Jim,

Diagnosing this from afar will be imprecise at best.

Sounds like you may have a genuine ground fault leakage problem when connected to the genset.

So, rules review:

1. All current that leaves a source will return to that source.  You can think of this as electron/ion flow, or conventional current flow, or any way you want, but all current that leaves a source will find it's way back to that source.
2. While you would like to think that current will return in an orderly manner on its designated return circuit, the fact is, current is disorderly, and will divide itself up to take all available paths to return to it's source.
3. The neutral and the safety ground should only be connected together at the "derived source" of your AC power.
4. When operating on shore power, the neutral and the safety ground should NOT be connected together on the boat.  The derived source with shore power is ashore, not on the boat.
5. When operating on an inverter, the neutral and safety ground must be connected together at the inverter.  The inverter is then the derived source in that configuration.  This is usually accomplished by a relay internal to the inverter.
6. When operating on the generator, the neutral and the safety ground must be connected together at the frame of the genset.  The genset is then the derived source in that configuration.
7. To accomplish all this, the generator transfer switch must be of the "double-pole" (120V) or "multi-pole" (240V) type, and must break the hot lines AND THE NEUTRAL LINE.  If the generator transfer switch doesn't break the neutral, then the neutral and the safety ground will be connected together on the boat when the boat is connected to shore power.  This creates a violation of rule 2.

GFCI devices compare the amount of current coming into them from their source with the amount of current coming back through them on the way back to the source.  Residential GFCI breakers are sensitive, and trip if there is any difference is excess of 5ma and lasting longer than 7 ms.

So, in your case, I would look for one of two cases.  One, a cross connected neutral-safety ground when the genset is switched online.  If there is a cross connection there, that sets up a parallel path for current to take.  Since current will take all available paths, some current that would normally flow only in the neutral can now bypass that neutral and flow in the ground.  The cross connection could be in the monitoring and control circuits for the genset itself.  I AM ASSUMING HERE THAT THE GENSET IS BUILT IN TO THE BOAT AND MEETS UL MARINE SPECS FOR A MARINE GENSET.  IF THIS IS A HONDA PORTABLE, IT SHOULD NOT BE USED ON OR AROUND BOATS.  Two, look for something that draws it's power downstream of the GFCI (load side of the device) but returns power upstream.  That would have to be something that is only connected when the genset is operating, like something in a genset remote start/stop, control or monitoring circuit.

The cross connection could be in the transfer switch wiring.  It could be as simple as an indicator or reverse polarity light being wired to the safety ground instead of the neutral.  To be blowing the GFCI, one point in the cross connection will be down-stream of the GFCI, whether tapping off power which returns upstream of the GFCI, or bypassing the neutral.  It's probably not going to be something that's plugged in to an outlet.  it's going to be something that's permanently wired into the boat.

You mention that the boat has 2 x 120V/30A inlets, but you don't say how the genset is wired.  It can be wired as a 120V machine or as a 240V machine.  the generator transfer switching implications are different depending on how the machine is wired.

I suppose it's even possible for this to occur because of a funky AC waveform from the genset.  Unlikely.

You may find that tracking this down will involve instrumentation that only a paid marine electrician can afford to bring to bear.

Find it and fix it; don't bypass it.  Whatever it is, it does represent a potential safety issue.

Lots of speculation above; hope it helps.

Jim

Peg and Jim Healy aboard Sanctuary
Currently at Rock Creek, Pasadena, MD
http://gilwellbear.wordpress.com
Monk 36 Hull #132
MMSI #367042570
AGLCA #3767
MTOA #3436

Jim, Jim Healy thought of everything that I can think of.

If this started to happen AFTER you installed and hard wired in some type of domestic (made to be put into a home) appliance into the port train of outlets, and did not
reroute out the appliance's ground to the boat's ground, and the shell of the appliance to the the neutral, then,
(Jim Healy correct me if I have written this backwards,)
then I would do that first to see if this issue can be resolved.

Otherwise, take your time and verify that the neutral and safety ground, in Jim's parlance, are only connected at the generator, and no where else.

Look for the obvious, the stuff that changed / upgraded just before you had this problem.

And, I have to agree with Jim, this is something you do need to fix. I would NOT put this ion my to-do list for next year, but would put it on the list to get it done before next boating season.

For now, unless you can resolve the problem in a somewhat expeditious manner, I would avoid using the port side outlets at all, at least while on the generator. ( I personally would de-energize that line until I could resolve it, but my Admiral accuses me of being over reactive on lots of issues.)

Lee
Marmaris

On 08 Sep 13, at 15:43, Jim Healy wrote:

Jim,

....
3.  The neutral and the safety ground should only be connected together at the "derived source" of your AC power.
....
6.  When operating on the generator, the neutral and the safety ground must be connected together at the frame of the genset.  The genset is then the derived source in that configuration.
....
The cross connection could be in the transfer switch wiring.  It could be as simple as an indicator or reverse polarity light being wired to the safety ground instead of the neutral.  To be blowing the GFCI, one point in the cross connection will be down-stream of the GFCI, whether tapping off power which returns upstream of the GFCI, or bypassing the neutral.  It's probably not going to be something that's plugged in to an outlet.  it's going to be something that's permanently wired into the boat.
...