The reason is, if the grounds were isolated and a short circuit were to develop between AC HOT and the boats DC system, DC ground or bonding system, there would not be enough fault current to trip the AC breaker on the dock. The two grounds should be tied together as recommended by the U.S. Coast Guard and the ABYC (American Boat and Yacht Council). This practice, however, raises safety concerns. If it was intentional, it was done to prevent galvanic corrosion while connected to shorepower. The danger of isolated AC and DC grounds In the electrical system on many boats, AC ground is intentionally or accidentally isolated from DC ground (DC negative). In fact it can occur on any boat as soon as a metallic part (the shaft and propeller) is in contact with water.Galvanic corrosion will quickly dissolve your sacrificial anodes, and attack the shaft, propeller and other metal parts in contact with water as soon as the boat is connected to the shore-side supply. It might therefore be tempting not to connect the ground conductor: this is however extremely dangerous because GFCI’s will not work nor will a fuse blow in case of a short circuit to a metal part on the boat. It is a misunderstanding that galvanic corrosion occurs only in metal and aluminium hulls. The rate of galvanic corrosion is a function of several variables including area ratios, conductivity of the fluid, temperature, nature of the materials, etc. In general, the more active alloy of the couple corrodes preferentially while the less active (more noble) material is cathodically protected. Seawater and, to a lesser extent, fresh water are such fluids. Galvanic corrosion occurs when two dissimilar metals in electrical contact are simultaneously exposed to an electrically conducting fluid. Bringing only the live and neutral wire on board results in an unsafe situation because GFCI’s will not work nor will a fuse blow in case of a short circuit to a metal part on the boat.
Connecting the ground wire of the shore-side supply to the metal parts of the boat will result in galvanic corrosion (see below). A fuse will blow or a GFCI (Ground Fault Current Interrupter) will trip in case of a short circuit or current leakage to ground. Safety is taken for granted in case of a normal on-shore installation. The isolation transformer eliminates any electrical continuity between AC shore power and the boat. It is essential for safety and eliminates the need for galvanic isolators and polarity alarms
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