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

[James Maynard]

Right, Bob.

What you say is true of AC residential wiring.

Boats are different.  The so-called "main" AC breaker on a correctly 
wired boat breaks both the "hot" wire(s) -- black or in 120/240 VAC 
single phase, black and red) and the "neutral" (white) wire -- coming 
from the boat's shore power inlet. Also, boats are required (by ABYC 
E-11) to have a reverse polarity indicator or alarm to alert the user if 
he plugs his shore power cable into an incorrectly wired shore power 
pedestal outlet at the marina.

The marina's shore power outlet at the pedestal near your slip is, from 
the point of view of the requirements for residential or factory wiring, 
a BRANCH outlet, and one in a particularly damp and hazardous location, 
where a Ground Fault Circuit Interrupter would be appropriate.  (Most 
marinas lack GFCIs however.)  The shore power coming on board a boat at 
its shore power inlet has its neutral and safety ground conductors tied 
together only at the main power inlet for the whole marina.  At the 
shore power pedestal at your slip, there is usually a noticeable 
different in AC voltage between the neutral and safety ground.  (If 
indeed, there *is* a safety ground -- too often marinas are wired 
incorrectly!)

This can be a particular hazard in marinas in fresh water.  If there 
were a wiring fault aboard your boat - or a nearby boat - there can be 
an AC potential gradient in the fresh water surrounding the boats, and 
anyone swimming nearby can be electrocuted by that AC voltage gradient.  
(People are more conductive than fresh water, but less conductive than 
salt water. So-called ESD -- electro-shock drowning -- can and does occur.)

So I am wiring my boat acccording to the current ABYC E-11 standard 
rather than according to the National Electric Code that applies to 
residential wiring.  I am installing ELCIs (Equipment Leakage Circuit 
Interrupters) for each shore power inlet, and I am also installing an 
isolation transformer, wired according to E-11, with the case connected 
to the boatside ground, and the shield between the windings connected to 
the shoreside neutral (white wire coming from the marina's shoreside 
pedestal).

You are correct, Bob, in describing residential wiring.  But boats are 
different!

Jim Maynard, K7KK

At 2011-04-02 05:14 AM (Pacif Daylight Savings Time), Bob Naumann wrote:
> Jim,
>
> You said:
>
> " 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."
>
> The neutral or white wire in USA residential wiring is never "cut" by a
> circuit breaker.
>
> Both neutral and protective ground circuits are continuous and tie together
> only in the main circuit breaker panel (typically) and neither are "cut" by
> a circuit breaker.
>
> In the case of normal residential 120/240v single phase circuit panels, both
> "hot wires" are indeed broken by a "Main" 2-pole circuit breaker. 240v
> branch circuits (2 hots) also use 2-pole breakers. In 120v branch circuits,
> only the single "hot" wire(black) is "cut" by a circuit breaker.
>
> 73,
>
> Bob W5OV
> (retired electrician)
>