Battery-potential stray current is another
matter, and it can cause profound metal corro-
sion in as little as 24 hours! If rapid corrosion
is noted and confirmed, it’s important to take
quick action to prevent further damage to
through-hulls and other bonded underwater
metals. This procedure is best done with a
helper, as jumping in and out of the bilge can
get a bit tiresome.
Identifying Leakage Current
with a DC Amp Clamp
To detect the low-level leakage resulting from
bonding and grounding systems, you need a
high-resolution DC amp clamp, such as the
Extech 380942. If you can’t measure down to
0.1 mA AC and 1.0 mA DC, you may not be
able to detect leakage current that is strong
enough to cause profound corrosion and, as
discussed earlier, be potentially lethal to swim-
mers around the boat.
If you suspect that DC stray current may
be causing problems on board your boat, find
the source using process of elimination (see
illustration page 118):
1 Start the test with the engine off.
2 Attach the amp clamp around the bond-
ing conductor.
3 Activate the circuits on the DC panel-
board one at a time.
4 If you suddenly get a reading on the amp
clamp, you have identified the problem
circuit.
a basic corrosion survey 117The high-resolution Extech 380942 amp clamp can mea-
sure down to 0.1 mA AC and 1.0 mA DC.Model 380942, Extech Instruments, http://www.extech.com
Model 100-LCD-200-3/4''Sea Clip, Swain Meter Company, http://www.swainmeter.comHIGH-RESOLUTION DC AMP CLAMPS5 Use a TDR to pinpoint the location of the
leak (Chapter 3). The TDR is the best
tool to find the location of a short circuit
to ground within a circuit.
6 Repeat the test with the engine running
to check a faulty alternator or starter
motor.Identifying Leakage Current
with a DVOM
It is also important to check the bonding sys-
tem for current flow at several locations. It is
possible for leakage current from different loca-
tions, such as bilge pumps and float switches,
to “dump” itself out through the bottom of the