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

[James Maynard]

On 2011-04-02 01:03 AM, Dave, G4AON wrote:
> With car electrics (and a boat may be similar), the reason is that if
> the engine grounding strap fails (or is removed and not replaced), the
> starter motor can draw current via the radio negative lead which leads
> to damage and possibly fire.
>
> 73 Dave, G4AON
Thank you, Dave, your answer reassures me.

In fact, boats are quite different from cars.  In cars, it is common to 
use the chassis as a common ground return.  In boats, this is forbidden.

To avoid ground loops, the various types of "grounds" on a boat (the DC 
safety ground [green], the AC safety ground [green or green with yellow 
stripe], the lightning ground terminal, the bonding ground for 
prevention of electrolysis, the RF radio  ground, etc.) have only one 
common point, which is usually the engine negative terminal.  In boats, 
only the starter motor is connected directly to the engine negative 
terminal.

Other DC circuits on a boat have three (3) wires: the "ungrounded" 
normally current-carrying conductor (red, DC+), the "grounded" 
current-carrying conductor (black or yellow, DC-) and the "grounding" 
conductor which normally does not carry current (safety ground,  green 
or bare wire).

Similarly for AC circuits: the "ungrounded" normally current-carrying 
conductor (in the USA, black, in the EU, brown), the "ungrounded" 
current-carrying conductor (in the USA, white, in the EU, light blue), 
and "grounding" conductor that normally does not carry current (in the 
USA green, in the EU, green with yellow stripe).

Both the DC and AC safety grounds have only one common point: the engine 
negative terminal.  Other circuits, such as the ham radio circuits, are 
on the other side of a "main" circuit breaker which cuts both the DC+ 
(red wire) and the DC- (black or yellow wire).  Likewise, AC branch 
circuits are on the other side of a "main" circuit breaker which cuts 
both the "hot" conductor ("ungrounded", black in US, brown in EU) and 
the "neutral" conductor ("grounded", white in US, light blue in EU).  
The safety ground (green with yellow stripe) is never switched.

So in the scenario that you describe, if the engine grounding strap 
should fail or be removed, my boat's main DC circuit breaker will trip, 
disconnecting both the DC- (black or yellow) and DC+ (red) conductors 
but leaving the "grounding" safety ground (green wire) connected to the 
safety ground bus.  The branch circuits downstream of the DC main 
circuit breaker then have fuses or circuit breakers only in the 
"ungrounded" (red) conductors.

Thank you, Dave;  your explanation reassures me - because I know how 
boat wiring differs from automobile wiring.

73,  Jim, K7KK