galvanic isolators can be used to effectively
block galvanic currents up to about 1.4 V. By
putting two diodes in series, you can block 1.4
VDC. Galvanic isolators with capacitors are
designed to offer extra protection for the
diodes in the event of a continual, low-level AC
leakage current running through the grounding
wire. This is not uncommon, as discussed in
Chapter 10. AC passes through a capacitor; DC
will not. The capacitor provides a path for any
AC leakage, or fault current for that matter, to
bypass the diodes, but still effectively blocks
DC current flow. When we discuss galvanic
current, we are referring to direct current.Tracking Stray Current
So far we’ve only touched briefly on stray cur-
rent, and evidence of its existence is based on
various hull or dock potential readings with
the silver/silver chloride reference electrode
(excessively high negative meter readings can
indicate DC stray current). Stray current is
what I like to refer to as battery-level voltage
and current, as compared to the galvanic current
we have been talking about up to this point—
voltages less than 1.4 VDC.116 electrical systems troubleshooting
an open circuit and immediate replacement is
needed.
Step 3. Determine if a short circuit to
ground exists:1 Connect one meter lead to either of the
green wire terminals and the other lead to
the isolator case (you may need to scrape a
bit of paint off the surface to acquire a
good connection).
2 Any continuity reading indicates a short
circuit to ground; replace the isolator
immediately.
3 A large variation in the comparative read-
ings on the capacitor charge would also
indicate replacement, but I have never
seen this happen.Due to the inherent voltage drop that occurs
across a diode—typically 0.6 V to 0.7 V—Testing the continuity in the opposite direction. First “flash” the capacitor by
shorting it from one side to the other with a jumper lead, and reverse the
DVOM leads. Again the meter reading will rise gradually as the capacitor
charges. Once the meter reading stabilizes, compare it to the first reading.Testing for a diode short circuit to ground. Connect one
meter lead to either of the green wire terminals entering
the device and the other meter lead to the case of the iso-
lator. Continuity indicates a short circuit, and the isolator
must be replaced.
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