properly wired, the fault current would
have been safely conducted to ground.
Why, then, is it important for the
aforementioned ground systems to
be interconnected? Take the above
scenario and substitute a raw-water
strainer for the microwave, and
imagine that a shorepower hot
conductor chafes against the strainer.
If the strainer is bonded and the
bonding system is connected to the
AC safety grounding system, as it
should be, the fault will be conducted
safely to ground, where it will trip an
In residential and commercial
installations, it’s common practice for
the AC safety grounding conductor
bus to be connected to the neutral bus.
While this practice is fine on land, it
most certainly does not work aboard
a boat, and it’s where many do-it-yourselfers and experienced land-based electricians make a potentially
fatal mistake when installing or
servicing marine electrical systems.
In a properly wired system, current
traveling aboard on the hot wire
returns solely through the white wire.
However, because a boat, unlike a
house, floats in a conductive medium,
interconnecting the green and white
wires aboard affords current that’s
returning to its source—in this case,
the transformer ashore—three returns
paths (rather than a single path): the
white wire, the now-connected green
wire, and the water surrounding the
boat (via bonded underwater metal
hardware such as the engine/propeller,
strainers, seacocks, and so forth).
Contrary to popular belief, electricity
takes all paths back to its source, not
just the path of least resistance. This
presents a clear hazard to swimmers,
and those aboard also may be at
risk, depending on the nature of the
It is vitally important, therefore, that
the connection between the AC safety
grounding wire and the neutral wire
occur only at the source of power that
is in use: at the transformer ashore
that is supplying power to your dock
shore cord, or at your generator or
inverter when and only when it is
supplying power to your boat.
If you have any doubts about the
aboard your boat or about your AC
safety grounding system, have it
checked out by an ABYC-certified
marine electrician.—Steve D’Antonio