Not every system requires laminated bus bars. In some rack-mounted
systems, current levels don’t rise above the capacity of
a backplane. In other cases, there are relatively few taps off the power
bus, so a wiring harness is the best approach. Also, in applications
where only a few cabinets need to be wired, a wiring harness may be
sufficient, even when the number of taps off the power bus and wiring
complexity are high. But with a complex power distribution in production
volumes, the advantages of a bus-bar design are compelling.
Laminated bus bars are typically more costly than wiring harnesses
because of how they’re made. Their manufacturing process involves
cutting strips of copper and insulation
to shape and then combining
them into a multilayer structure
with mounting holes, tabs, or connectors
added for interface.
As Rick Whistler, national sales
manager at bus-bar manufacturer
Eldre, Rochester, N.Y., describes
the process, fabricated copper
conductors and integral interconnections
and insulation are assembled
in a fixture and then subjected
to pressure and high temperature
over time. The insulation—one of
several materials such as Mylar,
Nomex, Tedlar, or Kapton—has
been customized with a b-stage
resin. When heated, this material
bonds the insulation to the copper.
The copper conductors are sized
to achieve the required current ratings
based on the acceptable temperature
rise for the conductors.
The Copper Development Association
publishes current ratings for
various sizes of bus bars at a temperature rise of 30ºC, a limit applied
in UL safety tests (see http://busbar.copper.org/ampacity/busT1.htm, Table 1, "Ampacities of Copper No. 110 Bus
Bars"). Whistler notes, though, that Eldre sizes conductors around a
more conservative temperature rise of 20ºC, in part to account for the
presence of insulators on the copper.
Although its material cost will likely be higher than that of an equivalent
wiring harness, a bus bar’s overall cost may be less after taking
assembly, reliability, and field service requirements into account. In
the factory, the interconnect components can be assembled much
faster with a bus-bar design than with a wiring harness. Moreover, the
likelihood of miswiring with bus bars is less. One reason for this is that
with bus bars, the location of the interconnects is fixed. They also are
readily labeled using a variety of methods, including rubber stamp,
silk screen, and adhesive labels. Plus, the benefits of easier assembly
translate into faster and easier component repair in the field.
Bus bars offer other advantages
as well. A bus-bar design is generally
more compact than a wiring
harness and may eliminate the restriction
of air flow posed by wires.
Reliability is better too, particularly
when a system is exposed to
harsh environmental conditions.
Hervé Dauvergne, a market development
engineer at Eldre, has
noted that while a wiring harness
might fail in five years, a bus bar
might last 30 years or more.
Beyond these benefits, bus bars provide a platform for greater mechanical integration at the system level. Advanced designs are exploiting this capability (see the figure). As Whistler says, "The greatest innovations are custom solutions that integrate as many components as possible." In addition to integrating power connectors, bus-bar designs are taking
on other power-related components, including fuses, circuit breakers, filters, and capacitors. Because the bus bar is essentially a custom design, there’s an incentive to exploit its ability to clean up and simplify assembly. Another opportunity for customization lies in the combination of power and signal interconnect within a bus bar that incorporates small wires or flex circuits
to route signals. Advanced bus-bar designs, like this one developed by Eldre,
distribute power neatly and efficiently and integrate many of the discrete components associated with power distribution.