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Summary:
"Low
Voltage Busway: An Overview"
With
trilingual summary
By
Richard L. Nailen, EA Engineering Editor
The
most flexible and economical means of electric power distribution
within buildings, particularly when total current reaches 800 amperes
at 600 volts or below, is often low-voltage busway. Its application
is governed by NEMA Standard BU 1, UL 857, and Article 368 of the
National Electrical Code. It consists of a metal channel enclosing
bus bar conductors of copper or aluminum, in standard lengths of 1.5
to 6 meters, usually suspended from ceilings but also usable as
vertical risers.
A
common version is "feeder" busway used to transmit power
from one area to another. Individual phase, neutral, and ground bars
may be insulated and stacked together ("sandwich"
construction), or left bare and spaced apart by insulating supports.
The sandwich design minimizes circuit reactance and voltage drop, and
limits arc migration during a fault. However, inspection is
difficult; bar insulation can deteriorate; and the busway itself does
little to limit short-circuit current.
Also
common, particularly in factory production areas, is
"plug-in" busway. It allows attachment of power taps or
plugs at intervals along its length, supplying individual loads that
are easily moved as processes or machines are changed. A variation
permits direct mounting of overhead lighting fixtures to the busway.
Taps may involve simple receptacles or a variety of control devices
such as circuit breakers or motor starters. Circuit protective
devices in such accessories need careful coordination with those
protecting the busway, as well as any in the individual loads.
Installation
should provide ample support. Ceiling hangers are required every 1.5
meters. These must also allow for horizontal expansion and
contraction of busway during load swings (typically 1 to 2
centimeters per 30 meters of length). Some types of vertical busway
may allow internal spread of fire and smoke, requiring fire barriers
at each floor through which they pass.
Standards such as NEMA BU 1.1, NFPA 70B, and ANSI/NECA 408 offer
guidelines for busway maintenance. Annual inspection is suggested, to
look for loose joints, moisture contamination, obstructed
ventilation, or overheating (infrared scanning is recommended).
Copyright 2003, Barks Publications, Inc., Chicago.
Reproduction by any means prohibited.
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