Page 961 - Industrial Power Engineering and Applications Handbook

Recommended practices buses and making bus joints 29/91 1
For making joints between aluminium and copper. care to eliminate any possibility of contact oxidation and to
should be taken that both surfaces are properly cleaned ensure an almost uniform current distribution through
and dried and applied with a thin layer of grease, before the contact area without an excessive heating. Such a
jointing, to eliminate electrolysis between the two metals practice may, however. be of little advantage to maller
in the presence of moisture. It is recommended that wch ratings in view of cost. It will be worth while only for
a surface (particularly of copper) be tin or sliver plated higher currents. say, 2500A and above and also for higher
to acoid electrolysis, which may take place with the operating temperatures. It is seen that the oxidation of
passage of time. Such a situation is predominant when aluminium or copper starts at about 85-90°C. It is for
making aluminium connections to the main switchgear this reason that the operating temperature ofa bus system
devices and components such as breakers. switches, fuses. is limited in this region. as discussed already (Table 28.3).
contactor%. relays and all other current-carrying and At higher ratings it is therefore recommended either to
switching devices. The connecting terminals of all these weld the edges ofthe joints (straight-through or flexibles)
cornponents are invariably of copper or bronze alloy. As to seal the openings and prel'ent any oxidation during
standard practice. these terminals, are either silver plated operation, or to silver plate the joints. Silver oxide is a
or tin plated to facilitate a direct jointing or connection good conductor of heat and electricity. With the use of
with aluminium links. However, use of grease at every silver joints or welded joints, a higher operating
joint and precautions, to eliminate the presence of moisture temperature of the busbar\ and the joints is permissible
at the joints is mandatory to ensure a good joint. up to 105°C (Table 2x2) as against 70°C as in Table
Some application engineers may, however, prefer a 28.2 (IEEE-C-37-20) or 85-90°C as in Tablc 14.5 (IEC-
bimetallic joint (e.g. a Cupal joint) for jointing between 60439-2) in ordinary joints. With the use of silver plated
copper and aluminium. A bimetallic (Cupal) joint has joints therefore, the rating of a bus system niay be
copper foil on one side and aluminium on the other. The improved and the use of metal optimired. See also Section
basic purpose of such a joint is to climinate electrolysis 2X.S.l.
during normal operation. It becomes superfluous, when
proper care is taken in making the joint as noted above. 29.3 Bending of busbars
It is a misconception that such a joint can deal with
differential expansions of the two metals.
Bending a busbar also requires utmost care. Sniallcr
Checking a joint sections niay not matter as much as larger and thicker
sec ti on s . The me tal s (part icu I ar 1 y a 1 u in i n i u m ) . be i ng
It is important to check the fitness ofa bus joint made in brittle. niay show up cracks. particularly on the outer
a factory or at site. Thi\ can be done with the aid of a d.c. surfaces. when bent, as a result of excessive tensile force
millivolt drop or measurement of the joint resistance at this surface. The cracks may reduce the current-carrying
(mil) test. Such measurements arc taken on a number of capacity of the busbar at this section. bedes rendering
similar joints and the results tabulated and compared. it mechanically weak, to withstand the electrodynamic
Values in the same rangc may be considered good joints, forces on a fault. Sharp bends are therefore not recom-
while those with wide variations will be indicative of a mended. When it becomes necessary to haw sharp bends
poor joint. Such joints may then be investigated and to meet locational requirements, it is recommended that
improved. the particular area of the bus section be heated first. to
make the metal somewhat soft and then to bend it gently
29.2.6 Silver plating of joints* while the metal is still hot. A more appropriate method
will be to use a hydraulic bending machine which can
It I\ wmetinies preterred to sll\er plate aluminiuinjoints exert pressure evenly and more gently to prevent crack$.

Siltcr and copper do ti01 make ;I chemical bonding with aluininiuni
through the electrolytic proces\. Alurninium thcreforc cannot be
silbcr coated dii-ectly. Thc rccoinincnded procedure is to hr\t apply
a %n ctrating (1 -2 micron\). then a coating ofCu (1-7 microns) and
then ii coating of As (13-15 iiiicrrrns).