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Generally, having all loads tied to a common
return (ground) buss is the best way to avoid common-mode coupling. This
occurs when there is a small voltage difference between various separated
grounding points. As little as several millivolts difference can raise havoc
with the high gain amplifiers in avionics when one unit is tied to one buss and
the other is tied to another, separated, buss. My preferred method of
wiring in non-conductive aircraft is to use shielded-twisted pair wires from the
power and ground busses to each load. I terminate the shield at the buss only.
The ground buss is easily made from 3/8" copper tubing pounded flat with 8-32
screws inserted about every 1/2". I anchor these screws with an internal
lockwasher under the head and one under the nut. A practice that is used
with aluminum conducters works well also here; after drilling the holes for the
screws, coat the buss with petroleum or No-Oxid grease, abraid the surface
with crocus cloth or steel wool through the grease,
then insert the screws with the lockwashers and tighten them. The teeth of the
lockwashers will penetrate the grease and the surface and form a gas-tight seal
to prevent oxidation. Brass screws and nuts may also be better for this
than steel. It is also helpful to segregate the loads on the buss, keeping
high current loads towards one end and low-level signals toward the other. For
instance, attach the battery minus to the buss so it feeds current one way to
high power loads toward one end and feeds current to low power
signals toward the other end. Don't mix them on the buss. Your alternator
return should be wired directly from the alternator
case to the battery (-); please dont
depend on the dubious connection provided by the alternator mounting screws to
serve that purpose. But if your alternator return goes to the buss then to
the battery, connect it at the buss next to the battery return on the high
current load side of the buss. That will minimize the three-phase
ripple current on the buss. Also, provide a piece of 3/4" wide braid wire
from the engine to the firewall. Remember, and this is important! There is no
such thing as a zero resistance/impedance buss, and all loads require two
conductors, voltage and return!
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