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Comparatively speaking, seawater is a pretty good conductor of electricity. So all the underwater exposed metal parts of your boat are essentially electrically connected to all the underwater exposed metal parts of any other boat or boats within a reasonable distance. (A conductor's ability to pass electricity is greatly reduced by distance, and that holds true whether its wire or water doing the conducting.) So think of these nearby boats as being in contact. It's somewhat akin to the example of dissimilar metals that I mentioned above.
 If you look closely, you'll see that this boat's owner has clipped a wire to the shroud base; the end in the water has a sacrificial zinc attached. This is a common practice among racing boats that aren't otherwise fitted with zincs to deter damage from electrolysis. | |
If you thought the corossion from dissimilar metals was easy to fix, the stray currents that threaten your boat's metal parts are even more easily handled. You've already taken the first step by bonding all of the metal components of the boat. Now, assuming that your electrical wiring is installed according to the National Electrical Code, the DC wiring is grounded to the bonded metal system created by connecting everything metal together. The AC wiring must also be bonded to this system to assure the proper operation of the overcurrent devices that protect crew and equipment aboard.
When shore power is connected to the vessel, the possibility of stray current electrolysis corrosion could exist if a ground or neutral faults to a current-carrying conductor ashore or on another boat, and ‘leaks' an amount of current too small to trip a breaker or blow a fuse. This tiny current flow may not be detected, and could exist for years with no damage or indication at the point of the fault. Electrical outlets that are wired incorrectly can create reverse-polarity, and this is commonly part of the problem. Such tiny current flows that lead to electrolysis may also be present when partial faults in the ground wires exist. This condition is avoided by doing three things:
 Unless it's properly protected and connected, the shore power cord plugged in on the left side of the dock can lead to problems for all of the nearby boats. | |
- Always check the polarity of shore power before connecting your vessel to the power. Check it separately, or install a polarity indicator on the line feeding your panel from the plug mounted on your vessel. Have any polarity discrepancy resolved before connecting and running your boat and its equipment.
- Install a properly wired ground plate or ground shoe on the exterior of your hull. The purpose of this component is to provide a reference point that connects the entire ground system to a single potential (the surrounding seawater). This ground plate is something that's required for proper lightning protection, so I'll offer more specifics on it when I cover that topic in a future article.
- Install a galvanic isolator in the ground wire between your vessel's panel and the shore-power plug on your boat. This is a device that will not allow the small destructive current to flow on your AC ground wire (and your bonded ground system), but will carry the large surge current of a fault, allowing the overcurrent protection device to open the circuit and protect your life and property.
We now have protected the vessel from corrosive destruction caused by chemically generating its own voltage (dissimilar metal corrosion) or conducting stray voltages at your marina. By implementing these fixes, you'll have a head start with lightning protection, which I'll discuss in my next article.
Onboard Electrical Guidelines You may want to further investigate the standards and guidelines for electrical wiring on boats, and if so here are a few sources that you can consult:1. NFPA 302 Fire Protection Standard for Pleasure and Commercial Motor Craft 2. NFPA 70 National Electrical Code 3. ABYC standards E-2,4,8,9, &10 www.abyc.com/Publications/Standards/descript.htm |
