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John Schroeder wrote:
<<<We are running out of tabs on B&C's 48 and 24 forest of tabs. Is is OK to
join a couple of grounds like those from the trim system and then run a
single line up to the ground?>>>
John, as long as the entire trim system has a dedicated return to the
central ground, it's unlikely to be much of a problem. Other things that may
not be a problem with grouped returns are circuits that draw current on a
steady-state basis such as indicator lights. Often these can be brought
together and returned to the central ground on a single wire.
What you need to avoid (by using the "Holy Point" grounding) is creating a
situation where two or more systems are using the same piece of wire for
their ground return. Even though we think of a length of wire as essentially
resistance-free, it just isn't so - even good copper wire has some
resistance. It's measured in ohms/ft for a given gauge, and though very
small (often found in a wire handbook stated in ohms/kfeet to give a number
without a lot of leading zeros) it is finite, and like any resistor, will
develop a voltage across it's length in accordance with Ohm's Law.
For some numbers: AWG 20 copper wire, at 77 deg. F has 10.4 ohms/kft. A 10
ft run of this gauge will have a resistance of ~0.1 ohm. If this wire is
used to return, say 2 amps it will develop about 0.2 volts - which could be
sufficient to upset something else sharing the same return.
Summing up: As a result of the resistance, ground return currents from one
system will produce small but measurable voltage drops across the wire
(proportional to the current and inversely proportional to the wire size)
that can and may produce unexpected effects on other systems simultaneously
sharing the same return wire. Some electronic systems can be quite sensitive
to such voltages. Also it must be remembered that ground returns very often
carry fluctuating currents developed in active circuits within a system as
well as "noise" currents from shields and etc., which are often the culprits
in inter-system interference.
Didn't mean for this to become a treatise on Holy Point grounds but there it
is, FWIW.
Dan Schaefer
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