Reply-To: <glcasey@gte.net>
Reply-To: lancair.list@olsusa.com
From: "Gary Casey" <glcasey@gte.net>
To: <lancair.list@olsusa.com>
Subject: Ground loops of the electrical kind
Date: Wed, 6 Mar 2002 07:44:38 -0800
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<<I vaguely remember someone on the list cautioning about having a "loop"
in the ground wiring circuit.  What is a ground loop and what is the
problem with it?  Any info would be appreciated since I am in the wiring
process now. >>

A so-called "ground loop" exists when a wire or device is grounded at two
places at the same time.  It happens most often in metal airplanes where a
case of, say, a radio is grounded to the airframe and then a well-meaning
technician runs a ground wire from the case to a "good" ground.  Now the
ground wire can conduct current from one location to the other on the
airframe.  All grounds are supposedly connected together, but all are not at
exactly ground potential, due to the currents running through them.  The
easiest to see is high-current requirements like for a starter, but a more
common problem is RF interference and this can occur in a ground loop.
Picture this - an engine analyzer uses grounded EGT sensors (one side of the
sensor grounded to the probe shell).  During the wiring the main ground wire
for the instrument is attached to a local ground under the panel.  The
inrush of the starter will be hundreds of amps, supposedly all going through
the main ground wire from the engine to the battery, but because of the
resistance of the ground strap there will be a voltage difference between
the engine and the battery ground.  Meanwhile there is this EGT ground wire
going from the engine, through the analyzer and back to the battery.  Some
of the starter current will go through this ground, not what the
manufacturer had intended.  Also, this "loop" could look like an antenna and
pick up all kinds of RF noise.  Sorry about the mechanic's explanation.
Hope it helps.

Gary Casey
ES project


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