Shore Power Problem - Ground Fault or Other Issue ?

[SailFastTri]

Troubleshooting advice needed -
My shorepower AC panel has built-in GFCI. When I plug in any AC load the power to the outlets and square green indicator light on the panel cuts off. I can measure 120VAC using a multimeter, but it goes off only when there is a load. This happens on either circuit (water heater or batt. charger) whether or not the other is turned off. I think it's got to be something common to the shore power or master breaker... otherwise I should have been able to isolate the problem on one interior circuit.

I ordered a replacement master breaker RCD and installed it -- it behaves the same as the original one at left in the photo. So now I assume the master GFCI breaker is good.

Going back today to the boat to check resistance measurements between leads starting at the shore power cord (will measure disconnected and connected) and work my way into the interior. I will also check to be sure the boat yard power outlet is wired properly, as this wasn't a problem until after I hauled out for the season.

Any troubleshooting insights or suggestions would be appreciated.

[Redcoat]

Have you tried a new shore power cord? I almost bought a new inverter because of a bad connection in the cord. lucky for me I had a good neighbor in the slip next to me. He hooked his cord up and the problem was solved.

[Vasco]

Check the hot water heater. Even when not on it can cause a problem if the connections to the element are a bit dirty.

[zeehag]

did you twist the cord when you plugged it into the box on dock?? i found mine didnt work correctly until it was twisted to contact the ground. good luck may your problem be embarrassingly easy to repair. other wise, check all the connections on your negative bus bar.

[SailFastTri]

Originally Posted by Vasco

Check the hot water heater. Even when not on it can cause a problem if the connections to the element are a bit dirty.

None of the loads can be causing this. The problem occurs on either interior circuit with the other interior circuit breaker turned off. I have tried plugging in various types of loads on either circuit. e.g. an electric heater (on low setting) or the battery charger.

[SailFastTri]

Originally Posted by zeehag

did you twist the cord when you plugged it into the box on dock?? i found mine didnt work correctly until it was twisted to contact the ground. good luck may your problem be embarrassingly easy to repair. other wise, check all the connections on your negative bus bar.

Will test shore power later today, and report results.

[DotDun]

Check the inside of the breaker box to make sure neutral and ground are NOT connected together!

[zeehag]

i lost my ground one day when workers were working on the dock power and water---i had to go out and twist my power cord plug back into the socket, as is spozed to be done-- the workers merely plugged it--didnt turn it-- and my ground was gone. easily fixed. good thin gi have a monitoring system for ground problems.

[SailFastTri]

Update:
I tested for opens and shorts, everything as it should be. Measurements between hot and ground, neutral and ground, and neutral and hot all in the 2-4 Meg-ohms ranges. What was interesting though is my shore power cord showed about 110 ohms round-trip resistance with test clips shorting the prongs. With test clips shorting the prongs of the shore-power inlet the resistance measured at interior outlets was 0.0xx ohms(through the breakers and interior wiring). So I brought home the shore power cord and will re-wire the connectors (they're not the molded type, so I can open them and inspect/re-do). Will also clean all prongs and then see what happens when I bring it back to the boat.
Today the boatyard outlet was showing 121v at 60 cycle, I tested it for proper wiring (reversed leads, opens and ground-faults) and it checked OK. Last time I measured voltage at the interior outlets it was 117v but today I measured several different readings between 90v and 101v. I measured voltage on the shore power cord on the end where it connects to the boat and it still showed 120v, but as noted above it had high resistance. At this point I suspect a marginal connection or hot-spot in one of the cord connectors, or possibly on the prongs where it plugs into the boat.... a marginal connection might show a good voltage measurement with the multi-tester but drop when any load is on it. (Power still drops out when I plug in a battery charger, heater or fan.) If anyone has other thoughts on this I would appreciate input.

[FlyingCloud1937]

Originally Posted by SailFastTri

Update:
I tested for opens and shorts, everything as it should be. Measurements between hot and ground, neutral and ground, and neutral and hot all in the 2-4 Meg-ohms ranges. What was interesting though is my shore power cord showed about 110 ohms round-trip resistance with test clips shorting the prongs. With test clips shorting the prongs of the shore-power inlet the resistance measured at interior outlets was 0.0xx ohms(through the breakers and interior wiring). So I brought home the shore power cord and will re-wire the connectors (they're not the molded type, so I can open them and inspect/re-do). Will also clean all prongs and then see what happens when I bring it back to the boat.
Today the boatyard outlet was showing 121v at 60 cycle, I tested it for proper wiring (reversed leads, opens and ground-faults) and it checked OK. Last time I measured voltage at the interior outlets it was 117v but today I measured several different readings between 90v and 101v. I measured voltage on the shore power cord on the end where it connects to the boat and it still showed 120v, but as noted above it had high resistance. At this point I suspect a marginal connection or hot-spot in one of the cord connectors, or possibly on the prongs where it plugs into the boat.... a marginal connection might show a good voltage measurement with the multi-tester but drop when any load is on it. (Power still drops out when I plug in a battery charger, heater or fan.) If anyone has other thoughts on this I would appreciate input.

Yep...you have to measure under load...but be careful. The only way to measure V-drop is under load.

Lloyd

[senormechanico]

Betcha you find the high resistance in the neutral lead of the cord.
That would trip the GFI and also account for the voltage dropout with load.

[Tellie]

Agreed. Usually, other than dead shorts or a bad GFIC, GFIC tripping problems are often found in the neutral.

[SailFastTri]

Re-wired the shore power cable connectors. Now tests zero ohms round-trip with clip-on test leads shorting the prongs at one end. Will try it when I can get back to the boat.

[senormechanico]

Which one was loose?
Inquiring minds want to know !

[SailFastTri]

Originally Posted by senormechanico

Which one was loose?
Inquiring minds want to know !

Don't know. I cut off the ends and re-did the connections in the cable.

Another update -- I made a quick stop at the boat and plugged in with the re-done shore power cable. Everything stayed "up". I would say the problem is solved but it's not... I plugged in an electric heater (1500w) and measured the voltage in another outlet on the same circuit:

1) Fan only -- 121v
2) low heat -- 108v
3) high heat -- 105v

The good news is the heater didn't shut down, but now I'm thinking maybe the outlet in the boat yard is marginal (it is outdoors against a fence). Next time I go back I'll plug the heater right into the boat yard outlet (without going through my boat's shore power) and take measurements on another receptacle in the same quad outlet.