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APPLICATION OF ALUMINIUM TERMINAL EXTENDERS FOR
MOULDED CASE CIRCUIT BREAKERS
By S.S.A. Rajapaksa MIET, MIEEE
Electrical engineer Rotax ltd Colombo 04
Introduction
As per the data available from theLondon metal exchange the
world copper prices
increased by 334% with comparision to aluminium as at july
2010.This price escalatingtendency of copper will prevail in future
as well due to the difficulties faced by mine
owners to unearth copper material.Therefore the electrical
manufacturers have
progressively commenced the manufacture of current carrying
components made of
copper replaced by aluminium alloys.It was observed that the
properties of aluminiumwill significantly changed for the
electrical use by adding alloys(aluminium alloy 6101).
This paper discuss the possible problem areas encountered during
the above transition
and solutions adopted by them.
Comparision chart for properties of Aluminium vs; Copper
properties Copper (%) Aluminium (%)
weight 100 55
Conductivity(electrical) 100 56
Conductivity(thermal) 100 55
Tensile strength 100 64
Increase of cross section forsame conductivity
100 156
Thermal expansion 100 143Specific resistance 100 180
Properties of conductor materials
Aluminium
Aluminium is the most freely available of all metals and never
found in its pure statebecause of its likeness to associate with
other metals.This feature of Aluminium enables a
wide range of alloying characteristics, however it also
increases refinement costs and
lowers the conductivity..The other properties of Aluminium are
good ductility(easilyworked by rolling,drawing,forging etc),very
good corrosion resistanance(except in salt
layden atmosphere) by forming a tough very hard invisible and
high resistanance
oxide,resists pitting(eroding) from arcing effects, galvanic
action where aluminium actsas an anode when placed with
copper.Aluminium alloy 6101 comprices of .5% silicon
and .6% magnesium in addition to its aluminium content.
CopperCopper a reddish material found in its almost pure state
therefore refining costs are
minimized with comparision to aluminium. The other properties
are good ductility,
corrosion resistance action by forming a thin easily removable
copper oxide layer faced
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with atmosphere.When copper is heated more oxides are formed and
continue to increase
the resistanance of the location of heat.
Solution based problem solving
Problems encountered Solutions adoptedTensile strength of
aluminium is less thancopper.Therefore meeting the stresses
under short circuit fault conditions has to
be addressed
With the increase of cross sectional area ofAluminium to meet
the current carrying
capacity the tensile strength of the
Aluminium material will automatically get
corrected
Connecting Copper and Aluminiumtogether with moisture in the
joint due to
humidity will form a galvanic
cell.Therefore aluminium as the anode will
start pitting(eroding)
Tin plating the Copper and Aluminiummaterial will prevent the
formation of a
galvanic cell. The steel bolts also shall be
nickel plated.
The thermal expansion of Aluminium is
greater than Copper.Therefore with the
increase and decrease of temperature at the
joint(due to the current flow) the clampingforce of the mounting
bolt can get effected.
Selection of nickel plated spring/lock
washers and tightning the bolt to the mccb
manufacturer,s recommendation using a
torque wrench will solve this problem
Formation of oxides in both metals willincrease the
resistance
Tin plating both meterials
ConclusionMany people understand the cost difference between
copper and aluminium but do nothave personnel experience with the
usage of moulded case circuit breakers comprises of
aluminium terminal extension bars and are hesitant to change. A
typical example is the
introduction of auto transmission and constant velocity joints
for the motor vehicles
where people found reluctant to buy such vehicles for years due
to no valuablereasons.Therefore we are compelled to explore the
possibilities of usage of Aluminium
for the current carring components of Mccb without delay.
References
1.Prior,Schlobohm, and Brownellsenior specification engineers of
GE consumer and
industrial facility a comparision of alumenium vs. copper as
used in electrical
equipment
2.facilities engineering branch US bureau of
reclamation-electrical connections for
power circuits
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