ELECTRIC BUS WAYS CURRENT RATING BASIC INFORMATION

To apply busways properly in an electric power distribution system, some of the more important items to consider are the following.

Current-carrying capacity
Busways should be rated on a temperature-rise basis to provide safe operation, long life, and reliable service.

Conductor size (cross-sectional area) should not be used as the sole criterion for specifying busway. Busway may have seemingly adequate cross-sectional area and yet have a dangerously high temperature rise.

The UL requirement for temperature rise (55 deg C) (see ANSI/UL 857-1989) should be used to specify the maximum temperature rise permitted. Larger crosssectional areas can be used to provide lower voltage drop and temperature rise.

Although the temperature rise will not vary significantly with changes in ambient temperature, it may be a significant factor in the life of the busway. The limiting factor in most busway designs is the insulation life, and there is a wide range of types of insulating materials used by various manufacturers. If the ambient temperature exceeds 40 deg C or a total temperature in excess of 95 deg C is expected, then the manufacturer should be consulted.

Short-circuit current rating
The bus bars in busways may be subject to electromagnetic forces of considerable magnitude by a short-circuit current. The generated force per unit length of bus bar is directly proportional to the square of the short-circuit current and is inversely proportional to the spacing between bus bars.

Short-circuit current ratings are generally assigned in accordance with ANSI/NEMA BU1-1988 and tested in accordance with ANSI/UL 857-1989. The ratings are based on (1) the use of an adequately rated protective device ahead of the busway that will clear the short circuit in 3 cycles and (2) application in a system with short-circuit power factor not less than that given in table 13-1.


If the system on which the busway is to be applied has a lower short-circuit power factor (larger
X/R ratio), the short-circuit current rating of the bus may have to be increased. The manufacturer should then be consulted.
The required short-circuit current rating should be determined by calculating the available short-circuit current and X/R ratio at the point where the input end of the busway is to be connected. The short-circuit current rating of the busway must equal or exceed the available short-circuit current.
The short-circuit current may be reduced by using a current- limiting fuse or circuit breaker at the supply end of the busway to cut it off before it reaches maximum value. Short-circuit current ratings are dependent on many factors, such as bus bar center line spacing, size, strength of bus bars, and mechanical supports.

Since the ratings are different for each design of busway, the manufacturer should be consulted for speci√ěc ratings. Short-circuit current ratings should include the ability of the ground return path (housing and ground bar if provided) to carry the rated short-circuit current.
Failure of the ground return path to adequately carry this current can result in arcing at joints, creating a fire hazard. The ground-fault current can also be reduced to the point that the overcurrent protective device does not operate. Bus plugs and attachment accessories also should have adequate short-circuit interrupting and/or withstand ratings.

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