Why Can't I Just Plug My Boat into the Dock?

First published 10/26/17
By guest blogger Fred Kaufhold, of M/V Dreamweaver, â€‹Anacortes, Washington. This post is an addendum to "Ground Your Boat for Swimmers' Safety."I thought my isolation transformer would be enough.
   Recently, while cruising our boat in SE Alaska, we entered Petersburg where we intended to stay for two months for various reasons not germane to this story.  Because of our extended stay, we wanted to be in the north harbor which is in the center of town rather than the alternative south harbor.  After receiving our slip assignment, I dutifully plugged my shore power cable into the impressive new power pedestal on the dock, turned on my own shore power breaker on the boat and promptly tripped the dock breaker.  

The culprit: my inverter​   Now, I am aware of other cruisers experiencing this problem at the north harbor in Petersburg in the recent past, but my boat is of rather recent vintage and is equipped with all the required electrical safety gizmos like isolation transformer, etc. so I thought it would not be a problem for me.  Not so fast.
   The harbormaster, a friendly and helpful soul, suggested I call the local electrician who has dealt with similar problems in the past.  I decided to call my home port mechanic instead and he enlightened me on the subject.  He said that sometimes the problem is with the inverter and how it is used.    Here's why the dock breaker tripped:​It turns out that when the inverter is turned on while underway, away from shore power connections, it correctly connects the ground and neutral wires together which is fine in the absence of external alternating current (AC) input.  When it senses the presence of an external AC source, such as shore power from the dock, it disconnects this neutral/ground bridge and all is well.  Unfortunately, in newer marinas or those recently upgraded, the ground fault interrupter (GFI) circuits on the dock are much more sensitive and react much quicker than some inverters can disconnect the ground/neutral bridge.  This results in the dock breaker tripping.
Fortunately, there was an easy solution.
​​   The solution is simple and free.  Before plugging into the dock, I turned my inverter off in addition to all the other AC loads which I turn off at the AC panel.  After connecting to the dock outlet, I slowly turn on the master AC breaker and then individual loads as required.  Turning on the breaker for the installed AC outlets may also cause the dock breaker to trip if the boat outlets are GFI equipped.  Some experimenting will usually result in a solid connection between the boat and the dock shore power.  In my case this resulted in a fully powered and pleasant stay in the desired marina.
   For a more detailed and informative dissertation on the subject, check out the Wagonner Cruising Guide blog post "Don’t Get Tripped-Up by New Shore Power – Updated."​
​   Also BoatUS's "Docks with Bad Wiring Continue to Prove Deadly." ​  

Fred Kaufhold and his wife Stephanie have traveled extensively aboard their Dreamweavers. â€‹Their first Dreamweaver was a hardy blue-water cruising sailboat they sailed from Europe to the US. Their second Dreamweaver is a Mariner 35 upon which they and their pup Brigantina (Tina) recently cruised to Southeast Alaska. The Kaufholds belong to Fidalgo Yacht Club, an organization that prides itself in safely and adventurously cruising the Pacific Northwest's Inside Passage, from Olympia, Washington, to Glacier Bay, Alaska.  

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