current going, or, for that matter, where is it
coming from? That’s the tricky part, because
the answer to the question is, it depends...
Electrical theory tells the electrician that
current will always follow the path of least
resistance, which is essentially true. Therein lies
the problem. With a boat plugged into shore
power, there are multiple paths for residual cur-
rent to travel. In a perfect world, residual cur-
rent will travel down the grounding conductor
and back to the power source somewhere at the
head of the dock. Unfortunately, it is not a per-
fect world, and dock wiring problems are ram-
pant. We cannot rely on the integrity of the
dock’s grounding system (which is why I use a
SureTest device to help confirm or deny this
integrity—see Chapter 4).Tracing the Source of Residual Current
Tracing the exact source of residual current is,
unfortunately, a complex process. It requires
process of elimination, which can be exacting,
tedious, and at times extremely inconvenient.96 electrical systems troubleshooting
Use a milliamp AC leak tester to check residual current by clamping it around the shore-power
cord. This reading of 6.62 mA is fairly low, but still more than desired. I prefer to see readings
of 3.0 mA or less.I say inconvenient because the possibility
exists that another boat may be the source of
the current, which is using the superior
grounding (compared to the dock) of your
boat as the quickest path to the water. If that is
the case, then some of that current is surely
leaking out through the bottom of your boat.
The potential for current to leak into the
water is, ironically, related to ABYC standards,
which call for the boat’s AC and DC ground-
ing systems to be linked at the boat’s ground-
ing bus or at the engine negative terminal. If
the AC grounding system on the dock has
excessive resistance, some or all of the residual
current will exit the boat right through the
bonded through-hull fittings as it seeks a path
to earth. In other countries, this bonding of
the AC and DC grounding systems is not a
common practice, nor is it required under
international standards. The reason is that
they employ the 30 mA whole-boat protection
system, which will simply shut down the boat
if the residual current hits 30 mA.