David Vogel <dbv_au@...>
Reading the great advice from Kent (If it still reads -750mv, you may have a DC current leak on your boat) reminded me of some other interesting tips I've found out recently whilst studying this topic . . .
Test the galvanic potential of the silver reference anode in both AC and DC volts - if there is any AC reading (shown by any Hz reading on a clamp-style Digital Multi-Meter - DMM), then this indicates that there could be a problem with AC current leaking into the water nearby and affecting our boat - if your own installation is up to scratch, then it could be a shore-power problem on a neighboring boat. Better to know than not.
Another way to test for STRAY CURRENT coming on-board via the shore-power connection, is to take a DMM and to set it to DC AMPs, and then clamp your shore-power cable. If everything is as it should be, there will be no net current flow - the AC current flow via the active wire is cancelled out via the AC current flowing via the neutral wire - the resultant reading should be zero. If however there is any DC current flow, this tells us that some current is 'leaking', presumably from the shore-power supply to the water, via our boat and its zincs, grounding plane, or other immersed metal bits. To help locate the source, which may be a neighboring boat, simply clamp their shore-power cords in the same way ... non-intrusive testing like this may find the culprit.
In the same way, clamping our boat's green bonding wire at various locations with the DMM set to measure DC AMPs, can show stray current travelling via the bonding system, and may help locate the problem area. Switching between DC and AC AMPs whilst clamping can also help identify the source. Generally AC current (Hz on the DMM) indicates shore-power, but can also point to bad diodes in the alternator, dirty current from battery chargers, leaking AC appliances, or power cord leaks.
DC current flows point to the boat's DC system and equipment. Do the same for any single or bundle of duplex wires - it should always read zero. If there is an current leak, then this will show up via an amp reading - searching on single circuits (pairs of wires) will help locate the source and possible problem areas.
Similarly, clamp the positive and negative leads from/to the battery bank. These should read the same (but in a reverse sense). If not, then it would be prudent to start to track down the difference, which is a potentially boat-destroying, or at least very expensive, current leak.
Hope this helps,
(not yet an owner)