On Sun, 14 Oct 2018 10:26:55 +0100, Jeff wrote:


I inspected the electrical system and found that fuse on the negative
lead had blown. Why manufacturers persist in providing a negative
wire fuse will remain a mystery as there are very few positive ground
vehicles still in service and even marine radios with floating grounds
are scarce. I have no idea where the radio was getting its ground
return for reasons that will soon be obvious. I replaced the fuse and
continued looking for problems.

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.

"Wiring and Grounding"
http://www.k0bg.com/wiring.html
And as shown, the negative lead fuse should not be removed.
The reason is, if the grounding point should lose its integrity,
excessive current could flow through the transceiver's
negative lead. It also prevents a minor ground loop between
the leads.

Most vehicles have a ground strap between the engine block and frame
and another ground cable between the frame and negative battery
terminal. I sometimes see a third cable from battery to engine block.
In this arrangement, any one of the three wires could be disconnected
and one would still have a tolerable grounding system. Fiberglass
body automobiles have duplicate ground wiring since there is no frame
ground.

"How To Properly Ground An Automotive Electrical System"
https://www.hotrodwires.com/how-to-ground-automotive-electrical-system.html

"Is it necessary to have two ground wires (1 to engine and 1 to"
car-frame)?
https://mechanics.stackexchange.com/questions/5903/is-it-necessary-to-have-two-ground-wires-1-to-engine-and-1-to-car-frame

--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558