Advanced Marine Electrics and Electronics Troubleshooting A Manual for Boatowners and Marine Technicians

level it takes to trip a conventional circuit
breaker, nominally rated at 15 or 30 amps, but
more than enough to be lethal.
How much is lethal? Table 10-1 describes
the typical effects of exposure to DC and
AC current on healthy humans (individual
responses will vary). Note that these figures are
not in amps, but in milliamps!

Using a Clamp-On AC Leak Tester

To protect against this potentially fatal prob-
lem, we need to accurately measure very low
levels of AC current leakage, in resolutions of
milliamps. The amp clamp provided with the
typical multimeter does not have the required
level of resolution; typically they can only read
down to a minimum of about 0.1 amp. A spe-
cial tool is required, namely, a clamp-on AC
leak tester, which can read down to 0.001 amp.
I use a Yokogawa clamp-on leak tester,
model 300-31, for which I paid just over
$400. Similar devices are available from other
suppliers, but I like this one because its clamp
has a 40 mm inside diameter, which will fit
around the largest shore-power cords.
Before you begin using a milliamp AC leak
tester, it is essential to understand that you will
now be measuring amperage in a way that may
seem dead wrong. Years ago, we were taught to
measure amperage by connecting the meter in
series with the load, sometimes using a shunt.
We also learned that we could check only one
wire at a time. Later, as inductive amp clamps

testing for residual current 95

became popular, we learned that you could

only measure amperage by clamping one wire

at a time.

Now, however, you will initially clamp your

meter around a boat’s shore-power cord to

check residual current. What’s being measured

here? It’s the differential between what’s going

down the AC hot wire and what’s coming back

up the neutral conductor. This differential is

current leaking into the grounding conductor,

measured in amps. If the reading is zero amps,

then no leakage exists, and the hot and neutral

are balanced perfectly, as they should be in a

perfect situation. But if there is measurable

current, the question becomes, where is this

Model 300-31, Yokogawa, http://www.yokogawa.com

Model 380942, Extech Instruments, http://www.extech.com

CLAMP-ON LEAK TESTERS

The Yokogawa model 300-31 AC leak tester.