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Why does my shore power breaker trip?

2018 November 19

Dear Boat Doctor,

Every couple days the shore power breaker on the dock blows when my boat is connected to it. This started happening when the marina replaced the power pedestals. How do I know if this is a problem on my boat or with the marina’s system?

Rich Lewinski

Rockport, Maine

Dear Rich,

In the last few years regulations on electrical systems have changed, with the intent of preventing stray current-induced drowning in marinas. The new systems significantly improve safety, but can make life difficult for boaters. In some cases, the system is preventing a dangerous situation, in others it just causes nuisance trips.

Your marina likely added a ground fault circuit interrupter in the shore power pedestal. In a proper electrical circuit, current is supplied on the hot conductor and returned on the neutral. If current returned is not the same as the current supplied, the interrupter will open the circuit, which looks like a “blown” breaker. 

The first step in diagnosing the problem is to understand what component is actually causing the problem. You can isolate the problem by turning all of your AC systems off and systematically turning them on.

Start out by connecting the shore power cable with all the boat and shore side AC breakers off, including your inverter.

Next, turn on the shore side breaker. If it blows, you know you have a problem with the cable or cable adapters.

Next, switch on the inverter. If the shore breaker trips, you know the problem is in the inverter.

Then systematically turn on each AC breaker, checking the shore breaker after each one. If the shore breaker trips, the last circuit energized is causing the problem.

Inverters appear to be the most likely culprit in nuisance breaker trips. A marine inverter connects the boats neutral to ground when it is supplying current, but separates them when passing shore power through. As the inverter senses shore power it may pass a bit of current before separating the ground and neutral, this will appear as leakage to the shore power breaker. To prevent this from occurring, be sure that all AC breakers are turned off before powering up the inverter. Allow 30 seconds to a minute for the inverter to start up and synchronize before closing any AC circuit breakers.

Non-marine inverters or incorrectly installed units can have neutral to ground connections permanently. Resolving this may require rewiring or in some cases replacement of the unit.

After testing, if you appear to have a circuit that actually leaks current, be sure to contact an ABYC-certified electrician for assistance. You may have a dangerous situation on yours hands. Leaking current is dangerous to swimmers, and can cause significant electrolysis damage to your boat. The leakage could also cause damage to the boat electrically and be a danger to people on the boat.