It is related to difference between maximum conductor operating temperature of cable and bus bars. The operating temperature of the bus conductors is restricted to 85-90°C for all ratings; because of following reason but according to standards the maximum conductor operating temperature for insulated cables is higher (refer to Table below).
Aluminum and copper conductors start oxidizing at about 90°C. The oxides of aluminum (A12
O) and copper (CuO) are poor conductors of electricity, they may adversely affect bus conductors, particularly at joints, and reduce their current-carrying capacity over time, and lead to their overheating, even to an eventual failure. Universal practice therefore is to restrict the operating temperature of the bus conductors to 85-90°C for all ratings, at least in the medium range, say, up to 3200 A. Silver oxide (Ag2
O) is a good conductor of electricity and can also be used for welding joints. It can seal the inside surfaces from the atmosphere and can also prevent the contact surfaces from oxidation. If the joints are silver plated or welded, the bus system can be made suitable to operate at higher temperatures. In aluminum conductors, for instance, they can be operated up to an optimum temperature of 125˚C, until aluminum begins to lose its mechanical strength. Similarly, copper conductors can operate at still higher temperatures. The entire bus system can now be operated at much higher temperatures.
However, operation at such high temperatures may impose many other constraints, such as high temperature in the vicinity which may endanger the operating personnel. It may even become a source of fire hazard.
Such a high temperature may also damage components mounted inside the enclosure, which may not be able to sustain such high temperatures. It may also cause limitations on gaskets and other hardware of the bus system to operate at such high temperatures continuously.
Accordingly, the maximum operating temperature of a bus system aluminum or copper with silver-plated or welded joints is also permitted up to 105°C only. The enclosure temperature is still restricted to 80°C or up to110°C at locations that is safe and inaccessible to a human body.