Busway Construction
Originally a busway consisted of bare copper conductors
supported on inorganic insulators, such as porcelain, mounted within a
nonventilated steel housing. This type of construction was adequate for the
current ratings of 225Ð600 A then used.
As the use of busways expanded and increased loads demanded
higher current ratings, the housing was ventilated to provide better cooling at
higher capacities. The bus bars were covered with insulation for safety and to
permit closer spacing of bars of opposite polarity in order to achieve lower
reactance and voltage drop.
Feeder Busway
Feeder busway is used to transmit large blocks of power. It
has a very low and balanced circuit reactance to minimize voltage drop and
sustain voltage at the utilization equipment Feeder busway is frequently used
between the source of power, such as a distribution transformer or service
drop, and the service entrance equipment.
Industrial plants use feeder busway from the service
equipment to supply large loads directly and to supply smaller current ratings
of feeder and plug-in busway, which in turn supply loads through power take-offs
or plug-in units.
Available current ratings range from 600 - 5000 A, 600 Vac
or Vdc. By paralleling runs, higher ratings can be achieved. The manufacturer
should be consulted for dc ratings. Feeder busway is available in single-phase
and three-phase service with 50% and 100% neutral conductor. A grounding bus is
available with all ratings and types.
Available short-circuit current ratings are 42 000 - 200 000
A, symmetrical rms (see 13.8.2). The voltage drop of low-impedance feeder
busway with the entire load at the end of the run ranges from 1-3 V/100 ft,
line-to-line, depending upon the type of construction and the current rating .
Feeder busway is available in indoor and outdoor
construction. Outdoor construction is designed so that exposure to the weather
will not interfere with successful operation.
Plug-in Busway
Plug-in busway is used in industrial plants as an overhead
system to supply power to utilization equipment. Plug-in busway provides tapoff
provisions at regular intervals (approximately every 2 ft) over the length of
the run to allow safe connection of a switch or circuit breaker to the busway.
Load side cable connections can then be short and direct. Plug-in tapoffs (bus
plugs) can be connected to their loads by conduit and wire or flexible bus drop
cable.
Bus plugs can be removed, relocated, and reused. The use of
flexible cable permits the bus plug and machine it serves to be relocated and
put back into service in a minimum of time Bus plugs are available in several
types.
They include fusible switches, circuit breakers, static
voltage protectors (potentializer), ground detectors (indicating), combination
motor starters and lighting contactors, transformers, and capacitor plugs. Many
can be equipped with additional accessories, such as control power
transformers, relays, indicating lights (blown fuse), and terminal blocks for
remote control and indication.
Busway is totally enclosed and can be of the ventilated or
non-ventilated design. Plug-in busways have current ratings ranging from
100-5000 A. Plug-in and feeder busway sections of the same manufacturers above
600 A are usually of compatible design and are interchangeable, allowing for a
section of plug-in to be installed in a feeder run where tapoffs are desired.
Bus plugs are generally limited to maximum ratings of 800 A
for fused-switch type plugs and 1200 A for circuit-breaker type plugs.
Short-circuit current ratings vary from 10 000Ð200 000 A symmetrical rms. The
voltage drop ranges are approximately from 1- 3 V/100 ft, line-to-line, for
evenly distributed
loading. If the entire load is concentrated at the end of
the run, these values double.
A neutral bar may be provided for single-phase loads such as
lighting. Neutral bars usually are of the
same capacity as the phase bars.The bus housing may be used
as an equipment grounding path.
However, grounding bus bar is often added for greater system
protection and coordination under ground fault conditions. The grounding bus
bar provides a low-impedance ground path and reduces the possibility of arcing
at the joint under high-level ground faults if the housing is used as a ground
path.
Lighting Busway
Lighting busway is rated a maximum of 60 A, 300 V-to-ground,
with two, three, or four conductors. It may be used on 480Y/277 V or 208Y/120 V
systems and is specifically designed for use with fluorescent and
high-intensity discharge lightingTapoffs for lighting busway are available in
various types and include those with built-in circuit protection by either fuse
or circuit breaker.
Accessories include special mounting brackets and tapoffs
for surface or close coupling attachment of fluorescent lighting fixtures to
the busway. Lighting busway can be surface-mounted, recessed in dropped
ceilings, or suspended from drop rods. Hangers are available to accommodate
each method.
Lighting busways provide power to the lighting Þxture and
also serve as the mechanical support for the fixture. Auxiliary supporting
means called strength beams are available for increasing supporting intervals.
The strength beams provide supports for the lighting busway
as required by the National Electrical Code (NEC) (ANSI/NFPA 70-1993). Lighting
busway is also used to provide power for light industrial applications.
Trolley Busway
Trolley busway is constructed to receive stationary or
movable take-off devices to power overhead cranes, monorail systems, industrial
doors, and conveyor lines. Trolley busways are not suitable for outdoor
application.
They are used on a moving production line to supply electric
power to a motor or a portable tool moving with a production line, or where
operators move back and forth to perform their specific operations.
Trolley busway is available in current ratings ranging from
60Ð800 A, up to 600 V ac or dc, and 3, 4, and 5 wire. The steel casing serves
as the ground. Tapoffs (moving trolleys) range from 15Ð200 A and can be
equipped with circuit breakers, fusible protection, starters, contactors, and
relays.
No comments:
Post a Comment