Wiring Systems and Accessories
Wiring system:
 It’s an assembly of parts used in the formation of one or more electric
circuits. It consists of the conductor, insulation, its protection against
mechanical damage (sheathing or armouring), and wiring accessories for
fixing the system.
Factors affecting the choice wiring systems
a) Safety.
The wiring should be designed to protect the user and the equipment from
any hazard.
b) Load.
The design and specification of the wiring system should withstand load
demands.
c) Durability
The wiring system should not deteriorate from exposure to long term effects
of fumes, moisture, chemicals and weather conditions and human activities.
d) Appearance
The wiring should be easily identifiable and have a decent aesthetic
appearance.
e) Cost of The Wiring
The wiring should be cost effective without neglecting safety.
f) Flexibility
The wiring system should be designed such that it can be upgraded or
modified to meet the changing requirements of a consumer.
Sheathed Wiring System (All Insulated Wiring Systems)
 It’s a method of wiring consisting of an insulated conductor provided with
sheath which serves as measure of protection against mechanical damage.
 The insulating material include: impregnated paper, rubber, plastics and
mineral insulation.
 The sheathing material include: lead. Tough rubber, plastics, aluminium and
textiles.
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Types of sheathed wiring cables:
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TRS (Tough Rubber Sheathed)
PVC (Polyvinylchloride Sheathed)
PCP (Polychloropene Sheathed)
LAS (Lead Alloy Sheathed)
HSOS (House Service Overhead System)
PILC (Paper Insulated Lead Covered)
MIMS (Mineral Insulated Metal Sheathed)
TRS (Tough Rubber Sheathed)
 Its termed as all insulated because the sheath is of an insulating material.
The insulation is vulcanized rubber. The sheath is tough rubber.
 There is no metal associated with the wiring thus lessens the need for
extensive earthing arrangement to be carried out to ensure safety against
electric shock.
Advantages
 It’s cheap.
 Flexible.
 Easy to install: it’s through use of buckle clips.
Disadvantages
 Rubber does not withstand severe mechanical damage.
 Rubber is affected by oil.
 Limited usage where there are high temperatures.
Polyvinylchloride Sheathed
It’s also termed as all insulated. The conductors are insulated with PVC and the
sheath is PVC.
The PVC system is available as single core (circular). Flat twin, flat twin with
earth and three core with sheath
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Advantages
 Less affected by oil
 Easy to install by use of buckle clips
Disadvantages
 Inferior to rubber due to its elasticity properties
 Less insulation resistance compared to rubber
 Softens at high temperatures
Polychloropene Sheathed
Suitable for farm wiring where steam, ammonia fumes, lactic acid, milk fats, direct
sunlight, and heat are to be taken into account.
The cable consists of conductors insulated with vulcanized rubber and sheathed
with PCP.
Lead Alloy Sheathed
 The conductors are insulated with vulcanized rubber, taped and sheathed
with lead alloy (lead with a small proportion of tin and antimony).
 The lead sheath can be used as circuit protective conductor.
 Sheathing must be continuous throughout the installation either by soldering
or clamps.
 Liable to corrosion when they come into contact with lime in damp
situations.
 Cable is available in single core (circular), twin core, three core, and twin
with cpc.
 Fixed by use of buckle clips.
 The system can be used with conduit system.
House Service Overhead System
 The conductors of this wiring system are insulated with vulcanized rubber
and taped.
 The core of the cable is varnished and calico taped. The whole cable is then
braided and coated with weather resisting compound.
 Used for consumers’ distribution wiring between buildings.
 They are either fixed to the external walls or supported by catenary wire.
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 Fixed by use of buckle clips.
 The cable is resistant to the adverse effects of all kinds of weather.
 Largely replaced by PVC.
Paper Insulated Lead Covered
 Used for external underground distributions systems.
 Widely used for internal distribution in factories and other industrial
premises.
 The paper must be impregnated with oil to protect against ingress of
moisture, hence use of cable sealing boxes.
 Fixed by use of cleats.
 Further protection against mechanical damage is provided by armouring in
the form of helical wound steel wire or tape.
Mineral Insulated Metal Sheathed
 These cables consist of copper or aluminium conductors contained in a
copper or aluminium sheath.
 The insulation is compressed mineral magnesium oxide.
 The most common is MICS with copper as main metal for conductors
and sheath.
 The advantage of this cable is that it requires no further protection against
high temperatures and fire.
 They are impervious to water and oil.
 Immune to condensation because the conductor, sheath and insulation are
inorganic.
 Ageless thus has an indefinite life.
 Installation is simple though the ends of the cable must be sealed off
against ingress of moisture by special termination.
 Fixing is by clips or saddles.
 Higher current capacity than PVC, TRS or PILC because of good heat
resisting properties
 Applicable in industrial installations, hazardous areas
 Offers excellent self-contained cpc
 MIMS cables can be used in earth concentric wiring in which metal
sheath acts as a circuit protective conductor and also as a neutral
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 Care must be taken with the use of MIMS cable to ensure that bonding is
efficient at all boxes and outlets.
 In the wedge type of seal, the pot has an earth tail welded to the inner
part of the wedge.
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WIRING SYSTEMS
CONDUITS
Conduit systems
A conduit is a tube designed so far as electrical work is concerned to
carry electric cables.
There are two distinct methods for installing conduits used namely:
Surface or concealed
Conduits are either metallic or non-metallic
Steel conduits include:
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Heavy gauge screwed steel conduit
Light gauge lug grip steel conduit
Aluminium conduit
Copper conduit
Flexible steel conduit
Steel conduits are available in two types or classes
 Class A plain end conduit
 Class B screwed end conduit
Class A conduit
 This conduit is known as light gauge, plain, slip, pin grip, lug grip according
to size.
 It has thin walls and is available as close joint, brazed or welded joint and
solid drawn.
 The conduit cannot withstand threading and presents problems where earth
continuity must be maintained.
 Various methods for connecting conduit to its associated accessories are
available including lug grip in which the fittings are held together by
slipping the conduit end in it. The conduit must be prepared first by
removing enamel. If the contacts are not cleaned the electrical contact
resistance is very high.
 Close joint conduits cannot be bent or set because the seams tend to open.
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 Care must be taken when slight bends and sets in are made in brazed or solid
conduits.
 Fixing is by use of saddles
 Conduits are erected before cables are drawn into them
Usage
Limited to situations which are not damp.
Where high degree of protection isn’t required against mechanical damage.
Class B conduits
 It’s known as heavy gauge or screwed conduit.
 It’s available as seam welded and solid drawn.
 Alternative finishes are:
 Black enamel: for use in dry situations.
 Silver grey finish: for use in dry situations & internal use where the
conduit requires to match the decorations.
 Hot galvanized or sherardized( coating steel with zinc by heating):
for external use where the conduit will be subjected to dampness or
condensation
 Solid drawn is expensive than seam welded and it’s used gas tight and
explosion proof installations.
 Welded seam conduit is used for most good quality installation. Conduits
joining is by use of threads thus giving good mechanical strength and
electrical continuity where the conduit is used as CPC.
 Sizes range from 16mm to 32mm outside diameter.
 A full range of system accessories are available for screwed conduit: they
include bends, elbows tee and boxes.
 Fixing accessories include saddles clips and crampets.
Advantages
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Gives good protection against mechanical damage
It allows easy wiring
Minimizes fire risk
Provides good earth continuity
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Disadvantages
 Its costly compared to other types
 Difficult to install in under wood floors
 Under certain conditions condensation may occur inside the conduits
i)
Flexible metallic conduits
 Used for final connection to machinery (e.g. motors) where vibration
and the possible need to adjust position of the equipment.
 Used for short runs where mechanical damage is unlikely to occur.
 Available as spirally wound, partially interlocked light gauge
galvanized steel strip with water tight or non- water tight.
 Can be obtained with PVC sheath
 As a conduit it’s not a satisfactory CPC itself thus a separate CPC
must be run between the special brass adaptors used to join flexible to
ordinary screwed conduit.
 Sizes range from 16mm to 50mm internal diameter.
Aluminium conduit
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its light in weight
offers good earth continuity
resists corrosion except when in contact with dissimilar metals
Easy to handle, cut and screw.
Its non-magnetic
Must be coated with bitumastic paint to protect against damp cement
Must be clean and free from oil and grease.
Easy to install
It’s a screwed end conduit
It’s not strong mechanically and may require further protection where the
risk of mechanical damage is great.
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Copper conduits
 Resists corrosion
 Provide excellent earth continuity
 A long life system
Its only disadvantage is that it’s expensive
IEE REGULATIONS ON METALLIC CONDUITS
 Must be fixed before any cable is drawn in
 Inspection boxes should remain accessible thorough out its life of service.
 Space factor must not be exceeded which is 45% of the csa of the conduit
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𝑡𝑜𝑡𝑎𝑙 𝑐𝑠𝑎 𝑜𝑓 𝑡ℎ𝑒 𝑖𝑛𝑠𝑡𝑎𝑙𝑙𝑒𝑑 𝑐𝑎𝑏𝑙𝑒𝑠
𝑐𝑠𝑎 𝑜𝑓 𝑡ℎ𝑒 𝑐𝑜𝑛𝑑𝑢𝑖𝑡
𝑥 100%
 When metal sheathed cables are used conduits should not come into contact
with other services such as gas, water pipes etc.
 cables of extra low voltage system should not be drawn in the same conduits
of power and lights exceeding low voltage
 Where conduit passes through flammable or explosive dusts, vapours or
gases, flameproof box must be inserted in the installation.
 Ensure that all conduits are mechanically sound and electrical continuity
across all joints is maintained
 Where conduits are to be concealed, they are installed while the building is
being erected and can be buried in floors and walls in such a manner the
cables can be drawn into them after the building has been completed.
Non-metallic conduit
Two main types
 Flexible conduits
 Rigid conduits
Flexible conduits
 They come in both round and oval section
 Material used is PVC
 Bent without use of special tools
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Rigid conduits
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They come in standard sizes
Closely resemble steel conduits
Bent by use of flame or springs
Fixing is by saddles or clips
When threads are required they cut themselves when screwed in a threaded
portion but can be threaded normally by use of thread stock and die.
 Heavy gauge PVC can withstand a fair amount of rough treatment both in
erection and in service
Advantages
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Reduction in risk of electric shock
Absence of fire risk due to breakdown in continuity
Easy manipulation without use of special tools
Resistance to corrosion from most industrial liquids
No internal condensation
Disadvantages of non-metallic conduits
 Does not offer mechanical strength
 Does not offer earth continuity thus an extra conductor is required for
earthing
Trunking system
Advantages
 Lighter than conduits of the same capacity
 Fewer fixings are required for one trunking length than a run of multiple
conduits
 Wiring is easier and quicker as cables are “laid in” instead of being “drawn
in”.
 Erection time is reduced
 Easily adaptable wiring system.
 Multi compartment trunking is available where segregation of services is
required.
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 Trunking is available in sections (square and rectangular).
 Lengths are joined by couplers normally secured by screws.
 Earth straps fixed between each section to ensure earth continuity
along the trunking run.
 Available in light and heavy gauge forms.
 Finishing is enamel but a galvanized finish can be supplied for certain
installation.
 Fittings for trunking are: blank ends, tees, bends elbows couplers four
way boxes, fire resisting barriers, pin racks etc.
Where trunking is used to contain busbars it becomes
 Overhead busbar trunking
 Rising mains trunking
Overhead busbar trunking
 Used for distribution of electrical energy to machines in factories. The
usual arrangement is steel trunking containing copper busbars
mounted on insulators.
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 At intervals along the length of run a tapping off is provided with
three HRC fuses mounted in a sheet steel case.
 Connections to machines are taken off at tap off points and then taken
by flexible conduits, steel conduits or other wiring system to the
machines
Advantages of overhead busbar system
 The system provides a three phase supply
 Has a plugin tap at 3ft interval into which tap off box can be fitted
thus it’s easy to provide a supply position whenever its required
 Available system in which lighting fitting and 13A socket outlets
can be attached at any required point.
 Can be installed before the final position of any machine is decided
and that position can be easily altered without incurring more
costs.
 Easily adaptable in industries.
Rising mains
 Often employed in multi storey buildings.
 In this system busbar are run vertically through the walls of the building.
It’s fed and controlled usually from the bottom at the service entry.
 At each floor there will be a tap off point to supply power.
 Distribution boards are fixed direct to the trunking.
 Fire proof barriers are fitted at each floor level since in the event of fire
hot gases and flames tend to travel up the trunking due to convection
 Where the busbars passes through these barriers they must be fitted with
insulating sleeves.
Forms of trunking
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Flush trunking
Multi compartment
Skirting trunking
Lighting trunking
Cable tap trunking
Pvc mini trunking
Flush trunking
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Fits flush with the walls
Multi compartment
Square or rectangular trunking with segregated compartments so that cables
carrying different voltages can be accommodated in the same trunking unit.
Skirting trunking
Designed to carry power, telephone and lighting cables in its various
compartments. Socket outlets and telephone points can be easily fitted as an
integral part of the trunking.
Lighting trunking
Designed to use where long rows of continuous lighting are required.
The steel enclosure carries the fluorescent, tungsten fittings control gear and
supply cables
PVC mini trunking
It’s a very small PVC trunking ideal for surface wiring in domestic and
commercial installations such as offices.
Cable tap trunking
Carries insulated supports which can accommodate VRI or PVC cables.
From which it supplies to machines and lighting circuits through tap off
boxes.
Ducting systems
 A system in which an installed network of under floor ducts in
solid concrete are made during the erection of the building.
 The duct is used to accommodate many other systems e.g.
electrical circuits, telephone circuits TV etc.
 Ducts are arranged to feed ceiling roses for the floor below and
socket outlets for the floor above.
 Junction boxes are provided with cover plates fixed flush with
the floor finish.
 Circuits are fed through surface conduits.
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 Because ducts are laid in straight lines between junction boxes,
it’s always possible to locate the runs.
 Available in form of sheet metal or an insulated fiber material.
Advantages
 Ducts are built at an early stage during the construction of the building
 Accommodation for wiring can be conveniently installed at an early
stage
 A concealed wiring system is provided without need for necessary
long chases on the concrete floors
 Easy to alter the position of outlets where necessary
 The system takes a form of intersection grid that every part of the
floor area is within a reasonable distance from the duct
Special systems
This area deals with installation developed to meet specific requirements
It’s a combination of wiring systems.
Bare conductors
Bare conductors or lightly insulated are installed in building for the following
purposes only.
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Earthing connections
External conductors of earth concentric wiring systems
Protected rising mains and busbar systems
Conductors for extra low voltage systems
Collector wires for travelling canes and trolleys
IEE REGULATIONS FOR BARE CONDUCTORS
i.
ii.
When installed in extra low voltage system, the insulation must be adequate
and protection against fire risks must be provided.
In rising mains and busbars systems, conductors should be accessible to an
authorize persons. Insulation used to support the busbar must be strong
enough to support them firmly. Conductors must be free to expand and
contract due to temperature changes.
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iii.
iv.
Where bare conductors pass through floors, walls. Partitions, or ceilings
they must be protected by enclosing in a non-absorbent incombustible
insulating material unless earthed metallic trunking is used.
Collector wires for travelling canes, the cables should be protected by
screen or barrier. Warning notice indicating presence of the wires must be
fixed along the length of run not exceeding 12 metres.
Catenary systems
The most common system uses a high tensile steel wire pulled taut between
two fixing positions (e.g. between two buildings).
The cable is run through the wire being supported in the same way.
The extension is this wiring is known as GRID SUSPENSION TYPE. It
consists of a central galvanized steel, high tensile stranded wire surrounded
by a number of PVC insulated conductors. The whole is enclosed with PVC
tape and PVC sheathing. Textile filling is used to obtain a circular section.
Wire is secured at each end by eye bolts and strainers. Special connecting
boxes are used at tees and right angle turns.
Uses
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Industrial buildings where other wiring systems are difficult to install
Supply feeds for lighting fittings along the length of run
Overhead street lighting
In factories instead of busbar system where the power requirement of
machines is small and the expense involve in installing busbar is high.
Advantages
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Water proof
Weight imposed on building is reduced
Installation can be done after completion of the building
Reduced installation costs
The system absorbs vibrations
Lamp replacements costs are reduced
Cables used are: PVC, PCP, TRS, and HSOS. These are useful for outdoor
use only.
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IEE REGULATIONS ON CATENARY WIRING SYSTEMS
 Cables which are supported by a separate catenary wire shall be
continuously bounded up with the catenary wire attached to it.
 The weight of the conductor must be fully supported by the catenary wire
 Catenary wire must be securely fixed at each end
 The clearance between the cable supported by the catenary and the ground
must be adequate. A pole may be used necessary.
EARTH CONCENTRIC SYSTEM
It’s also known as “Combined Protective and Neutral”
Basically it’s a form of protective multiple earthing in which the neutral
conductor is also the circuit protective conductor.
It’s a single core cable. The copper sheath carries the neutral and any earth
fault current which might occur should a fault appear on the circuit.
Continuity of the sheath is of utmost importance.
In wedge type of seal, the pot has an earthing tail welded to the inner side
of the wedge to enable continuity connections to be made easily.
IEE REGULATIONS ON CONCENTRIC WIRING SYSTEMS
 Must be connected to an a.c supply system on which P.M.E. has been
authorized
 No fuse or non-linked switch gear or circuit breaker shall be inserted on the
earthed external conductor
 The external conductor must be earthed.
OVERHEAD SYSTEM
There many situations require power to supply by overhead means rather
than underground cable.
These include: distribution between farm buildings, garage, and green house.
The wiring includes rubber or PVC insulated cables supported by a separate
catenary wire. PVC covered conductors on insulators, cables with an integral
catenary wire
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