Way back in the '90s, when I served on the SAE automotive RF immunity and
emissions (EMI & EMR) committees, questions about why we have
negative-lead
fuses in ham radio gear came up from time to time. (Many of us on the
committees were hams.) The reason we were given is that there was a
scenario in which the negative battery lead ground failed, and the radio
negative became the best path to ground for the starter motor,
particularly
if the radio negative was screwed to the negative battery post. In
retrospect, this seems far-fetched, but the OEMs don't like to hear
stories
about electrical fires in their vehicles, so somehow, this was
communicated
to the radio manufacturers, and negative-lead fuses became the rule.
Any good connection to the chassis anywhere on a unibody vehicle is far
better than a connection to a battery negative. Motorola is smart enough to
tell installers to ground to the chassis, not the battery. In the UK the
directive is to use the chassis or a manufacturer supplied terminal, and it
specifically prohibits connecting to the battery negative pole.
The negative fuse is just foolishness. If it opens, all the radio current
goes through small wiring. If the fuse opens, there goes the radio or
something connected to the radio via a port. All of the radio current will
go through some small wire.
My shop bench radio has an open foil on the CW key line and the mic because
of a fuse holder failure, and that isn't the first radio that has that
happen. :)
The entire problem centers around use of the battery pole or battery
connector as a source, and this carries over into our station desks. There
is a ground loop similar to that in a car created between the power line
ground, the power supply case, the negative lead, and the radio chassis back
to ground. As in the car, if this stuff was built or wired correctly, the
12V bus would only be grounded at one point and there would be no negative
fuse.
In our houses, many of the problems blamed on RF feedback are actually
ground loops caused by grounded cabinets common to negatives and voltage
drops on negative leads.
At least some places in Europe got their together and banned battery
negative connections because of the hazards.
73 Tom
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