There are all kinds of reasons for this issue. There is test equipment available thru Blue Sea but it is only distributed to ABYC electrical certified technicians. An inductive ammeter that can test for the current leak .004 amps will work but the meters are expensive.
I had this issue from day one after purchasing our C26. I found that when the boat was built the technician failed to tighten the wire connectors at the shore plug receptacle. I could plug in fine but when I started to use power from different 120V devises it would trip the ELCI. The loose connection would cause arching when a larger amp load was applied. When I checked the connections during my troubleshooting I found the connections burned. I replaced the connectors and receptical and this fixed the issue. Before I troubleshot the issue I had the Cutwater dealer troubleshoot the issue (warranty) and he kept telling me I'm not plugging it in right and all I have to do is reset the breaker??? Make sure if you have a technician troubleshoot the problem he is ABYC trained electrician. Many dealerships do not employ ABYC trained technicians.
It could be your inverter not switching properly and leaving the ground and neutral connected
It could be a house hold devise that you are plugging into an outlet that is not wired properly.
Rather than going through a list I found this portion of an article from Passagemaker. It is a good informative article.
Likely Culprits
Let’s look at the most common situations that create neutral-to-ground leaks:
Household appliances can come with a direct neutral/ground connection. Houses don’t have the same safety issues as boats. If you are installing a household appliance (refrigerator, washing machine, etc.) on your boat, it must be checked for a neutral ground connection and then the connecting point must be identified and disconnected. An ohmmeter can be used to check for any continuity between neutral and ground on the unplugged appliance.
Inverters provide an abundance of potential problems. If the industry presented an award for the device most likely to be improperly installed, inverters would win and there wouldn’t be a second place. First and foremost, the inverter must be marine rated. Earlier we talked about the need to connect the neutral and grounding wires at the source of power, but not on the boat. Inverters (generators and isolation transformers) become the source of power when producing AC power from the batteries and must have a neutral-to-ground connection.
Inverters must have a way to make that connection when in use and to disconnect it when shorepower is present. The inverter senses the presence of shorepower and then goes through a synchronization process that switches over to shorepower and breaks the neutral-to-ground connection. Some AC circuits might be in use during this switching process, and that can cause a momentary shot of amperage from neutral to ground, tripping the ground fault device. If this problem occurs on your boat, try turning off all AC loads when plugging in (you should do this step regardless), allow the inverter a full minute to go through its switching sequence, and then turn the loads on.
Electric water heaters can create leaks as the insulation on the element breaks down. AC current leaks into the water and to the case of the heater and finds its way into the grounding circuit. Exterior leaks can also create problems as moisture or corrosion on the electrical connections creates a path to ground.
Air conditioning compressors contain insulated electrical windings. Over time the insulation breaks down and can lead to electrical leaks. If turning the air conditioning on trips the ground fault device, you might need a new compressor.
Shorepower cords and shorepower inlets can suffer from exposure to salt water. Corrosion can build up, providing a path for leaks between neutral and ground. Inspect the ends of the cords the receptacle/inlet for green crusty buildup, discoloration from excessive heat, and pitting from arcing.
Incorrect wiring by untrained technicians or boat owners often leads to neutral-to-ground connections. Aftermarket products that are not intended for marine use and/or are improperly installed can lead to trouble.
Solutions
The onboard ground fault issue can be resolved through proper installation of an isolation transformer. Unfortunately, that option is not practical for most cruisers due to cost and space requirements. If your boat has 50A/220V AC shorepower and does not have an isolation transformer, now would be a good time. For everyone else we can expect a few more years of transition as the industry responds to evolving codes and standards. If you have not run into this problem yet and have a trip planned, consider hiring a marine electrician to give your boat a shorepower health check. The money will be well spent, resulting in a safer boat and one that can travel and still find access to shorepower.
