Bus-bars are current carrying conductors. They are designed to carry certain normal current continuously.
The bus-bars are designed to carry certain rated normal current continuously within the permissible rise in temperature. The value of cross-section so obtained is verified for temperature rise under short time short-circuit current. This figure is further used from the protection point of view while considering the relay setting. In fact, the relay should be set such that under fault condition no damage is caused to these bus-bars.
Bus-bar conductors are separated on post insulators or strain insulators. These insulators experience electrodynamics forces depending upon the current being handled. these forces produce bending moments on the insulators. These bending moments, therefore, become the ruling factor to decide the spacing between insulators (span).
The following factors are taken into consideration while designing a bus-bar :
(i) Bus-bar material,
(iii) Cross-section of conductors,
(iii) Temperature rise during continuous normal current,
(iv) Temperature rise during short circuit current of 1 sec. of 3 sec.
(v) Design of insulator creepage distance and clearance,
(vi) Distance between phase conductors,
(vii) Force on insulators during peak short circuit current,
(viii) Span of insulators support, and
(ix) Enclosure design.
Copper and Aluminum are two preferred materials for electrical conductors. Considering certain advantages of lightness, cost, resistance to corrosion, temperature coefficient of resistance etc. Aluminum has got an edge cover copper. As such, most of the bus-bar conductors now-a-days are of Aluminum (E 91-EWP Class).
Though a table has been prepared keeping certain parameter fixed (sec table-3) but to select the cross-section of the bus-bar we may first have to find out the derating factors.
Derating factors depend on :
(i) Allowable bar temperature over an ambient of 35 C,
(ii) Enclosure of the bus-bar.
Though a table has been prepared keeping certain parameter fixed (sec table-3) but to select the cross-section of the bus-bar we may first have to find out the derating factors.
Derating factors depend on :
(i) Allowable bar temperature over an ambient of 35 C,
(ii) Enclosure of the bus-bar.
Derations for these two factors are obtained from the table 1 & 2 given below and multiplied to get the overall derating factors.
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