[Elecraft] K3 - stuck at MCU LD we upgrading software

[Don Wilhelm]

Tom,

The size of the fuse at the power supply end should be consistent with 
the size of the wire.  For #10 wire, fuse at 30 amps, for #12 wire, fuse 
at 20 amps.  Note that this recommendation has nothing to do with the 
current draw of the equipment, only with the wire size.
I fuse #16 wire at 10 amps and #18 at 7 or 5 amps both in the shack and 
at the workbench.  I do not use anything smaller than #18 wire on power 
cables.

As I indicated, this has nothing to do with the current draw and 
protection of the connected equipment - it is only a consideration for 
the wire size.

If you compare it to household wiring and circuit breakers in your 
electrical panel, you will see a parallel.  #10 wire, use a 30 amp 
breaker, #12 wire, use a 20 amp breaker, #14 wire use a 15 amp breaker. 
It is the same thing at DC - fuse at the power source for the wire size.
If the equipment needs a smaller fuse than that afforded by the fuse at 
the power supply, add an additional fuse or circuit breaker at the 
equipment end.  It is common to see equipment using AC power lines 
protected by a fuse in the equipment cabinet, and it will be at a lesser 
rating than the rating of the wiring - that latter fuse is to protect 
the equipment, not the wiring - two different "animals" - safety to home 
and personnel vs. protection for the equipment.

As far as fusing both the positive and negative leads, that is 
controversial and depends on how the negative is wired.  It goes back to 
mobile installations where if the negative is connected directly to the 
battery and a fault in the vehicle grounding system should fail, then 
the fuse in the negative will protect the equipment.
However, the better solution is to connect the mobile radio negative 
solidly to the chassis (near the radio) and fuse only the positive side. 
  Ask any professional installer of mobile radios how it should be done, 
and they generally agree that the negative should connect to the chassis 
near the radio, and not be routed all the way to the battery negative 
(where it would need a fuse).

73,
Don W3FPR

73,
Don W3FPR

On 1/29/2017 6:03 PM, Tom wrote:
> Thanks Don.  I though the K3S was protected but I did a quick search of the
> manual for fuse and only found a reference to fusing both positive and
> negative cables if using a battery.