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Ralph Mowery · October 14th 18, 06:39 PM posted to uk.radio.amateur,rec.radio.amateur.antenna

In article ,
says...

The negative fuse is nothing to do with positive ground vehicles (and a
radio with the case connected to negative would not work in a positive
ground vehicle anyway without additional isolation).

The negative fuse is there to stop high currents, such as the starter
motor, being drawn through the radio wiring and coax should the battery
to chassis connection be high resistance or open circuit, and prevent a
possible fire.

This is at greatest risk if the radio negative is wired directly to the
battery.

Jeff

That seems reasonable. However, I've never seen that happen.

More common was blowing or removing the negative power cable fuse to
the radio. That makes the DC ground return for the radio go through
either the car frame, which will produce alternator noise on the
transmit signal, or through the coax cable, which will produce a
smoking coax cable in transmit. I've seen both about 5 times each in
the last 50 years. I would consider these faults to be a greater risk
than a disconnected battery to chassis (or engine) ground cable.

I don't know anything about the tractor trailer wiring, but around 1972
I picked up extra money repairing the CB radios. Several truckers
brought in rigs that were blown up when switching from the car to the
truck. None of them had a fuse in the negative power lead. By blown
up, most had a diode across the poer wires inside the transceiver and
that diode had shorted and blew the fuse. I seem to remember then they
talked about the tractor haveing a positive ground. Probably ran on 24
volts also.

I have only ran FM ham rigs in a car. I used to ground the rig to the
frame and run the positive to a relay that comes on when the car is
started. Never had any alternator whine or problems. For about the
last 10 or more years I just plug into the lighter or accessory socket
in the car. My Toyota power on those sockets only come on when the car
is started.

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