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Date: Fri, 06 Apr 2007 21:28:15 -0400
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From: "Paul Lipps" <elippse@sbcglobal.net>
Subject: Grounding
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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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<DIV><FONT face=3DArial>Generally, having all loads tied to a common=20
return&nbsp;(ground) buss is the best way to avoid common-mode coupling. =
This=20
occurs when there is a small voltage difference between various =
separated=20
grounding points. As little as several millivolts difference can raise =
havoc=20
with the high gain amplifiers in avionics when one unit is tied to one =
buss and=20
the other is tied to another, separated, buss. &nbsp;My preferred method =
of=20
wiring in non-conductive aircraft is to use shielded-twisted pair wires =
from the=20
power and ground busses to each load. I terminate the shield at the buss =
only.=20
The ground buss is easily made from 3/8" copper tubing pounded flat with =
8-32=20
screws inserted about every 1/2". I anchor these screws with an internal =

lockwasher under the head and&nbsp; one under the nut. A practice that =
is used=20
with aluminum conducters works well also here; after drilling the holes =
for the=20
screws, coat the buss&nbsp;with petroleum or No-Oxid grease, abraid the =
surface=20
with crocus cloth or steel wool <STRONG><EM>through</EM></STRONG> the =
grease,=20
then insert the screws with the lockwashers and tighten them. The teeth =
of the=20
lockwashers will penetrate the grease and the surface and form a =
gas-tight seal=20
to prevent oxidation. Brass screws and nuts&nbsp;may also be better for =
this=20
than steel. It is also helpful to segregate&nbsp;the loads on the buss, =
keeping=20
high current loads towards one end and low-level signals toward the =
other. For=20
instance, attach the battery minus to the buss so it feeds current one =
way to=20
high power loads toward one end and feeds current to low power=20
signals&nbsp;toward the other end. Don't mix them on the buss. Your =
alternator=20
return should be wired directly from the alternator=20
<STRONG><EM>case</EM></STRONG> to the battery (<STRONG>-</STRONG>); =
please dont=20
depend on the dubious connection provided by the alternator mounting =
screws to=20
serve that purpose.&nbsp;But if your alternator return goes to the buss =
then to=20
the battery, connect it at the buss next to the battery return on the =
high=20
current load side of the buss. That will minimize the three-phase=20
ripple&nbsp;current on the buss. Also, provide a piece of 3/4" wide =
braid wire=20
from the engine to the firewall. Remember, and this is important! There =
is no=20
such thing as a zero resistance/impedance buss, and all loads require =
two=20
conductors, voltage and return! </FONT></DIV></BODY></HTML>

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