Tom,
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.
This is precisely the kind of problem that the single-point ground rule
would solve, but following it can be tough. Batteries are often grounded
to a fender liner or the bulkhead, which makes one of these the ground
return point for the vehicle electrical hardware. The starter motor is
bolted to the engine, which makes it the common for the high-current
devices in the vehicle. Where to put the radio ground? Certainly not to
the engine block, sooooo...
And BTW, think about the coil-on-plug ignition systems most vehicles have
now. The top of the primary of the coil goes to the battery through a long
loop around the engine and to the battery or the Engine Control Module.
The bottom ends are pulled to ground to fire the plugs by transistors in
the ECM. How does the secondary find ground? I'm told that newer vehicles
ground the secondary to the engine block, but some older ones simply
floated the secondary and essentially let it find ground through the
capacitance of the wiring harness. Either scenario makes it tough to fight
ignition noise.
Brad KV5V
On Sat, Oct 31, 2015 at 10:23 AM, Tom W8JI <w8ji@w8ji.com> wrote:
> Yes, or where other premises grounds are tied together. Systems like
>> Telco, CATV, satellite, even lightning rods if there are any, building
>> steel if there is any.
>>
>>
> It doesn't matter where in the world we live, electrons all follow the
> same physical laws.
> There are three things that should apply to advice:
>
> 1.) What really works and actually is a good thing to do
> 2.) What is a bad thing to do
> 3.) What still meets but safely works around local codes
>
> The USA and Japan does some really stupid things with 12V radio systems,
> like negative lead fuses and power sources and radios with negative 12V
> rails tied to ground at both ends.
>
> The UK is smarter on that, with requirements a mobile negative NEVER go to
> a battery negative post and never have a fuse. This should, through common
> sense, apply to a 12V supply on a desk 12V powered Ham radio. We are,
> however, locked in a loop of doing things the wrong way because that is how
> we do it.
>
> The USA is good with the use of a common entrance point ground for
> everything, where all utilities like CATV, Telco, safety, and power have
> one single common "entrance" ground point, although that is sometimes
> missed. The ground rod means very little compared to the importance of the
> common bonding.
>
> Our local fire station caught fire from a lighting strike because the
> fools they hired to do a wireless internet system just run cables in
> willy-nilly through plastic pipe because it "meets code". Occasionally they
> will have problems at different county buildings in storms because they
> have utter nonsense in the wiring, but it "meets code". So far they have
> had a fire in a fire station (and destroyed much of the computer gear),
> lost the tag office, and lost things at the sheriff's dispatch system.
>
> But the system that will keep burning up, and maybe eventually even kill
> someone, does meet code. That is what matters. :)
>
> There is little that can ever be bad about using a single point ground,
> but adding a second path to ground (which no longer makes it single point)
> can quickly become a disaster.
>
> It doesn't matter if we are in London or LA, electrons and grounds work
> the same way. The issue we face is making sure it works correctly, while
> not violating codes. Meeting codes is NOT a cure, and can cause a problem.
>
> 73 Tom
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