2.2
Aluminium Alloy
2.2
Aluminium Alloy
Over the last few decades, Aluminium welding
technology has vastly improved bringing down the cost significantly. Furthermore, the cost of the dies for
extrusion of Aluminium has drastically come down.
Aluminium sheets are available in various alloy
compositions and it is possible to select a specific alloy for the desired
application. Alloy sheets are available
with a conductivity of 61% IACS with less mechanical strength to 30% IACS with
60% of the strength of Steel. Alloy
Sheets are available with excellent resistance to corrosion. This flexibility has made Aluminium sheets and
extrusions, the preferred material for bus enclosures.
Aluminium tubes are extruded up to a diameter of
600 mm (24”). Extruded tubes are
preferable to rolled tubes. Marginal
loss of conductivity can be compensated by an increase in thickness for an envisaged
resistance. Extruded pipes have better
mechanical properties. The process of
extrusion demands a minimum t/d (thickness to diameter) ratio. For long route length of bus enclosures with
a larger diameter, as may be required for gas insulated transmission line, a spiral
welding sheet is adopted.
Grades 1050 &1350 have excellent
conductivity while grade 6101 exhibits good mechanical properties at a marginal
compromise with conductivity. Grade 5052
displays good properties for Corrosion resistance, workability/formability, and
weldability. Grade 6061 is extruded and
displays good mechanical properties.
These alloys can be heat treated and are available in different tempers.
Depending upon the nature of specific
requirements, with respect to conductivity, mechanical properties, and
resistance to corrosion, enclosure material is chosen from a wide range of
treated alloys. Busbar Systems
manufactured with Aluminium enclosures are lighter. Aluminium enclosures provide excellent earth
continuity. Special Aluminium alloys are
used when enclosures are exposed to saline atmospheres as in installations in
coastal areas, ships, and off-shore platforms or when the gas insulated bus,
are buried underground.
2.3
Steel
Powder-coated Sheet Steel is used extensively as
a bus enclosure material.
Steel is economical, strong, and workable. The sheets are cold-rolled up to a thickness
of 4 mm (5/32”).
Since Steel enclosures are more susceptible to
corrosion, it has now become a normal practice to fabricate the enclosure with
galvanized sheets to provide better environmental protection. Powder coating of galvanized sheets, to the desired
colour and finish, provides additional protection.
Busbar trunkings, installed for the distribution
of power in an assembly area or machine shop, are invariably made with steel
enclosures. These are characterized by
low rated currents and low fault currents and therefore are not subjected to
significant electro-dynamic forces. The
overall cross-section size is small.
Since the weight of the busbar trunking (enclosure + conductor + insulating support)
is low and is installed indoors, it is not subjected to large mechanical forces. The enclosures for these are made from steel
of thickness as low as 1.0 mm (3/64").
The entire enclosure is formed (profiled) to provide adequate
strength. Tap off boxes with thin sheet
steel enclosures, equipped with isolating & protecting devices, are
attached to the main trunking.
2.4
Stainless Steel
There are some, extremely corrosive atmospheres,
where clients prefer stainless steel for Bus enclosures. Chlorine and saline atmospheres aggressively
corrode steel and Aluminium enclosures.
Stainless steel is the preferred material in off-shore platforms and
chlorination plants. Drug &
pharmaceutical industries specify this material when the enclosed bus runs
inside the plant.
Stainless Steel sheets and components are
manufactured in two grades Type SS 304 and Type SS 316. Both are austenitic Steel and have very low
magnetic susceptibility. Grade Type SS 304
exhibits more magnetic property than grade Type SS 316. As a crude approximation, both of these can
be called non-magnetic and hence do not generate hysteresis losses in presence
of the magnetic field generated by alternating currents.
Stainless Steel sheets are more difficult to
work with. Being a hard material, they
require special tools, drills, dies, and punches. Metal inert gas or Tungsten inert gas welding
needs to be performed during manufacture.
Stainless steel enclosures are expensive to manufacture and hence
specified only for very special needs.
Stainless steel enclosures, manufactured with grade
Type SS 316 have a higher corrosion resistance due to the presence of
Molybdenum and a higher content of Nickel than with grade type SS 304. The preferred material for stainless steel
hardware is grade Type SS 304 that has higher Chromium content and is therefore
much harder and wear-resistant. Stainless steel enclosures do not require
painting (unless it is required to improve emissivity for better heat
dissipation).
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