Shore power GFI blows

[SolarCat]

Hi, I recently installed a new 12,000 btu Mermaid AC unit in my Gemini 3400. I've been to four different marinas since and haven't had an issue. At my new marina in FL they have replaced all of the circuit breakers breakers with the GFI style because of code. I can run every device on my boat and all outlets have GFI. I've tested all outlets for proper wiring. When I turn on the AC it will run for a minute then blow the shore power GFI. I also tried a different power cord and same result. A fellow down from me had the same problem and he replaced the breaker with a non-GFI and I don't have any issues after plugging into his. Sooooo are there any ideas from people out there that have solved a similar problem? The thing that comes to my mind is a bad ground somewhere if it is on my boat or the marina? They did have some problems and rewired everything in the past. What is the easiest way to determine the problem???

Also, a guy near me is having Zinc issues.

Thanks ahead for any input.

[CharlieJ]

First off, your problem is not a "bad ground". GFIs operate by measuring the current going to the vessel on the L (hot or hots) and returning on the N (neutral) and will trip if there is a mismatch that exceeds the trip point.

What is the trip level of the GFI? GFIs typically have a trip level of 6mA, which is a tough standard to meet for the whole vessel. The ABYC recently adopted a standard for an Electric Leakage Current Interrupter (a whole vessel GFCI, on steroids) with a trip level of 30mA and 100mS.

If the GFI is operating correctly (even new units have been known to fail out of the box), then it is tripping because you have leakage from N > G in the air conditioner. To prove this, put a clamp meter around the shore power cord with the air conditioner off. The reading should be very close to zero. Turn on the air conditioner while observing the clamp meter. You will probably observe a reading that exceeds the trip level of the GFI.

Check the electrical installation of the air conditioning unit to ensure that the electrical connections are correct. Ensure that there is no N > G bond in the unit. If these checks are ok, then it is probably faulty insulation in the compressor motor.

[tuberider]

Are you saying that the 30amp shore power receptacle is a GFI? When you clamp the amp probe over the shore power cord are you looking for a reading in the 60Ma range to determine if you have a problem on your boat.

[Andina Marie]

Reading zero with a clamp on meter doesn't test for leakage to ground. The meter is measuring all the current going one direction and subtracting all the current going in the other direction. If there is leakage between Active or Neutral to ground on the boat and some of the current is coming back in the ground conductor it is still going back through the meter and the meter doesn't know or care WHICH conductor it is in. However the imbalance in the active and neutral will trip the GFI since the GFI is doing the same thing as your clamp on meter but ONLY on the active and neutral.

However if there is leakage to ground and sufficient voltage on the underwater metal to cause some current to leak, then theoretically the clamp on meter would read that.

First check is to measure resistance between the active cables and the ground cables of the A/C which should hopefully be greater than 100,000 ohms. This can only be done with it unplugged and doesn't necessarily check everything since it is not running.

[CharlieJ]

Reading zero with a clamp on meter doesn't test for leakage to ground.

It does, but, as with everything electrical, there are caveats.

The meter is measuring all the current going one direction and subtracting all the current going in the other direction.

Correct.

If there is leakage between Active or Neutral to ground on the boat and some of the current is coming back in the ground conductor it is still going back through the meter and the meter doesn't know or care WHICH conductor it is in.

(Emphasis added.)

Here is where it gets sticky. We are most interested, from a safety standpoint, in any leakage current that is returning to ground by other than the correct path which is through the neutral. This is the fundamental design concept for GFIs, GFCIs, RCDs, and ELCIs. Further, we are especially interested in any current that is returning to ground via the water path as this can be moderately dangerous to people in the water in saltwater and down right deadly to them if they are in fresh water.

However if there is leakage to ground and sufficient voltage on the underwater metal to cause some current to leak, then theoretically the clamp on meter would read that.

(Emphasis added.)

There is absolutely nothing theoretical about this test. It is a common test that is performed by any marine electrician performing an electrical or a corrosion survey. It is valid and it provides important results.

[Dsanduril]

I'm going to guess that the AC has either a start capacitor, a run capacitor, or both on the main motor. Capacitors, even new ones, can be terrible sources of leakage currents. And the older they get the worse they get.

There are a couple of options; you can try to install a new capacitor (but you indicate the AC is new, so presumably so is any capacitor), or you can try and insulate the capacitor body from ground. That can be as simple as a couple of wraps of electrical tape around the body where the mounting strap goes around it. Of course, that presents a safety issue if the capacitor is leaky, but it will tell you where the problem lies.

From a more general perspective, here's some info on leakage currents and testing using a clamp meter:

http://www.testequipmentdepot.com/fl...0_appnotes.pdf

or this on how GFIs work (including a note that RV AC units cause a lot of nuisance trips - seems pretty similar to a boat AC unit):

http://personal.cha.bellsouth.net/j/...les/rv/gfi.pdf