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CABLE
JOINTING: as the name implies is the process
of bringing two or more electric conductors together for continuity purpose.
CABLE
: This defined as a length of single conductor having one or several wires
stranded together or two or more such conductor each provided with its own
insulation and laid up together. the necessary requirements of cable are that
its should conduct electricity efficiently, cheaply and safely.
•cable
consists of three parts
a . conductor
b. insulator
c. mechanical protection
CONDUTORS
AND INSULATORS
A
conductor is material that allows the free flow of electric currents. E.g
aluminum, silver, brass, maganin, tungsten e.t.c.
Copper
:Is widely used for the manufacture of electric wires, cables and bus bars,
Brass:
Is used for terminals and connecting block it is harder than copper.
Nichrome:
Is used to form the elements in electric heaters.
Tungsten:
Is used to provide filaments for electric lamps.
COMPARISON
OF ALUMINIUM AND COPPER CONDUCTORS
STRANDING
OF CONDUCTORS: Conductors are stranded to ensure flexibility and ease of
handling. The sizes are often given by the number of strands and the diameter
of the individual strands.
INSULATORS.
This are materials that does not allow the free flow of current.
USES
OF INSULATORS
•To
confine electric currents to the conductors.
•To
provide mechanical protection to the cable.
•
Prevent leakage of current.
TYPES
OF INSULATORS
•polyvinyl
chloride (p.v.c):polyvinyl chloride is the general term for a class of
thermoplastic that has a tendency to soften when heated and to crack at low
temperature. It is impervious to chemical action. They are used where
temperature does not exceed 60 degree.
•Vulcanized
Rubber insulated: This is similar to p.v.c.It is a preparation of pure rubber
with a small amount of high sulphur. It is impervious to water, flexible and of
high resistivity.
•Synthetic
Rubber insulation: among these are
1.Polychloroprene
insulated insulated (p.c.p)
2.Butyl
rubber insulation
3.Silicon
rubber insulation : used temperature rise does not exceed 145 degree.
4.Ethylene:
propylene insulation.
•paper
impregnated insulation: paper is often used to insulate
the
conductor
in the underground cables.
Mechanical
jointing : These include connector blocks, joint boxes, mechanical crimping
Soldering
: This is the method of making joint in conductors together with the aid of a
solder at a lower
temperature.
CABLE
TERMINATIONS
A
termination is the entry of a cable end into an electrical accessory.
The methods of preparing a cable for termination are
•cut
off cable with a suitable tool, e.g pliers
•stripe
of insulation with a cutting knife
•cut
back the outer protective sheath from the insulation by approximately 12mm.
IEE
REGULATION FOR TERMINATION ARE
•all
cable terminations must be electrically continuous and mechanically sound
•accessible
for inspectionsl (unless buried)
•free
of mechanical strain.
Capable
of containing all the strands of the conductor
PROCEDURE
FOR USING A BLOWLAMP
1
Fill the lamp with gas (paraffin) not more than 2/3 of its capacity.
2
Turn the gas open when starting .
3
Start lamp with small rag dipped in methylated spirit .
4
Close the lamp as soon as the lampis hot and the pump can now be operated
5
The blowlamp should be put under an asbestos sheet until the flame is fully
established.
SOLDER:
The solder is a low point melting alloy of tin and lead .
Two
basic types of solder used in electrical work are
1.Fine solder (tinman’s solder )it consist of 60%of tin and
40%of lead .it melts more easily as tin has a lower melting point than lead.
2.Plumbers solder it consist of 30%and 70%lead.it does not
melt quickly because of the high percentage of lead .This alloy is to be used
for plumbing joints in cable.
METHOD
OF SOLDERING
There
are three method of soldering conductors
1.SOLDERING
BIT: In this method the bit which is made up of copper is heated, and
coated with solder. The tinned copper bit is now applied to the prepared
surface to bring them up to the melting temperature of the solder. While at the
same time, more flux and solder are applied. At the correct temperature the
tinned surfaces and solder will flow together freely and when cooled. It will
form a solid unit.
2.BLOW
LAMP: This is a heat source fueled by paraffin butane gas as the medium
for the soldering operation. The paraffin fueled type of blow lamp needs
preheating to vaporizer the paraffin and is then pressurized by a hand operated
pump. The butane gas type need no preheating and is often pressurize by it own
cylinder. It is more convenient but more expensive to operate. This method is
used for operation where a lager scale of heating is required. Such as the
joining of Large conductors and the fitting of terminal lugs or cable socket to
the conductors. In this method soldering ,the socket or surfaces to be joined
are heated to the required temperature by the blow lamp. The solder stick is
then dipped in the flux and applied to the socket or termination until it
penetrate the joint.
