SEAWORTHY
The BoatU.S. Marine Insurance and Damage Avoidance Report*
When Your Shore Power
Loses Its Cool
ot long ago, Kevin Ritz, an ABYC master technician
and author of the Seaworthy story “A Preventable
Dockside Tragedy” (October 2009), walked down the
docks at a local marina with an infrared camera and
snapped some shots of shore power cords. The pictures
he took (see page 12) are sobering; the camera shows
significant heat buildup that the naked eye can’t see.
N
Shore power cords have a tough life; they spend months
out in the elements, get tossed in lockers, and are
pulled on, stepped on and sometimes dropped in the
water. Despite the “abuse,” power cords are expected
to carry current to the refrigerator, water heater, lights,
battery charger, and maybe an air conditioner or heater.
Not surprisingly, shore power cords are occasionally
damaged, giving them the potential to overheat and
start a fire. Seaworthy combed the claim files to find out
how to avoid getting burned by shore power problems.
The Crux of the Problem:
Resistance (Impedence)
Plug a heater into a typical household extension cord
and there isn’t much chance you’ll have a problem. Now
plug two heaters into that same cord and suddenly you
have the potential for a
big problem. The cord
is designed to carry
a maximum amount
of current based on
the size of the wires
(about enough to
run one heater) and
if the current limit is
exceeded, the wires
begin to get hot. The
reason is resistance.
Resistance is like
In This Issue . . .
MAYDAY PROTOCOL . . . . . . . . . . . . . . . . . 6
PREDICTING SURGE . . . . . . . . . . . . . . . . . 8
COPING WITH SURGE . . . . . . . . . . . . . 10
BOATS AND BIRDS . . . . . . . . . . . . . . . 16
*The BoatU.S. Damage Avoidance Program is
dedicated to helping you enjoy accident-free boating.
Seaworthy looks at real claims and how they might
have been avoided. Material in Seaworthy may be
reprinted with credit to “Seaworthy, the BoatU.S.
Marine Insurance and Damage Avoidance Report.”
According to a study done by Seaworthy a few years ago,
11 percent of boat fires are caused by the AC system and
most of those are due to cord problems. Knowing where
the most common problems are and taking some time to
inspect your setup can prevent a catastrophe.
friction—run too much current through a wire and the
“friction” will create heat. It is increased by using smaller
wires or running higher current through the wire.
Conversely, resistance can be reduced by either running
less current through a wire, or using a larger wire. Too
much resistance in a shore power cord can cause the
cord or connections to melt or even ignite.
Question: Why doesn’t the shore power pedestal just
trip the breaker before a damaged cord overheats?
The answer is that the breaker will only trip when its
cutoff current—30 amps for standard cords—has been
exceeded. But if some of the strands of wire in the cord
are severed, the cord is effectively smaller and it may
only be able to safely carry, say, 20 amps. If you were to
then run 30 amps through the cord, resistance will cause
the remaining wires to get smoking hot well before the
breaker trips. According to Bill Drake, vice president of
Continued on page 12
Vol. 28 No. 3 ••••••••••••July 2010
