle. A
3-phase circuit is then run as three (or four) single-core cables. To minimise the electromechanical forces
between the cables under short-circuit conditions, and to avoid eddy-current
heating in nearby steelwork due to magnetic fields set up by load currents, the
three single-core cables comprising the three phases of a 3-phase circuit are
always run clamped in ‘Trefoil’ formation, as shown in Figure 2.5.
FIGURE 2.5
SINGLE CORE CABLES LAID IN
TREFOIL
At any instant in time
the net magnetic flux outside the group of cables due to the three line
currents in them approximates to zero because of the symmetrical cable layout.
Heavy current cable
runs, such as the low-voltage connections from a transformer, may consist of up
to four single-core cables in parallel per phase; all 12 cables are run bunched
into four 3-phase sets, each set laid in trefoil. In the case of 4-wire systems the neutral
conductor needs a smaller cross-sectional area than that of the phase
conductors and may be met by one or more smaller single-core cables in
parallel.
A power cable is
terminated in an air-insulated cable box in offshore installations; it enters
the box through a compression gland which grips the wire armouring and seals
the entry of the cable. The outer
sheath, armouring and bedding of the cable are stripped back, enabling the
cores to be spread to match up with the fixed bushing terminals, and the insulation
is removed to expose the conductors.
Such a cable box is often referred to as a ‘trifurcating box’.
In some high-voltage
onshore installations, especially outdoor ones, the cable box may be filled
with compound, a tar-like substance which is poured in hot and then sets hard
to exclude moisture. It can only be removed by heating.
FIGURE 2.6
LOW VOLTAGE CABLE TERMINATION
Conductors are
terminated either with lugs bolted to the fixed bushing stems as shown in
Figure 2.6 or, for heavier currents, with cylindrical ferrules which are clamped
into terminal blocks. In either case the terminations are crimped onto the
conductors using either hand or hydraulic crimping tools. To make a good
connection it is vital that the lug or ferrule is the correct size for the
particular conductor and that the correct die is used in the crimping tool.
FIGURE 2.7
HIGH VOLTAGE CABLE TERMINATION
Special
measures must be adopted, when screened high-voltage cables are terminated, to
prevent a concentrated electric field being developed where the copper screening
tape is cut back; this strong electric field could lead to the insulation at
that point being so overstressed that a breakdown occurs. Special stress cones are fitted which are
bonded to the screening tape; they control the electric stress and reduce the
resulting voltage gradients to a safe value.
This arrangement is shown in Figure 2.7.
The conductor of a
single-core cable and its surrounding metallic armouring act as a current
transformer having a 1:1 turns ratio; load current passing through the
conductor produces a magnetic flux which, linking with the wires of the
armouring, induces an emf in them. If a
circuit is provided between the armouring at one end of the cable and at the
other, a current flows in the armouring which, if sufficiently large, causes
heating. This is shown in Figure 2.8(a).
FIGURE 2.8
INSULATED CABLE GLANDS
voltage of the armouring it must be bonded to earth; this is done by
deliberately bridging the gland insulation using bonding links. The armouring can be bonded in one of two
ways. In Figure 2.8(a) it is bonded at
both ends of the cable run (shown in red); the emf induced in the armouring
causes currents (IA) to circulate in the armouring which in
heavy current circuits may lead to an undesirable temperature rise in the
armouring. Alternatively, the armouring
may be bonded at one end only as in Figure 2.8(b); there is no circuit for
current to flow, but a voltage (EA) is developed across the
gland insulation at the unbonded end.
Where one end of the circuit is in a hazardous area, it is customary to
bond this end so that any arcing that may occur due to emfs induced in the armouring
can only take place in the non-hazardous area.
There is one other
magnetic problem associated with single-core cables: where such cables enter a
cable box or pass through partitions the conductors must pass through holes in
the gland plate. If these plates are made of a magnetic material such as steel,
the magnetic fields due to the load currents in the conductors induce eddy
currents in the gland plate which may cause it to become very hot. For terminating or passing a.c. circuits
using single-core cables, gland plates of non-magnetic material must be used.
One make of
high-voltage, plug-in elbow connector used in some installations is the
‘Elastimold’ type illustrated in Figure 2.9, where the live conducting parts
are shown in red.
FIGURE 2.9
ELASTIMOLD HIGH VOLTAGE ELBOW
CONNECTOR
If a cable core screen
is cut or terminated abruptly, the electric field distribution within the cable
changes radically outside it. Both the
surrounding air and the dielectric material immediately in the vicinity of the
terminated screen then become overstressed electrically. The continuity of the earthed cable screen is
carried on by the metal elbow past the plug-in connector to the entry bushing on
the equipment. To prevent rapid breakdown
of the cable, a stress cone is applied at the end of the screen (bottom of
Figure 2.9). The cone has an insulating
portion to reinforce the primary cable insulation and also an earthed
conductive portion to mate with the cable core screen. This is to control the distribution of the
equipotential lines (that is, lines of equal voltage) shown in blue, so that,
when they finally emerge into the air, they are sufficiently far apart not to
cause too great a potential gradient and so not to give rise to ionisation and
possible electrical breakdown.
In Figure 2.9 the
dispersal of the blue equipotential lines at the cone area (shown as percentages
of the core voltage) is clearly seen.
Control cables are
also terminated using compression glands.
The sheathing and armouring are stripped back to leave tails of the
required length. Each core is identified
using plastic ferrules bearing the wire number, and terminated using a crimped
connector. The cores are either laced up
into suitable runs using plastic cable ties, or secured to cable racks within a
control panel.
2.7 OUTER SHEATH COLOURS
Standard colours are
used in some installations to identify the system to which the various cables
belong; they are:
Red
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High-voltage system.
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Black
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Low-voltage system.
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Green
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Control and
instrumentation system.
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Orange
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Fire and gas
detection and telecommunications systems
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Blue
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Intrinsically safe
systems.
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Yellow
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Thermocouple
circuits.
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