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Old 05-09-2007, 17:22   #1
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AC Mystery

I've just finished adding some wiring that enables me to switch AC power coming to the boat between shore, an inverter, or a small portable Honda generator on the aft deck. The power then goes through the AC panel to the various outlets and AC-powered equipment as before. Of course, I have to make sure that I don't do something stupid like running the battery charger from the inverter or the water heater from anything but shore power. This arrangements works well when powering boat AC from shore or the small Honda, but powering it from the inverter isn't working right and has me baffled.
Here's what happens: After tuning on the inverter, the "reverse polarity" indicator lights up, then if you switch on the main breaker it immediately trips. (Yes, i've checked oll the wiring to make sure that hot and neutral weren't switched somewhere, and also tried purposely switching hot and neutral to see if it would make a difference - it did not)
Then I took a closer look at the AC panel and found that the main breaker is a 3-pole - 1 for the hot, 1 for the neutral (white), but the third one connects the ground bus-bar (green wires) to the neutral (white). I thought that on a boat, the green and white wires are supposed to remain separated and are only connected at the source - either the marina's distribution panel, or at the genset, etc. I experimented with removing the wire that connects green to white through the main breaker third pole, and then the inverter powers the panel as expected without tripping the breaker, but the reverse polarity indicator still lights up. The reverse polarity indicator is connected between the neutral (white) and the ground (green) bus. Although I could get the panel to work from the inverter by leaving this wire disconnected, I don't want to do that unless I understand why it was there in the first place.
Csn anyone give me some insight into this mystery?

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Old 05-09-2007, 19:12   #2
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ground-to-neutral etc.

Your 3-pole switch is there for the case when switched off of shore power there might be a generator present AND a separate GEN/shore switch exists IN ADDITION to the main breaker. The dual functions are built in to that triple pole breaker that you have.

For your situation eliminate the third pole that connects neutral to ground and add a separate two-pole (if you already do not have one) that allows the generator to be switched in or out (assuming that the generator already has a bond between neutral and the green wire "ground").

Often reverse polarity indicators will illiminate in a dim fashion when a pulse-type inverter is the power source. To verity this pay particular attention to the level of illumination when the source of power is shore versus the inverter. If you notice that when the shore power present illuminates the reverse-polarity indicator (when you have purposefully reversed the neutral and hot for this test only) MUCH greater than does the inverter then you can ignore the faint indication when the iverter is working. You may also notice that when the inverter is operating with a heavy load that the reverse-polarity indicator even gets dimmer (depending also upon other factors).

How is it going so far?

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Old 05-09-2007, 19:31   #3
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I agree with Rick.

An inverter (even a sine wave) will put out enough high frequency noise to cause rather dim illumination of such lights.

Steve B.
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Old 05-09-2007, 20:19   #4
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Thanks for the reply Rick and Steve.
I had thought of that, but the light seemed pretty bright. Have just tried swapping hot and neutral from shore power, and the reverse polarity light shines with the same intensity as with the inverter.
I couldn't find any three position, 2-pole switches I felt were suitable except for rotary type, so I used a less elegant and much cheaper solution: mounted three 3-conductor twist-lock outlets which connect to shore, inverter, and genset, and put a plug on the cable going to the AC Panel which gets plugged into the appropriate source..
A closer look at that 3-pole main breaker reveals that 2 of the poles have labels indication I = 30, V = 250, HZ = 50/60, D = 62, TA = 37.5 I suppose these refer to current, voltage, freq, D?, and trip-amps. But the third pole (connecting white to green) indicates I = SP, V = 65, HZ = 50/60, D = 60, TV = 65AC. It also has a resistance of about 1250 Ohm in the closed position whereas the other two meassure almost no resistance in the closed position like they should. Still scratching my head!
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