2.1 Introduction
The enclosure material for an enclosed Busbar
System is generally Aluminium alloy sheet/extrusion or sheet
steel. Stainless steel and fiberglass
enclosures have also been used for very specific requirements. The enclosure
contributes a significant part to the overall cost of the product and must,
therefore, be optimized.
The enclosures have to be fabricated, painted,
assembled with components, installed at the site, and should require minimum
maintenance for the entire life of the enclosed bus. To be cost-effective, the enclosure material
properties must consider:
2.1.1 Workability & Weldability
Fabrication of enclosure comprises shearing,
punching, folding, and welding. The
process can be manual or automated and contributes significantly to the cost
and performance of the product.
2.1.2 Strength
The enclosures are expected to withstand
internal forces generated by a short circuit and external forces caused by
wind, snow, ice, vibrations, and seismic activity
2.1.3 Resistance to Corrosion & Erosion
Enclosures are subjected to corrosion &
erosion depending upon the service conditions.
Surface coats provide additional protection and enhance life. (Gas insulated bus with Aluminium enclosure
may be buried below ground. Apart from
providing surface protection with coats, sacrificial anodes, and cathodic
protection are provided to counter the effect of corrosion.)
2.1.4 Electrical Conductivity
Enclosure material with a higher conductivity is
advantageous as it will contain losses and act as an electromagnetic
shield. In many designs, where enclosure
also serves as an earth conductor, higher conductivity is preferable. Enclosures of isolated phase bus carry the
current almost equal to that of the conductor and to limit the losses and
temperature rise, they are fabricated from high conductivity Aluminium.
2.1.5 Losses
All metallic enclosures of the Busbar
Systems will generate eddy current
losses and those with steel will, in addition, generate hysteresis
losses which contribute to a temperature rise of the enclosure. All international Standards specify a limit
on the temperature rise of the bus enclosure.
2.1.6 Weight
The enclosure contributes significantly to the overall weight of the enclosed Busbar System. Manufacturers have a preference for Aluminium over Steel. The lower overall weight facilitates ease of installation and lower cost of the support structure.
Over the years, the industry has matured and the enclosure material for different current ratings of the enclosed bus, application, and environment have been identified and there is an overall agreement on the selection. Many alloys of Aluminium have been specially developed to enhance strength and increase resistance to corrosion. However, there are many project-specific requirements for an enclosed bus.
Continued..........
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