[Elecraft] K3 inline fuse

[Don Wilhelm]

Tom,

You are absolutely correct.  Take a mobile situation - the radio's power 
ground is connected to the transceiver chassis ground, as is the coax 
shield, microphone shield and other stuff.  All that "stuff" can (and 
will) connect to the vehicle chassis at some point.  I know the coax 
will connect at the point where it attaches to the antenna.

Now imagine a situation where the negative lead fuse is open.  The 
vehicle chassis is still connected to the battery negative - so the 
radio gets it positive lead directly from the battery (since that fuse 
is still good), but the negative battery terminal voltage flows through 
the vehicle chassis to the point where the coax shield connects to the 
vehicle chassis (presumably at the antenna), and then follows the coax 
shield to the transceiver.  The transceiver still works, so you (the 
operator) are not aware that the negative fuse has opened, but the DC 
current paths will give rise to "strange happenings".

Bottom line, follow Tom's advice and fuse only the positive lead, and in 
a mobile situation, take the negative voltage from the vehicle chassis 
close to the radio.

The fusing of the negative lead started when some were concerned about 
the vehicle's battery to engine block (or vehicle chassis) connection is 
broken, which could lead to massive starting motor currents being 
conducted from the coax ground at the mobile antenna, through the radio, 
and onto the battery negative terminal.  Yes, that situation is quite 
real, but can be circumvented by periodically checking the engine block 
to chassis grounding in the vehicle, as well as the battery negative 
connection to the vehicle chassis.

If the radio has an isolated (floating) negative connection (I believe 
only commercial mobile radios do that), then fusing of the negative lead 
is OK, but for most ham radios, the fusing of the negative lead can 
cause as much damage as the reasons given for inserting a fuse into the 
negative lead.  Take your pick.

73,
Don W3FPR

Tom W8JI wrote:
> Positive lead fuses are a good idea.
>
> Negative lead fuses never were a good idea unless the radio has a totally 
> floating negative buss.
>
> If the negative lead fuse to the radio opens for any reason all the negative 
> lead current for the radio will flow through the negative lead of any 
> accessories connected to the power supply.
>
> They are a terrible idea for any system with the negative lead common to 
> ports in and out of the radio, or to the chassis. You certainly won't catch 
> me using one!The negative lead needs to be connected solidly to the power 
> supply negative for a multitude of reasons, the most prominent of which is 
> if the negative lead fuse to the radio opens you can blow up accessories or 
> open ground traces including traces in the radio.
>
> They are not even recommended in vehicles any longer in some countries 
> because of the fire and damage hazard they create.
>
> 73 Tom
>
>   
>> In both Neg and Pos leads, supply a fuseholder and fuse.  I use the
>> "stubby" fuses from Motorola @ 30A, you should use something like
>> that.  These are T-rated, meaning they're designed to open with a DC
>> load.  Don't use a fuse out of your home's fusebox...
>>     
>
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