Download Sheathed and unsheathed cable

Contents
Introduction
Assessment criteria
2
2
Parts of a cable
4
Electrical cable types
5
Sheathed and unsheathed cable
5
Flat and circular sheathed cable
5
Number of cores
6
Sheathing colours
6
Special cable types
7
Cable design
Conductor stranding
9
9
Common cable configurations
10
Cable insulation
11
Cable identification
12
Conductor colours
12
Applications of cables
14
Fixing and supporting sheathed cables
14
Limitations on use of sheathed cables
15
Insulated cables without sheath
15
Jointing and terminating cables
16
Soldering
16
Terminal blocks
17
Crimped connections
17
Poor connections
18
Summary
20
Answers
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Introduction
All electrical circuits consist of cables of one type or another. You must be
able to identify the types of cables you are expected to use, so that you can
select the correct cable for the particular application.
In this section, we’ll look at the types of insulated cables used for electrical
wiring, identification of cables, termination of cables, and accessories.
Sheathed cable is the most commonly used cable in electrical installation
work. It is used almost exclusively in domestic installations, very widely in
wiring in commercial buildings and extensively in industrial wiring.
Sheathed cable is made from cores of building wire enclosed by another
material.
Assessment criteria
At the end of this section you should be able to:

list common cables and state typical applications

explain the terms conductor material, stranding, insulation type, voltage
rating, screening, sheathing, armour and serving

state the Australian and International colour standards for cords and
cables

describe the construction of common cables

identify cords and cables by conductor size, type and rating

describe typical applications for given cord and cable types

identify hardware used in terminating cords and cables

demonstrate correct preparation for termination of cords and cables
correctly terminate cords and cables using crimp lugs, tunnel connectors,
soldering and solderless lugs
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Resources
You will need the following resources to complete this section. All
resources should be available from your campus bookstore:

AS/NZS 3000:2000 Electrical Installations: Australia and New Zealand.
We will refer to this Standard as ‘the wiring rules’.

Electrical Wiring Practice by Pethebridge and Neeson. We will refer to
this reference in the text as EWP.

The resource manual for this module (with a yellow cover).
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Parts of a cable
You need to be familiar with the terms describing cables that are used in the
wiring rules. These terms include the following:
Term
Meaning
Armour
Armoured cables are use in environments where the cable must be
protected from mechanical stress. These cables have a layer of steel
wire just beneath the serving.
Conductor
The metallic, conductive material that performs the main function
of the cable. May include the insulation, depending upon the
context.
Core
One of the insulated conductors in a cable
Insulation
Each conductor is surrounded by a layer of insulation of polymer
material such as PVC or polyethylene, preventing electrical contact
between the conductors. The sheath also a acts as further insulation
between conductors in different cables, or between a cable
conductor and an earthed enclosure for example.
Serving
The final sheath forming the outer surface of the cable.
Sheath
A part that surrounds and protects the cores of the cable. May be of
polymer like PVC or metal such as lead or steel. In common
sheathed cable for general use, the sheath is typically PVC.
Some underground or undersea cables may have inner sheath.
Shielding
A conductive layer of metal tape or braid which prevents electric
fields and electromagnetic interference from leaving or entering the
cable.
Strands
A conductor may be composed of multiple strands of wire, to
improve the flexibility of the cable.
See EWP Figures 4.6, 4.7 and 4.8 for illustration of these terms.
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Electrical cable types
Sheathed and unsheathed cable
The vast majority of installed cable is sheathed – in other words, it has a
sheath or outer wrapping. Sheathed cable consists of conductors surrounded
by thermoplastic insulation, and then surrounded again by a protective outer
sheath, normally made of polyvinyl chloride (PVC). This type of cable is
commonly called TPS (thermoplastic sheathed) cable.
The sheathing provides protection for the single-insulated cores inside the
cable. Where it is installed in a position that is not subject to mechanical
damage (as defined by the wiring rules), no further protection is required.
Unsheathed cable is just the insulated conductor, essentially identical to one
core of a sheathed cable. The wiring rules refers to unsheathed cable as
‘insulated cables (without sheath)’, but common unsheathed cable is known
in the trade as building wire, or TPI (thermoplastic insulated) cable.
In most situations unsheathed cable must be installed within an enclosure
(eg conduit), so it is less commonly used.
Flat and circular sheathed cable
Sheathed cable can be made in flat or circular configuration. Flat cable has
these characteristics:

it is less expensive, because additional filler material is required to
make a circular cable,

it is easy to attach to building structures such as rafters and studs,

it becomes increasingly difficult to handle as the conductor cross
sectional area (CSA) increases.
For these reasons, flat cable is typically used in residential installations,
where low cost and ease of installation are important, and where lower
amperage circuits are the norm.
Circular cable has the following characteristics:
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
is easier to install, especially in the larger sizes, because twisting is not a
problem

is more expensive

it is easier to handle, especially when a large CSA is required

is a natural shape for cables that require more than three cores, such as
those for three phase circuits or communications and instrumentation
circuits.

is a natural shape for the manufacture of special cables such as high
temperature cable, and armoured cable.
For these reasons, circular cable is used extensively in industrial or
commercial applications, where cable trays or cable cleats are required, and
where heavy currents and three phase circuits are common.
Note that single core sheathed cable, or unsheathed cable is naturally
circular anyway.
Multi-cored circular sheathed cables are available in a wide variety of sizes.
In 3-phase multi-core cables rated greater than 200 amps per phase, the
neutral core may be reduced in size to as small as half that of the active
cores. Cables of this type are referred to as 3 12 core TPS cable.
Number of cores
Sheathed cable may have only one core or a number of cores. There are
common names applied to the various configurations.

When the sheathed cable has only one core it is referred to as single
double-insulated (SDI) cable.

Two core sheathed cable is referred to as twin sheathed cable, or just
‘twin’.

A cable containing cores for active, neutral and earth is called twin and
earth sheathed cable or just ‘twin and earth’.

Cable is also made in three and four cores, both with or without an
earthing conductor.
Sheathing colours
The most common sheathing colour for domestic applications is white.
White cable can be easily coloured with water-base paint if it is in an
exposed position. It also allows it to be more easily seen in places where
there is little light (for example, ceiling spaces). Other standard sheathing
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colours are black and grey and many electricians prefer these for surface
wiring where appearance is important.
The standard industrial electrical colour is orange and some industries do
not allow other sheath colours. Circular cable is therefore almost always
orange in colour. Flat sheathed cable can also be purchased with orange
sheathing.
A red sheath is used almost exclusively for fire alarm systems
in buildings so that cables are not confused with low-voltage wiring.
Another example is the use of blue sheathed cables for instrumentation
wiring.
In a large installation, several sheath colours may be used. Distinctive
sheathing colours makes for easy identification and reduces the possibility
of wrong connections to a specific type of circuit.
Caution
Two sheath colours which cannot be used are those reserved for earthing
conductors (that is, green, yellow and green/yellow). However, if a single core
double-insulated (SDI) cable is used as an earthing conductor, you must use the
green/yellow combination for the sheath.
Special cable types
We have only discussed cables for low voltage installations, and for general
use. There are many other types of cables for use in different fields such as
communications, for arduous conditions or special applications such as
mining, undersea, or use in high temperatures, and for high voltages.
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Student exercise 1
1
What is one desirable feature of wiring with sheathed cable?
____________________________________________________________________
2
What is sheathed cable with one core often called?
____________________________________________________________________
3
What two configurations are sheathed cables made in?
____________________________________________________________________
4
What specific use is made for cables with the sheath colours of:
(a) red? ____________________________________________________________
(b) blue? ___________________________________________________________
Check your answers with those given at the end of the section.
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Cable design
Conductor stranding
As you know, current passing through a conductor causes heating, energy
loss, and a voltage drop across the conductor. Ideally, we would use
conductors with zero resistance, but this is not possible with today’s
technology.
Instead, we must compensate for these effects by increasing the cross
sectional area of the conductor as the required current increases. The ability
of a cable to carry current, without overheating, is termed its current
carrying capacity. The more current a cable must carry, the larger we must
make the cross sectional area of the conductors. You will learn how to select
cables later in your course, but here we will look at the practical
implications for cable manufacture.
If we use a solid conductor, the cable will become very stiff as we increase
the cross sectional area, and will become unmanageable. To avoid this, the
cable cores are stranded: a core can be comprised of many fine wires wound
together. A cable of 1 mm2 CSA is always a solid single conductor, and
those of 1.5 mm2 and 2.5 mm2 can be either solid or stranded. All other sizes
are stranded.
The number of strands in cores of increasing CSA follows a definite pattern.
This pattern is:
1, 7, 19, 37, 61 and 127.
Referring to Figure 4.4 in EWP, you will see how these numbers arise. They
are simply determined by the number of strands that will fit neatly around
the previous layer.
As the cable core is created, each successive layer of strands (each ply) is
wrapped in opposite direction. This helical wrapping keeps the cable
together once it has been made. After all the layers have been applied, the
cable is covered with thermoplastic insulation.
Standing is specified as two numbers separated by a slash (/). The first
number is the number of strands and the second is the diameter of each
strand in millimetres. For example, a cable marked as 7/0.67 has 7 strands of
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wire, each 0.67 mm in diameter. Table 1 shows a sample of conductors and
how they are constructed.
Table 1: Stranding of conductors
No of strands
Dia of strands
(mm)
Cross-sectional area
2
(mm )
1
1.13
1
1
1.38
1.5
7
0.67
2.5
7
1.35
10
19
1.35
25
19
1.78
50
37
2.03
120
Common cable configurations
The main types of flat sheathed cable that are made as standard items are
shown in Table 1.
Table 1: Sheathed cable construction
No of
strands
Diam of
strands
(mm)
Crosssectional
area (mm2)
No of cores
in cable
7
0.50
1.5
2 core + earth
flat
7
0.67
2.5
4 core + earth
19
1.35
25
4 core
7
1.35
10
3 core + earth
circular
19
1.53
35
1 core
37
2.52
185
1 core
19
1.53
35
4 core + earth
circular + 3 12 +
earth circular
7
0.50
1.5
2 core
There is a practical limit to the conductor size for multi-core sheathed cables
because the multi-core types become too heavy and awkward to handle. In
addition, practical considerations are such that flat cables of more than four
cores and earth are not made. More cores can be fitted in the circular
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configuration, but it is rare for any stock cable to contain more than four
cores and earth.

See EWP Figure 4.10 for some commonly used flat cable types used for
general wiring.

Some general information on accepted installation methods and cable
sizes is given in Tables 3.1, 3.2 and 3.4 of the wiring rules. Installation
electricians must select the correct size cable both for the current it will
carry and its location. This information may be found in the
AS/NZS 3008.1 series
Visit a cable manufacturer’s website for a catalogue of common cable
types. For example, visit www.olex.com.au and follow the links to
products/low voltage. Then click on the link to the current product
handbook.
Cable insulation
The properties of the insulation largely determine how a cable can be used.
The electrical properties of the insulation include its dielectric strength and
breakdown voltage. These will determine the maximum voltage of the cable
for example.
There are many other important properties including the strength, thermal
conductivity, resistance to chemical attack and so on.
One important property is the resistance to elevated temperatures. Whenever
there is a current in a cable, heat is produced. This heat cannot be so high
that the cable insulation begins to sag or ages prematurely.
Different grades of insulation will withstand different temperatures. Normal
thermoplastic (such as polyvinyl chloride PVC) insulated cables operate at a
maximum temperature of 75C. Other PVC cables, under certain conditions,
may be operated at 90C and 105C. Other special cables with fibrous
insulation may be operated to 200C or higher.
Read EWP section 4.3 for a treatment of cable insulation and sheathing
materials for high or low temperature, fire resistance, pest resistance and
chemical resistance.
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Cable identification
Sheathed cable is identified by a label on the cable drum. This label gives
information about the:

manufacturer

number of cores

core csa

strands in each core

size of the strands in each core

colour of the sheath

voltage and temperature rating.
A typical label is shown below in Figure 1.
ABC CABLE COMPANY
Twin and earth flat
1.5 mm2 stranded (7/0.50)
0.6/1 kV
75°C
White PVC/PVC
Figure 1: Typical label on a drum of sheathed cable
Conductor colours
In Section 3 of AS/NZS 3000:2000 the first entry concerning colour codes
is Clause 3.8.1 (page 87). Let’s look in detail at this part of
AS/NZS 3000:2000.
Table 3.5 lists acceptable colours for active, neutral and earthing
conductors.
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Earthing conductors
Green/yellow is the recommended colour for earthing conductors. This
combination was chosen because people who are mildly colour blind have
difficulty in separating red and green colours. Clause 3.8.1 requires that
where a combination (eg green and yellow) is used, each colour must cover
at least 30% of the surface area.
Note that in general practice the colour violet (or purple) is not used for any
low-voltage conductor as it identifies telecommunication cables.
Most electricians prefer to use a distinguishing colour for switch wires when
carrying out wiring work. These are conductors which are made active when
a switch is turned on. A common colour for these conductors is white. By
using this colour in single-phase wiring, there is little chance it can be
confused with the red active when making connections at lights. This is not
stated in AS/NZS 3000:2000 but is good working practice.
See EWP section 4.4 for a discussion of the identification for fixed wiring
and flexible cables.
Student exercise 2
1
What clause in AS/NZS 3000:2000 applies to sheathed cables buried in the ground?
____________________________________________________________________
2
What happens when sheathed cables are exposed to very high temperatures?
____________________________________________________________________
3
What clause in AS/NZS 3000:2000 deals with fixing sheathed cable?
____________________________________________________________________
Check your answers with those given at the end of the section.
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Applications of cables
Probably more sheathed cables are used than any other type of wiring
system since it is practically the only type of wiring used in domestic
installations. In timber-frame buildings, the cable can be run in wall cavities,
frames and ceiling spaces without further enclosure. This makes sheathed
cable very quick and easy to install.
Sheathed cables are used extensively in commercial installations for the
wiring of lighting and GPO circuits. They can be taped together in looms
and laid above suspended ceilings, which is much quicker than wiring in
conduit and building wire.
In industrial applications, sheathed cable can be used in areas where it is not
subject to damage. One application is the wiring of lights on roof trusses,
and also for cable tray and cleat wiring.
Sheathed cable is considered to be a wiring enclosure and is far easier to
install than cables in conduit. In addition, it may be buried directly in the
ground as long as clause 3.11.3.2 of the wiring rules is strictly followed.
Sheathed cable is also widely used in the installation of catenary wiring
systems complying with clause 3.13 of AS/NZS 3000:2000.
Fixing and supporting sheathed cables
Sheathed cables are to be fixed and supported in accordance with the
requirements of clause 3.9.3 of AS/NZS 3000:2000. The point to remember
is that sheathed cable must be fixed in place with suitable clips.
Clause 3.9.5.3 of AS/NZS 3000:2000 requires that cables likely to be
disturbed (as indicated in clause 3.9.5.2 of AS/NZS 3000:2000) are to be
fixed to prevent undue sagging.
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Limitations on use of sheathed cables
Sheathed cable is a most useful and versatile wiring system. However, it is
liable to damage by severe mechanical forces or high ambient temperatures.
High temperatures can cause the plasticiser in the sheath to evaporate and
leave the cable rigid and brittle.
Sheathed cables are not used where solvents could affect the sheath, as
solvents will cause similar problems.
Insulated cables without sheath
We have mentioned that standard unsheathed cable (building wire) can be
used in conduits for some situations. However there are other types of
unsheathed cable with specific applications, including:
•
single insulated flexible cables often used for wiring within appliances.
•
glass fibre insulated cables used for high temperature situations such as
ovens. The insulation is fragile and could be damaged when pulled
through conduits. Again, they are more expensive;
•
telephone cable or bell wire which has insulation only suitable for extralow voltage;
•
instrument or data cable only suitable for extra-low voltage;
•
insulated aerials conductors are not annealed and so are too stiff and
hard to draw into conduit.
None of these cable types is suitable for general purpose, low voltage
installation wiring.
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Jointing and terminating cables
All electrical cables must be connected to a circuit in some manner. This
connection is called the termination of the cable. There are many ways in
which a cable can be terminated including the use of connectors, soldering,
crimping, compression fittings and equipment terminals of many types.
The termination of a cable depends on the type of cable, its application and
its size. We will have a look at a number of terminating methods in this
section.
Three basic factors to consider when terminating a cable include the
following:

The connection must be good electrically, which means it should have as
low a resistance as possible.

The connection must be good mechanically. There must be no undue
mechanical stress on the connection, and the connection must not loosen
due to vibration or thermal cycling.

A connection should not interrupt the insulation any further than is
necessary to make the connection.
First let’s see what AS/NZS 3000:2000 has to say on the subject of
terminations. Clause 3.7.2.2 deals with the removal of insulation from the
cable. There are special conditions mentioned, such as: the insulation shall
not be removed any more than necessary, the connection shall be insulated
and damaged insulation replaced.
Clause 3.7.2 of AS/NZS 3000:2000 covers connection methods. Whatever
method is used, it must provide electrical continuity and adequate
mechanical strength.
Soldering
Soldering is covered in AS/NZS 3000:2000 by clause 3.7.2.6.
It should be observed that if flexible cords are soldered, care must be taken
to prevent the solder running up the flexible conductor by capillary action.
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This solder flow makes the flexible core stiff and if it is flexed at this point,
it could break off.
Another requirement in soldering is that corrosive fluxes must not be used.
A corrosive flux (eg hydrochloric acid) acts as an excellent flux but will
continue to corrode the conductors long after the joint has been made. It
could also damage enclosures or equipment.
Terminal blocks
Cables are commonly terminated in terminal blocks. These come in many
varieties of size, number of cables accommodated, current rating, terminal
layout and screw thread types. There are many hundreds of varieties and the
only way to find a suitable type is to consult a manufacturer’s catalogue.
Crimped connections
Crimping should only be done by a tool specially designed for the job using
a size suitable for the cable and by using a crimping sleeve or lug specially
made for the cable size.
Crimp connections are made using a wide variety of lugs, including:
 those with holes, or eyes, for slipping over terminal screws
 slotted ends for pushing under terminal screws
 quick connect table ends
 quick connect bullet terminals (as used in automotive and some appliance
wiring).
Compression joints or terminations are usually only done on cables from
about 10 mm2 or more. The compression type of termination is made by
squeezing the surrounding sleeve and the cable from all sides to remove
practically all internal spaces. A completed compression joint is basically
solid copper.
Connection of cables by connectors is usually limited to connecting cables
in junction boxes. Connectors of the normal type (tunnel terminals
surrounded by insulating material) are usually limited to cable sizes up to
6 mm2. When joining earthing conductors with tunnel connectors, the tunnel
connector must have two screws (Clause 3.7.2.10 of AS/NZS 3000:2000).
Another common termination procedure is to use solderless lugs. The cable
core is made into a small circle, and the lug is slipped over the cable end and
the lug is secured to the cable, usually with pliers. It is common practice to
use solderless lugs for terminating earthing cables.
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Poor connections
When terminating cables the colour coding of the cables must be
maintained. It would be a disastrous error to mistakenly connect an earthing
conductor to an active terminal. A licensed electrician could be charged with
manslaughter if this incorrect connection resulted in someone’s death.
High resistance joints can result from any of the following:
 poor soldering
 poor crimping or compression
 loose screws
 loose nuts.
All of the above problems are due to poor working habits and the electrician
must use the correct procedure for each connection.
The effect of a poor connection is not normally detected when the current
passing through the connection is small relative to the capacity of the circuit
conductor. It is usually when the rated load current through a conductor is
required that the effect of a poor connection is detected. The high resistance
joint between the contact surfaces at the termination passes the same current
as the circuit conductors. Since the heat produced in a conductor is
proportional to the resistance through which the current passes, the high
resistance joint will produce a localised ‘heat spot’. Heat spots may be
detected as abnormally high temperatures by thermographic photography or
visual signs of insulation ‘browning’ or burning. If a condition like this is
not detected the surface connection at the termination will deteriorate
leading to an even higher resistance joint and therefore a higher temperature.
The likelihood of a fire igniting the cable insulation is great.
When power cables are terminated it is essential that the contact surfaces of
the connecting surfaces be made secure and flush. Under-tightening or overtightening of a bolt connecting the two surfaces of a cable termination may
lead to poor surface connection where the cross sectional area of the
conductor is not maintained through the termination. Some suppliers of
electrical equipment may specify a particular torque setting for tightening
bolts so that correct tension is achieved at the contact surfaces.
See EWP sections 4.6 to 4.8 for information on joining and terminating
cables.
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You will learn more about making terminations in your practical sessions,
and on the job.
Student exercise 3
1 What part of AS/NZS 3000:2000 covers soldered joints?
_____________________________________________________________________
2 Where are solderless lugs commonly used?
_____________________________________________________________________
3 What can be the result of a high resistance joint?
_____________________________________________________________________
Check your answers with those given at the end of the section.
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Summary

In this section you have accomplished the following outcomes. During
your practical session some of these outcomes will be reinforced.

Sheathed cables are a double-insulated wiring system and need not be
enclosed further, if not subject to damage.
Sheathed cables are made in flat and circular configuration.


Flat cable is only available in smaller sizes, and cores of three or less.
Circular sheathed cables are made in large sizes with up to 30 cores for
special applications.

Sheath colours vary, but the most common colours are white and black.
The industrial sheath colour is orange, while red is reserved for fire, and
blue for instrumentation.

Sheathed cables are used extensively in domestic wiring, and also in
commercial and industrial wiring..

Sheathed cabled are liable to be damaged by severe mechanical forces or
high ambient temperatures.

Building wire is single insulated thermoplastic cable for use in lowvoltage wiring.
Building wire is suitable for voltages of 600 volts to earth and 1000
volts between conductors.


1.5 mm2 and 2.5 mm2 building wire can be either solid or stranded.


Building wire must be installed in enclosures.
Enclosures can be conduit, duct or troughing.

PVC conduit is made of medium duty and heavy-duty types.

The labels on cable drums or reels give the length, size, stranding,
voltage rating and maximum operating temperature of the cable on the
drum.

All earthing conductors must be either green or green/yellow
combination.
All neutral conductors should be black.


20
Active conductors may be any colour except the colours reserved for
earthing and neutral.
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
The conductivity of a joint must be no less than the cable itself or, in
other words, the resistance must be no higher than the same section of
cable. The insulating properties of joined cables must be maintained.

Loose tunnel-type connectors for earthing conductors must have two
screws.

Earthing conductors are often terminated with solderless lugs.

Colour coding must be maintained when connecting cables.

High-resistance joints can lead to overheating of the cable termination.

Cables may be terminated by:
–
–
–
–
–
connectors
soldering
crimping
compression
using terminals.

Quick-release spade and bullet connections are used in automotive and
appliance wiring.

Connectors are used for joining fixed wiring cables up to 6 mm2 in
junction boxes.

Crimping is normally applied to lugs used on cables to small power
equipment termination or control equipment terminations (ie Stop/Start
push buttons, PLC’s etc.)

Compression connections are usually made to cable lugs that connect
into medium to large motor terminal boxes. The lug that is compressed
normally has a hole in the flat palm that is slipped over the terminal
screw before being fastened.

The correct size crimp lug and crimping tool must be used for crimped
joints.

A good termination is as important as the cable itself.

Some ways of mounting accessories to masonry surfaces (bricks,
concrete) are by screwing into wall plugs, using nail in plugs or masonry
anchors, or using an explosive powered tool in conjunction with
specially designed fixings.
EGG221A: 3 Identify and terminate cables
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
To drill a hole in masonry, a tungsten carbide tipped drill bit is often
used and when using the power drill, it should be set to its
percussion/hammer function.

Cable clips are used to fix TPS cables to surfaces, whereas saddles are
used to fix conduits.

If an explosive tools is to be used you must first be trained and given a
certificate of proficiency.

There are many accessories used in the electrical industry and in order
that engineers, salespeople and tradespersons can work efficiently, we
must be able to identify the most common items used.
Check your progress
Now that you have worked your way through this section, it is time to have a look at your
module workbook (in the yellow folder), at the appropriate section.
Attempt the review questions now in the same way as you have been doing the student
exercises. The answers are at the back of your workbook. Yours teacher at the practical
session will check the work you have done and help with any problems.
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EGG221A: 3 Identify and terminate cables
 NSW DET 2017 2006/060/05/2017 LRR 5168
Answers
Student exercise 1
1
It can be done quickly; it’s cheaper.
2
SDI (single double-insulated)
3
flat and circular
4 (a) fire alarms
(b) instrumentation wiring
Student exercise 2
1
Clause 3.11.3.2
2 The cable could lose its plasticiser and become very brittle. (Heat also
softens PVC and that could also cause problems if there is any strain on
the cable.)
3
Clause 3.9.3
Student exercise 3
1
Clause 3.7.2.6
2
connecting earthing conductors
3
heating at the joint (it could glow red hot!)
EGG221A: 3 Identify and terminate cables
 NSW DET 2017 2006/060/05/2017 LRR 5168
23