Download Uses for TPS cables

Contents
Introduction
Assessment criteria
Description and applications of TPS cables
Acceptable types of TPS cable wiring systems
Installed in air and unenclosed cables
3
3
4
5
7
Segregation of cables
7
Cable support and fixing
8
Protection against mechanical damage
13
Conditions that affect current carrying capacity
16
Uses for TPS cables
Flexible cords
Testing TPS wiring systems
23
23
25
Earthing resistance
25
Insulation resistance
28
Polarity
29
Summary
31
Wiring rules
32
Answers to activities and check your progress
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Introduction
TPS cables are widely used in domestic, commercial and industrial
applications where you need an economic cabling system.
This type of cable is generally easy to install and very reliable, if you use
simple precautions making sure the cable always operates within its limits.
It could suffer mechanical damage if it is installed in an exposed position
where the cable could be stood upon or damaged during maintenance of
other services. You should not introduce a hazard by installing the cable
next to other services or by reducing the fire rating of barriers by
inappropriate penetrations.
This section will guide you to useful sources of information and outline
minimum standards designed to ensure reliable operation and safety.
Assessment criteria
At the end of this section you should be able to:

determine Australian Standards requirements for the installation of flat
TPS cable

install flat TPS cable in trunking and duct for the supply of socket
outlets

use flat TPS cable to assemble and install a lighting loom

test circuits to ensure they are safe and operate as intended

determine Australian Standards requirements for the installation of
circular TPS cable

install a final sub-circuit for lighting using circular TPS cables on a
cable tray

install a 5 pin socket outlet using circular TPS cable maintaining an
IP56 rating

test circuits to ensure they are safe and operate as intended
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Description and application of TPS
cable
Thermoplastic sheathed (TPS) cables have two basic forms—flat and
circular. Flat TPS cables form the last leg of most low voltage domestic and
commercial lighting and power distribution systems. Circular TPS cables
are more widely used in commercial and industrial applications especially
where you need weatherproof terminations. Both cable types satisfy the
requirements of The Wiring Rules Clause 1.4.57 for double insulation. TPS
cables are generally designated using the following criteria.
Shape
Flat or round
Size of active
conductors
Standard range of sizes such as 1 mm2, 2.5 mm2.
See AS/NZS 3008.1.1:1998
Number of cores
Single or multi-core—generally, you specify the
number of live conductors in the cable and whether
there is an earth conductor:
Colour of sheath

A single core cable with conductor and sheath is
called Single Double Insulated or SDI.

A cable with three actives, one neutral and an
earth conductor is a multicore cable described as
four core and earth.

Multicore cables with ratings in excess of 100A
may have a neutral conductor that is smaller than
the cable actives. This cable is sometimes referred
to as three and a half core and earth.
Circular TPS is generally orange (standard colour for
hazardous electrical service)
Flat TPS is generally white although you will find
other colours used for fire detectors (red sheath) and
other applications.
Other
characteristics
4
Twin active—Multicore cables with two live
conductors coloured red and white are often called
twin active.
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Neutral screened cable—These are circular cables
with a copper screen surrounding the actives that is
intended to be the neutral conductor
Screened cable—Circular TPS cables with copper
braided screens are commonly used for the
connections between induction motors and variable
frequency drives. The screen is connected to earth but
is not necessarily the earth conductor.
Multicore control cables—Multicore cables for
control uses may have 30 cores. Identification of each
core is by numbers printed on each core.
Flat TPS cables use less material in their construction than circular cables of
the same current carrying capacity. For this reason flat cables take up less
space in an enclosure and are cheaper to buy. Circular cables on the other
hand are easier to install neatly and larger sizes are easier to handle. Flat
cables are inclined to twist during installation.
Speed of installation is one of the advantages of TPS systems. You can
install these cables without further enclosure in some installations provided
you can maintain the safety and integrity of the cable and the surrounding
conditions.
Acceptable types of TPS cable wiring
systems
Sheathed cables are suitable for a wide range of installation conditions. The
Wiring Rules Table 3.1 permits the installation of sheathed cables in
situations that range from an unenclosed situation resting on a continuous
surface to burial direct in the ground.
Certain conditions must be met to make sure the cable system is going to be
safe and reliable in all of these conditions. Acceptable wiring systems for
TPS cables are:

resting on a continuous surface without fixings

in a wiring enclosure

supported on cable tray or ladder

supported by catenary system

buried direct in ground

buried in ground within a wiring enclosure.
AS /NZS 3008.1.1:1998 defines cables that rest on a surface without fixings
as installed in air, and cables supported by a cable ladder or tray as
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unenclosed. Cables which are unenclosed, but in contact with thermal
insulation, are in a different installation category.
Activity 1
The following diagrams are used by AS /NZS 3008.1.1:1998 to identify certain installation
conditions for cable systems. Write the description of the installation condition using the
terms from AS /NZS 3008.
A
B
C
D
E
F
G
The cable type and installation conditions have a significant impact on the
current carrying capacity of a particular size of cable and you must be able
to recognize risk factors.
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Installed in air and unenclosed cables
Unenclosed cable installations are common in domestic and commercial
installations. Cables incorrectly installed could be the source of electrical
fires or contribute to the risk of electrocution. It is important that you
understand and apply the basic safety and regulatory requirements to ensure
the correct operation of these circuits. We will outline these requirements
using the following points:

Segregation of wiring systems and cables

Support and fixing

Protection against mechanical damage

Installation conditions that affect cable current carrying capacity.
Segregation of wiring systems
Accidental damage to the cable by other activities nearby is one of the
dangers associated with unenclosed TPS cables. For this reason, cables
associated with low voltage electrical systems must be separated from the
wiring systems of other services such as telephones and computer networks
in accordance with The Wiring Rules Clause 1.10.4 Segregation.
The Wiring Rules Clause 3.9.9.3 allows for low voltage and extra-low
voltage systems to share enclosures provided certain conditions are met.
You should refer to relevant standards to check all requirements if you are
considering and installation of this type.
A minimum separation of 50 mm or a solid barrier is specified in the
Australian Communications Industry Forum (acIF) Standard S009.
Telecommunications wiring and wiring associated with low voltage
electrical work should not share a common hole in any building frame
member. Any accidental penetration of the hole by a nail, for example,
could impale both cabling systems and introduce a dangerous voltage into
the telecommunications system.
Any electrical wiring system shall not be installed in the vicinity of any
system that could cause detrimental effects to the cables. Systems such as
hot water reticulation could affect the reliability of the cable sheath and
insulation due to high temperatures. Read The Wiring Rules Clause 3.9.9.5
for more information about the proximity of non-electrical services.
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Cable support and fixing
Arrange all cables and conductors you install so as to minimise damage to
the sheath, conductors and terminations during use and maintenance. Also
you should install the cable so that the insulation or sheath is not damaged
due the cable’s own weight. This factor is particularly important if the cable
is operating at close to the temperature rating of the insulating material. Hot
insulation is generally soft and hard corners could easily deform the
insulation and reduce its effectiveness.
Changes in direction will also increase the stress on the insulation and
sheath if the corners are too tight. Manufacturers generally provide
information about the minimum bending radius of their cables but if
this information is not available use the recommendations of The Wiring
Rules Clause 3.9.7.
Read The Wiring Rules Clause 3.3.9 for the requirements for the installation
of cables.
Typical support systems include:

continuous surfaces

proprietary cable support systems such as cable tray or ladder

cable clips, saddles and brackets

enclosures such as rigid conduits and troughing systems which, in their
own right, require support.
Continuous surfaces
One example of a common installation system that uses a continuous
surface is the cabling installed above a ceiling for domestic lighting. You do
not need further fixing or support for these cables unless there is a danger
that the cables could be moved into a position where they may be damaged.
The highest danger of accidental interference occurs in situations where
cables are likely to be disturbed.
Some locations that fall into the ‘likely to be disturbed’ category are
mentioned specifically in The Wiring Rules Clause 3.9.5.2. Where cables
require support, The Wiring Rules provides no guidance about the separation
between support points although Clause 3.9.5.3 does say that cables that are
likely to be disturbed shall be fixed to prevent undue sagging. To achieve
this, supports should be separated by about 0.3 m.
Supports in other areas could be at 1.2 m centres although this is only
a suggestion.
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Refer to EWP Chapter 8.4 for more information about support for TPS
cables.
Activity 2
Use light shading on the house section diagram to indicate areas in which cables are ‘likely
to be disturbed’. Show relevant dimensions for these areas.
Cable support systems
If a continuous surface is unavailable you could always add one. There are
several cable support systems in common use. Two generic names are cable
tray and cable ladder. These systems are generally pressed from galvanized
sheet steel and assembled on site using manufactured fittings. You can cut
sections and bolt them together to form offsets and corners.
There are other systems that are manufactured from small diameter steel
rod. These systems have trade names such as ‘Cable Cage Systems’ and
‘Ramset Speed Track’. These systems, although slightly higher in material
cost, claim to be easier and faster to install. Savings in labour cost could
make these systems more economical to install in the long run.
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Tying cables
Where these systems are installed horizontally, you may not need to clip the
cable to the support. However, cables are often tied for these reasons:

Cables can jump off the tray during a fault that causes high currents and
powerful magnetic fields.

The phase and relationship of the magnetic fields that surround single
core cables is controlled throughout the entire length of the run.

You maintain separation from other circuits to avoid excessive heating
and derating factors.
If you do tie the cable to the support system you should make allowances for
linear expansion in the cable. Your ties should be arranged or loose enough
to allow some movement as the copper conductors expand due to
temperature rise. The temperature rise is particularly significant under short
circuit fault conditions.
Of course when cable tray and ladder are installed vertically you must tie
the cables to the support so they don’t fall off. When cable support systems
are part of the building riser, you can expect long runs of cable support
systems installed vertically. Suitably spaced support is essential to comply
with The Wiring Rules Clause 3.9.6 so the cable is not damaged due to its
own weight.
Cable clips and saddles
Figure 1: Cable support system glued to underside of a concrete slab
Pin clips and plastic cable clips are the most common clipping systems for
flat TPS cables. Plastic clips are designed for specific cable sizes. Pin clips
are an all metal clip with hardened nail. The clip will expand to fit a range
of cable sizes. See illustrations in EWP, Chapter 8.4.
These clips are widely used in timber frame construction and are placed
manually using a hammer. An alternative is the insulated staple. These
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staples are placed with a purpose built staple gun generally called a cable
tacker. Telephone linesmen have used the staple method for many years to
install telephone cabling in domestic installations. There are staple systems
for flat TPS cables up to about 2.5 mm2 and circular cables such as earth
conductors and coaxial cables.
What if you can’t use a nail? Try glue. The cable support or anchor is glued
to the underside of a concrete slab at suitable intervals. The gluing process is
a two-part method where the slab is primed and then the support with the
adhesive is applied. The installing electrician ties the circuit cables to the
anchor. This system is widely used above false ceilings in commercial
installations.
No support
Tempting but unacceptable wiring
support systems include:

suspended ceilings with
removable tiles

other services such as sprinkler
or gas pipework (see Figure 2)

support systems used by
telecommunications or data
systems.
Figure 2: Wiring loom supported
incorrectly on sprinkler system pipes
You should maintain adequate clearances from other services so that
maintenance to their systems will not interfere with your cables. Another
practice you should avoid is installing cables above rafters. The Wiring
Rules Clause 3.9.4.4 prohibits installation of cables in any space formed
between roofing material and its immediate support. Cables installed within
50 mm of the roofing material need increased protection outlined in The
Wiring Rules Clause 3.9.4.6. Lighting and power circuits in domestic
installations have RCD protection but circuits like consumers mains do not.
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Activity 3
List at least two other circuits that you may find in a domestic installation that are not
likely to have RCD protection.
Enclosures
Various enclosures provide added protection for cables. Types of enclosures
include:

conduit

troughing and trunking

switchboard surrounds

duct systems

smooth channels (chases) cut into brick or masonry.
The space formed in a framed wall is also a type of an enclosure. TPS cables
in framed sections of walls need no further support or protection provided
there is no danger of the cable being damaged by its own weight and the
cable is not fixed in position and does not pass through a close fitting hole
that is within 50 mm of the surface of the wall. Cables laid in chases also
require further attention.
Figure 3: Vertical grooves cut in masonry wall (chase) house cables for the switch
drop
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Activity 4
The following diagram represents an electrical accessory attached to a masonry wall. Show
areas where TPS cables with overload protection should not be concealed within the solid
wall.
Cables in areas other than those permitted by The Wiring Rules
Clause 3.9.4.5 require further protection
In concrete
Unarmoured sheathed cables are not permitted in concrete unless they are
enclosed in an appropriate wiring enclosure installed in accordance with The
Wiring Rules Clause 3.3.7 and 3.9.4. Refer to The Wiring Rules Clause
3.9.8.2.
Protection against mechanical damage
One protection method already mentioned uses a residual current device.
The other protection methods outlined in The Wiring Rules Clause 3.9.4.6
are:

earthed metallic armouring, screen covering or enclosure

adequate mechanical protection to prevent damage.
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Earthed metallic screens or covers provide protection by making sure the
normal circuit overload protection operates if the cable is damaged or
penetrated in some way. The circuit protection disconnects the circuit from
the supply making it safe. The cable is still damaged however and would
need repairs.
Protecting the cable from mechanical damage requires a different approach.
The Wiring Rules Clause 3.3.7 also directs you to protect cables from
mechanical damage. Cables that cross the top of ceiling joists could be stood
on and damaged. You can protect TPS cables in this situation by placing
battens across the tops of the joists and clipping the cables to the battens.
Figures 4 and 5: Unprotected TPS cables crossing ceiling joists require
mechanical protection
Use bushes or grommets to prevent damage to the sheath of cables passing
through openings in steel frames or metallic switchboard surrounds.
Read EWP Chapter 8 for more information about installing TPS cables.
Mechanical protection underground
The Wiring Rules Section 3.11 permits TPS cables in underground
installations provided certain conditions are met. Table 3.6 categorises this
type of installation as a Category B and therefore requires further protection.
In general, TPS cables should be laid on a bed of sand at least 50 mm deep
and covered with the sand to the same depth. You should also add extra
mechanical protection above the cable that complies with Clause 3.11.3.3.
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An orange marker tape should now be installed immediately above the
mechanical protection.
Activity 5
Use the appropriate clauses of The Wiring Rules to answer the following questions.
1
Identify three methods that are identified as suitable mechanical protection for
category B underground installations.
_____________________________________________________________________
_____________________________________________________________________
_____________________________________________________________________
2
What is the maximum allowed separation between the underground wiring and the
mechanical protection?
_____________________________________________________________________
3
What is the maximum allowed separation between the wiring system or mechanical
protection and the orange marker tape when used?
_____________________________________________________________________
4
What is the recommended spacing between underground wiring systems and other
underground services?
_____________________________________________________________________
Maintaining fire ratings
You must not reduce the fire rating of partitions and floors by cutting
openings for cables and leaving a path for a possible fire to follow to
another level or section of the building. Any gap left after installing the
cables must be filled with an appropriate fire stop material in accordance
with the relevant provisions of the Building Code of Australia. This includes
gaps in cable troughing systems and conduits.
Standard concrete mortar does not have a suitable fire resistance so you
should get specialist recommendations for suitable material. Many fire stop
materials expand to many times their original volume when heated filling
penetrations and forming a hard, fire resistant char.
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In general, there are a variety of materials you could use such as mortars,
putties, pillows and bags as well as boards, blocks and bricks.
Figure 6: Fire stopping of cable entry provided
by fire rated pillows
Figure 7: Fire stopping provided
by rated mortar
Read The Wiring Rules Clause 3.9.10.
Installation conditions that affect cable
current carrying capacity
The current carrying capacity of TPS cables is mostly limited by the amount
of heat generated by the conductors and the temperature rating of the cable
insulation. So any factor that reduces the cooling of the cable will affect the
current carrying capacity. In general, the heat generated by the cable has to
be absorbed by the immediate environment.
Thermal insulation reduces the effects of external ambient conditions but
has the effect of reducing the cooling rates of cables that are in contact with
the insulation. There are four general categories of installation conditions
recognised by AS/NZS 3008.1.1:1998:

cables installed in air

cables installed in thermal insulation

cables buried direct in ground

cables installed in underground wiring enclosures.
Each category relies on certain conditions, eg ambient temperature:
16

cables in air and cables installed in thermal insulation—40˚C

cables buried direct in ground and in underground enclosures—25˚C.
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Activity 6
Use AS /NZS 3008.1.1:1998 to categorise the following examples of cable installations
using the headings in the table. Write the letter that corresponds to the installation method
in the space in the table. Some items in the list do not belong to the headings in the table.
Unenclosed in
air
Enclosed in
air
Completely surrounded by
thermal insulation
Partially surrounded by
thermal insulation
Installation method
Letter
Lying across ceiling joists
A
In a switchboard enclosure
B
Embedded in plaster in a masonry wall
C
In conduit embedded in a concrete slab sitting on the ground
D
In conduit clipped to a structural roof member surrounded by thermal insulation
E
Unenclosed cable in contact with bulk thermal insulation on all sides
F
In conduit laying 0.5 m below ground level
G
Clipped beneath a ceiling
H
In an enclosed trench with removable covers
I
Unenclosed cable clipped to a roof member and in contact with bulk thermal insulation
J
In non-metallic conduit embedded in plaster on a masonry wall
K
The operating temperature of the insulation is a limit set by the
manufacturer. AS /NZS 3008.1.1:1998 Table 1 lists limiting temperatures
for various types of insulated cables. The temperature limits in this table do
not represent the maximum transient temperature limits that may occur
under short circuit conditions but are the sustained temperature limits. Read
the notes for Table 1.
The temperature limits for insulating materials in contact with the
conductors is listed in AS /NZS 3008.1.1:1998 Table 52. These
temperatures may occur under fault conditions. The circuit protection should
operate sufficiently quickly so that these temperatures are not exceeded.
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Activity 7
1
List the types of cable insulation that have an operating temperature under normal use
of 75˚C.
_____________________________________________________________________
2
What is the maximum temperature limit of a cable that has insulation formed from
cross linked polyethylene (XLPE) X-90?
_____________________________________________________________________
3
Why is the current carrying capacity of cables used for fixed wiring using V-90 PVC
compounds based on a temperature of 75˚C? (Read note 2 Table 1.)
_____________________________________________________________________
_____________________________________________________________________
Factors that contribute to cable heating
The heat generated by cables is due to the current the cable carries and cable
resistance. Doubling the current increases the amount of heat developed by a
factor of four. The actual operating temperature of the cable will then
depend on:

cable resistance which is determined by the cable cross-sectional area
and material

actual current flow

time the current flows

cooling or heating provided by the ambient conditions.
You generally select a cable cross-sectional area based on the current
demand of the load. The danger here is to assume that a cable so selected
will be suitable under all conditions.
For example, AS/NZS 3008.1.1:1998 Table 9 lists the current rating of a
4 mm2 twin and earth V-75, TPS cable installed touching a surface as 34A.
If the same cable is completely surrounded by thermal insulation the current
rating drops to 17A. If the cable surrounded by thermal insulation carries
34A, the cable temperature will exceed the limits set for V-75 and will be
damaged. The damaged cable could cause a fire or an electrocution.
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AS/NZS 3008.1.1:1998 Tables 3 to 14 have four general categories for
cable installation conditions:

unenclosed

enclosed

buried direct

underground, non-metallic wiring enclosure.
The cable ratings under each of these categories consider no outside
influences to reduce the expected rate of cooling.
Cable rating and de-rating factors
Variations to the stated installation conditions outlined in AS /NZS 3008 are
taken into account by applying a rating or de-rating factor to the cable
current carrying capacity. AS /NZS 3008.1.1:1998 Table 22 lists de-rating
factors for various installation conditions. For example, any cable clipped to
the underside of a ceiling so that it is in direct contact has its current rating
reduced to 0.95 of normal. That is, if the rating under normal conditions is
100 A, the maximum current it could carry without damage is now 95 A.
Where otherwise unenclosed cables are installed on a flat surface such as an
unperforated cable tray you must apply a similar de-rating factor. See
Table 23 and 24 in AS /NZS 3008.1.1:1998.
Bunching or grouping cables
Placing cables close to each other has two effects on the temperature of the
cable:

The airflow around the cables could be reduced.

The heat produced by one cable contributes to the final temperature of
another. This is called mutual heating.
Figure 8: Restricted airflow
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You should follow the guidance offered by AS /NZS 3008.1.1:1998 figure 1
if you wish to maintain the listed current rating of the cables and avoid
unwanted heating.
Where these recommended separations cannot be achieved, you have to
apply the de-rating factors listed in Tables 22 to 26. Short sections of cables
that are bunched, such as at a switchboard entry may be ignored provided
the length where the cables are grouped is not longer than:

1 m for copper cables smaller than 150 mm2

1 m for aluminium cables smaller than 300 mm2

3 m for larger cables.
Read AS /NZS 3008.1.1:1998 Clause 3.5.2.2 (b) for more information.
Ambient temperature
When the ambient conditions are different from the specified conditions
then you need to adjust the current ratings of the cables accordingly. This
may mean a reduction in current or sometimes an increase. For example
if a TPS cable is enclosed in a concrete slab that is part of a floor heating
system then the operating temperature could be almost guaranteed. If the
slab temperature was a constant 25˚C then a V75 cable rating could be
adjusted up to 1.21 times its normal rating under these conditions.
However any temperature over 40˚C results in a current rating reduction.
Take particular care with cables on cable tray that may be installed close to
the underside of sheet metal roofing such as in warehouses or factories. The
temperature in these locations may easily exceed 40˚C for long periods.
Underground cables
Rating factors for underground cables are based on a particular soil
temperature and thermal resistivity. The thermal resistivity is the resistance
offered to the transfer of heat from the cable to the surrounding soil and is
similar in concept to electrical resistance. The greater this factor, the slower
the heat transfer which results in a greater temperature rise in the cable.
AS /NZS 3008.1.1:1998 specifies a thermal resistivity of 1.2˚C.m/W. Dry
sand has a value that exceeds this figure and clay or peat soils are lower.
Two examples of soil compositions that comply with the average figure are
cement bound sand and a gravel–sand mix. Read AS /NZS 3008.1.1:1998
Clause 3.5.5 for a more complete explanation. See table 29, AS /NZS
3008.1.1:1998 for de-rating factors.
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Depth of laying
Cables buried below 0.5 m have de-rating factors that range from 1.00 at
0.5 m to 0.83. See AS /NZS 3008.1.1:1998 Table 28.
Parallel connections of SDI
Each of the tables in AS /NZS 3008.1.1:1998 that list current carrying
capacities apply to single circuit arrangements. Circuits may be either
single-phase (active, neutral and earth) or three-phase (three actives, neutral
and earth). It is not unusual to connect cables in parallel to increase the
current carrying capacity of a circuit. The Wiring Rules Clause 3.4.3 places
restrictions on the cables for this arrangement which include:

minimum conductor size—4 mm2

equal size and material for each conductor

conductors the same length following the same route

joined at each end by clamping or soldering.
When using single double insulated cables in parallel combinations AS/NZS
3008.1.1:1998 Appendix B recommends arranging the cables symmetrically
to maintain equal current distributions.
Even though this arrangement is a single circuit, as far as current ratings are
concerned, each group of cables is a separate circuit. So if nine conductors
are arranged to supply a three-phase load, the number of circuits is three and
you should apply the appropriate de-rating factors.
Activity 8
The diagram represents a section through a parallel combination of cables laid on a cable
tray. Identify the location of each phase of the supply using letters A, B, C so the
configuration complies with appendix B of AS/NZS 3008.1.1:1998.
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Single circuit – Two conductors in parallel
Eddy currents
The Wiring Rules Clause 3.9.11 requires you to take precautions to limit
circulating and eddy currents within the electrical installation.
Figure 9: Limiting eddy currents
A single core cable surrounded by a steel enclosure is a typical example of a
situation where eddy currents are generated and is not suitable for ac
supplies. An example of an installation where this could be overlooked is
where the cable passes through a metallic switchboard surround. The ac
field surrounding the cable will induce currents into the metal of the
surround. The circulating currents tend to heat the metal, which may cause
cable damage.
To overcome this problem you could remove the metal between cables and
replace it with a non-conductive plate that still maintains the fire rating of
the panel or cut between cable entries.
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Uses for TPS cables
TPS cables are widely used in the building industry for subcircuits and
submains. Ease of use and availability of accessories make TPS cables the
first choice for an economical cabling solution. Almost without exception,
all lighting and power in small commercial and domestic installations use
TPS cables. TPS cabling systems with suitable protection are the main
choice for builder’s temporary power supplies.
Commercial lighting systems where the cables are installed between the
false ceiling and the supporting concrete slab are an ideal application for
TPS cabling systems. In this situation the cables and accessories can be
assembled off site or at floor level and placed before the ceiling structure is
installed. There is a range of accessories designed specifically for this
application.
Read EWP, 6th edition, Section 8.5 more details of this practice.
Flexible cords
Flexible cords are defined in The Wiring Rules as:
A flexible cable, no wire of which exceeds 0.31 mm in diameter and no
conductor cross-sectional area exceeds 4 mm2. The maximum number of
cores is five.
You may use flexible cords as part of the fixed wiring system provided
it is classified as heavy duty or enclosed in a suitable wiring enclosure.
Remember the minimum size for a conductor used as fixed wiring is 1 mm2.
However a flexible cord used to connect a fluorescent lamp luminaire
mounted in a false ceiling to the permanent wiring system, is not necessarily
considered as fixed wiring.
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Activity 9
List conditions that must be fulfilled so that a flexible cord is exempt from the conditions
that apply to fixed wiring.
_________________________________________________________________________
_________________________________________________________________________
Current carrying capacity of flexible cords and cables
AS /NZS 3008.1.1:1998 Table 15 and 16 list the current carrying capacity of
flexible cords and cables. The figures in table 16 are based on an ambient
temperature of 25˚C and a conductor temperature of 60˚C. (See notes below
table.)
Flexible cords and cables used as fixed wiring and not subject to continuous
flexing have a current rating that may be found from tables 3 to 14 in AS
/NZS 3008.1.1:1998. Use Table 15 and 16 when the cables are continuously
flexing.
Read AS /NZS 3008.1.1:1998 Clause 3.3.2 for more information.
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Testing TPS wiring systems
Complete installation testing is outside the scope of this module however
you should be able to conduct the basic safety tests and identify unsafe
practices associated with the installation of TPS cables.
The Wiring Rules outlines four mandatory tests:

continuity of earthing system and bonding conductors

insulation resistance

polarity

correct circuit connections.
Read The Wiring Rules Section 6.3.3 for reasons for these tests.
Testing in this module will concentrate on the subcircuits you are likely to
install in the practical exercises. Before you can conduct these tests you
should know where each conductor is meant to be connected to and be able
to identify the location of the connections.
You should know how to use a multimeter, especially the ohms ranges and
an insulation resistance tester. You also need a fundamental knowledge of
the wiring rules that relate to the installation of cables and testing.
Earthing resistance
The resistance of the earthing system must be low enough to ensure that the
supply is automatically disconnected before the touch voltage generated by
the fault causes injury or harm to persons or livestock
Read The Wiring Rules Clause 1.7.4.3 for an outline of the aims of this
protection method.
If the earth conductor or more correctly, the fault loop impedance is too
high, the circuit protection will not operate as quickly as it should. Persons
or livestock in contact with any metal connected to the earthing system may
be exposed to a dangerous voltage that may cause an electrocution.
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Causes of high fault loop impedance are:

incorrect earth conductor size

circuits that exceed recommended lengths
Of course if the current rating of the circuit protection is higher than it
should be, you could still have an unsafe condition even if your circuits are
installed correctly.
What is a safe value?
The maximum value of the main earth conductor is not to be more than
0.5 Ω measured from the earth electrode and the MEN connection
(Clause 5.4.3). You will conduct tests of this section of an installation in
other modules.
The Wiring Rules Appendix B4 and B5 contain guides to the safe resistance
of the fault loop created in a subcircuit. The impedance values in table B4.1
include the complete fault path all the way back to the source of supply.
You cannot measure this value if supply is not available.
You can, however, make a simple adjustment to the value and use this as a
guide when you test without supply. Clause B5.2.1 asks us to assume that
there will always be 80% of the supply voltage at the circuit protection
device. This suggests that 80% of the total circuit impedance exists on the
load side of the circuit protection.
So, from Table B4.1, a circuit that is protected by a type C circuit breaker
with a rating of 20 A has a maximum circuit impedance of 1.53 Ω. The
value you should measure between the load and circuit protection should not
exceed 0.8 × 1.53. Further to this, you should determine the impedance
value when the cables are operating at normal temperatures.
If you are testing without supply it is likely that the cable temperature is the
same as the normal ambient temperature. You compensate for this low
temperature by a further adjustment to the reading. Table 4.1 Note 4
suggests a further reduction of 0.8 should be applied.
So, the impedance of the previous example should be:
1.53 × 0.8 × 0.8 = 0.979 Ω
This is a conservative figure. If your circuit impedance is equal to or less
than this figure you can guarantee the circuit protection will operate in the
required time.
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Conducting the test
You will need an ohmmeter that can reliably measure resistance values less
than one ohm.
Test your meter by shorting the test leads together and recording the
reading. If your meter shows a resistance value above about 0.2 Ω try to
adjust the zero error.
Consult the documentation for your meter for this procedure. If there is no
adjustment then your meter is probably not suitable for this test procedure.
Have the meter checked.
Without power and the MEN still connected, connect the active conductor to
the neutral link and use an ohmmeter at a suitable remote section of the
circuit to measure the resistance between active and earth.
The value measured should be equal to or less than the figure previously
calculated.
Remember to restore all connections before you leave the installation!
Figure 10: Testing fault loop impedance with an Ohm tester
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Insulation resistance
You must test insulation resistance to ensure that the live conductors and
terminals within the installation are correctly insulated from earth.
Read Clause 6.3.3.3 for an outline of this test. The main points of this
section are:

test voltage —500 V dc (+20%, –10%)

minimum acceptable resistance= 1 MΩ (A value of 10 KΩ is allowed
for sheathed heating elements).
The ohms range on your multimeter is not suitable for this test.
This test is conducted between live conductors and earth. Live conductors
include Active and Neutral conductors as well as conductors used for load
switching.
You do not have to use an insulation tester between active and neutral
conductors.
Figure 11: Insulation resistance test with MEN connection removed
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Conducting the test
Make sure all power has been isolated for this test. Warn any workers who
may be in contact with equipment associated with the circuit that you are
about to test. 500 V will give someone a nasty shock. Loads or appliances
should be disconnected for this test as they can be damaged, and will give
low readings for the test.

Remove the MEN link or disconnect the neutral conductor of the circuit
under test. (It is better to disconnect the neutral than the earth
conductor. If you forget to replace the earth conductor after the test,
there is no obvious symptom until it is too late.)

Connect one lead of your insulation tester to the installation earth and
the other to the live conductors.

Make sure all switches are in the ON position.

Operate the test switch and record reading.
Polarity
As the circuits we are working with are fairly basic in nature we will not
cover all the options for testing circuit polarity.
At this stage we wish to:

identify the earth conductor

identify the neutral conductor making sure there is only one connection
to the earthing system at the MEN link

identify the active conductor making sure that it is this conductor that is
connected to the circuit protection and any switches associated with this
circuit operate in this conductor.
Conducting the test
Make sure all power is removed and the MEN link is connected.
In this test the low impedance of the MEN connection identifies the neutral–
earth loop.
This test is only effective if the earth conductor at the socket has been
positively identified first. A trailing lead is useful for the earth identification
test.
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Use the low ohms range of the multimeter to measure the resistance between
the earth terminal of the load and the neutral terminal. A low resistance
indicates the MEN circuit. A high resistance indicates an open circuit
between the neutral connection and the MEN. This could mean the terminal
is connected to the active instead. Further tests need to be made to find the
fault in the wiring.
Figure 12: Polarity test using Ohm meter
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Summary
TPS wiring systems are the backbone of economical wiring installations
provided the cables are selected according to the prevailing installation
conditions.
The installation of TPS systems requires the installing electrician to identify
possible hazards and protect the cable accordingly. Some hazards such as
high temperatures may not be obvious at the time of installation but must be
allowed for during the cable selection process.
This section has introduced some of the tables and clauses of The Wiring
Rules and AS /NZS 3008.1.1:1998 and provided some examples of their
use. You should make yourself familiar with these standards because you
will need to refer to them regularly in this and other modules.
Testing is an extremely important part of installation work. How do you
know if you have completed the job successfully if you cannot test it at the
end? This module only provides an introduction to testing concentrating on
the mandatory tests outlined in The Wiring Rules
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Wiring rules
The Wiring Rules
1.4.57
Table 3.1
1.10.4
Segregation
3.9.4
Protection against
mechanical damage
3.9.5
Wiring systems likely to
be disturbed
3.9.9
Prevention of mutual
detrimental effects between
services
3.9.6
Wiring systems installed
vertically
3.9.7
Change of direction
3.9.8.2
Sheathed cables
(Armoured and
unarmoured)
3.3.7
Impact
3.11.3
Underground wiring
systems
3.9.11
Limitation of circulating
and eddy currents
3.9.8.4
Flexible cords used as
fixed wiring
3.9.10
Penetration of fire barriers
6.3.3
Mandatory tests
AS /NZS 3008.1.1:1998
32
3.3
Types of cables
Table 1
Limiting temperatures for
insulated cables
3.4
Installation conditions
3.5
External influences on
cables
Table 52
Temperature limits for
insulating materials in
contact with conductors
Appendix B
Recommended circuit
configuration for
installation of single core
cables in parallel
Tables 22 to
29
Rating and de-rating
factors for cables
Table 15 and
16
Current carrying capacities
of flexible cables and cords
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Check your progress
Refer to EWP or Australian Standards to answer these questions:
1
Describe the cable that is commonly referred to as twin active.
_____________________________________________________________________
2
What does SDI generally stand for when referring to cables?
_____________________________________________________________________
3
What is the common colour of the sheath of circular TPS cables?
_____________________________________________________________________
4
What is the minimum separation required between cables of a 230V power system and
copper telephone cables?
_____________________________________________________________________
5
What is a suitable distance between supports for a cable installed within 2 m of a
ceiling access hole?
_____________________________________________________________________
6
Name two building service systems that should not be used as support for TPS cables.
_____________________________________________________________________
7
What type or category of underground wiring system allows the installation of TPS
cables without an enclosure?
_____________________________________________________________________
8
What is the minimum width of the additional mechanical protection for a category B
wiring system?
_____________________________________________________________________
9
State the maximum separation allowed between a category B wiring system and the
added mechanical protection.
_____________________________________________________________________
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10
What is the normal operating temperature of V-90 cable insulation?
_____________________________________________________________________
11
State the minimum bend radius of an unarmoured sheathed cable with a diameter of
25 mm.
_____________________________________________________________________
12
What is the minimum spacing between the cables so no de-rating factor need be
applied to the current carrying capacity if D = 30 mm? The cables are supported on a
perforated cable tray.
_____________________________________________________________________
13
What is the minimum spacing between cables supported on a cable tray that is fixed
directly to a wall If the diameter of one cable diameter is 30 mm and the other is
15 mm?
_____________________________________________________________________
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Answers to activities and check your
progress
Activity 1
A
Unenclosed – Spaced
B
Unenclosed – Touching
C
Enclosed – Wiring enclosure in air
D
Enclosed – Wiring enclosure partially surrounded by thermal insulation
E
Enclosed – Completely surrounded by thermal insulation
F
Buried direct
G
Underground wiring enclosure
Activity 2
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Activity 3
Circuits that you may find in a domestic installation that are not likely to
have RCD protection—electric stove, electric hot water, submains.
Activity 4
Figure 3.1 AS/NZS 3000—make sure you have Amendment No. 1,
September 2001.
Activity 5
1
Any from Clause 3.11.3.3
2
75 mm
3
200 mm
4
100 mm
Activity 6
Unenclosed in
air
Enclosed in
air
Completely
surrounded by
thermal insulation
Partially
surrounded by
thermal insulation
A
D
F
E
B
I
C
K
J
H
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Activity 7
1
Thermoplastic; V-75 and HFI-75-TP; V-90 and HFI-90-TP; V-90HT
2
90˚C
3
Likelihood of thermal deformation in the temperature range of 90˚C to
105˚C if exposed to mechanical damage. This type of cable damage is
likely where the cable is under pressure from clamps and support
systems.
Activity 8
Single circuit – Two conductors in parallel
Activity 9
Clause 3.9.8.4
(a) Flexible cords for pendant fittings or for connecting appliances
(b) Flexible cords not open to view, less than 2.5 m in length and used for
the connection of a single appliance or luminaire provided the cord has
a current carrying capacity:
Equal to the current rating of the circuit protection
The actual load of the appliance or luminaire provided the cross-sectional
area of the conductor is not less than 0.75 mm2.
Check your progress
1
TPS cable having one red core and one white core. Commonly used for
switch drops.
2
SDI Single double insulated
3
Orange
4
50 mm
5
0.3 m, although this is only a suggestion. The wiring rules require
support to prevent undue sagging. The Wiring Rules Clause 3.9.5.3.
6
Sprinkler systems, support systems of other building services such as
data.
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38
7
Category B
8
150 mm; The Wiring Rules Clause 3.11.3.3
9
75 mm; The Wiring Rules Clause 3.11.3.3
10
75˚C; AS /NZS 3008.1.1:1998 Table 1and The Wiring Rules Table 3.3
11
6 × 25 = 150 mm; The Wiring Rules 3.9.7 (b) (i)
12
2D = 60 mm
13
6D = 180 mm; see AS /NZS 3008.1.1:1998 figure 1, note 4
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