[Elecraft] [K3] Why fuse the negative lead from a battery?

[Don Wilhelm]

  Where will the current will flow if there is a fault?  That is the 
question.

There are two things involved here - wiring the radio negative directly 
to the battery or wiring it to the vehicle chassis.  It does make a 
difference because of the way most vehicle batteries are tied on the 
negative terminal.

If one wires both positive and negative directly to the battery, the 
negative should be fused to protect the RADIO in case of a fault between 
the battery and the engine block.  It is not a safety issue.

Note carefully that the battery negative is wired both to the engine 
block and to the chassis by direct wires from the battery negative.  If 
the battery were tied only to the engine block and then the chassis were 
also tied to the engine block (as was pointed out in the case of the 
boat), the failure mode would not exist.
And it is all because the radio is grounded to the chassis through the 
antenna connection in addition to the negative lead.  If both these 
grounds go to the chassis instead of the battery negative, the problem 
solved by the fuse in the negative lead would not exist.

If the radio negative is connected to the vehicle chassis, the problem 
does not exist and the negative lead should NOT be fused.

73,
Don W3FPR

On 4/2/2011 9:40 AM, David Ferrington, M0XDF wrote:
> I'm not positive (forgive the pun), but I think ideas have changed on that in the UK and you again don't fuse the -ve lead, but I'll need to check.
> 73 de M0XDF, K3 #174, P3 #108