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Old 05-08-2018, 19:47   #1
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AC outlet weird voltages

First, hello! I've been lurking the last year or so, and have learned a great deal from the collective minds on CF, so thank you to all right up front.

Hoping someone who knows AC better than I do can help me understand what's going on here.

Last fall got my new (to me) boat "refreshed" in the electrical department by a local marine electrical outfit. The system had been professionally wired some years ago with good components, but a few things were showing their age (old Freedom inverter, etc). Got some new gear installed (new inverter/charger, isolation transformer, battery monitor, new batteries, and a few 'wrong' wiring instances were corrected at the same time).

The topology is now: Dock AC power -> Charles isolation transformer -> BlueSea AC distribution panel. At the BlueSea panel, one breaker controls power to a Magnum MMS1012 1kw inverter / 50A charger. There are two AC outlets on the boat: one is the inverter's outlet, and the other is a 'normal' outlet wired to a breaker on the AC panel labelled "AC outlet".

The inverter outlet behaves as I would expect: 0 VAC when inverter is off and when shorepower is disconnected or when main AC breaker is flipped off, and 120 VAC when inverter is on, or if shorepower is connected and the inverter is passing through the shore AC.

The other outlet that is wired straight to the AC panel, however, behaves strangely (at least, *I* think it's strange?) These voltages all measured at the outlet:

0.002 VAC - shorepower cable disconnected

0.15 VAC - shorepower cable connected, but shorepower main AC breaker (before isolation transformer) OFF

17.8 VAC - main AC breaker ON, AC distribution panel main breaker ON, outlet breaker OFF

37.4 VAC - inverter/charger breaker ON, outlet breaker OFF

124.5 VAC - outlet breaker ON


Why am I seeing 17 VAC at the outlet when the outlet's breaker is OFF? Why am I seeing a jump to 37 VAC when the inverter's breaker is flipped ON, with the outlet's breaker still OFF? Should I be calling the marine electrician? Any clues appreciated.


Dave
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Old 05-08-2018, 20:32   #2
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Re: AC outlet weird voltages

Possibly the ground/neutral are bonded somewhere other than the automatic switch within the inverter.
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Old 05-08-2018, 22:09   #3
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Re: AC outlet weird voltages

If I understand correctly, I should have two neutral-ground bonds: one as a relay in the inverter, which kicks on when the unit is inverting, and the other at my isolation transformer, where the secondary winding's neutral and ground are bonded to the transformer case. I admit I still don't understand how I measure 18V between hot and neutral at the outlet when the breaker is off, nor why the voltage rises to 37V when power is supplied to the inverter (which would be in pass-through mode so no ground-neutral bond there).
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Old 05-08-2018, 23:09   #4
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Re: AC outlet weird voltages

my first guess would be a common neutral between shore side and inverter output. this is commonly done wrong. the transformer output, inverter input, and ac outlet should be on a separate neutral bus then the inverter output and inverter outlet.

you should measure between all 3 wires to help figure out whats going on.

hot - neutral
hot - ground
neutrel - ground.

for all of your situations.
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Old 06-08-2018, 08:36   #5
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Re: AC outlet weird voltages

I will do that. Time to run through all the tests in Calder's book...
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Old 06-08-2018, 09:19   #6
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Re: AC outlet weird voltages

Quote:
Originally Posted by codger View Post
Should I be calling the marine electrician?
Yes .
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Old 06-08-2018, 09:23   #7
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Re: AC outlet weird voltages

With no load and a meter with a very high input impedance, all it takes to read a bit of voltage is a bit of salty dust across the breakers and wiring.
If you measured the "off" settings with your fingers across the meter leads, you would probably measure zero volts or very close to it.
Don't do that with all the switches on !
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Old 06-08-2018, 12:34   #8
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Re: AC outlet weird voltages

Yup, time to call the electrician...

Opened up the isolation transformer panel (Charles ISO-G2). The shore side is as expected (120 VAC on hot-neutral and hot-ground, 0 VAC on neutral-ground), but on the boat side of the transformer, I see hot-neutral at 124 VAC, hot-ground at 71 VAC, and neutral-ground at 54 VAC.

At the AC distribution panel, everything looks normal - the AC ground from the transformer is connected to the AC ground bus, which is bonded with the DC ground bus. Neutral is connected to the neutral pole of the double bole breaker and hot to the hot pole. With the AC input wires disconnected there is no continuity between neutral and ground.

Is this possibly a fault in the isolation transformer?
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Old 06-08-2018, 18:17   #9
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Re: AC outlet weird voltages

Quote:
Originally Posted by codger View Post
Yup, time to call the electrician...

Opened up the isolation transformer panel (Charles ISO-G2). The shore side is as expected (120 VAC on hot-neutral and hot-ground, 0 VAC on neutral-ground), but on the boat side of the transformer, I see hot-neutral at 124 VAC, hot-ground at 71 VAC, and neutral-ground at 54 VAC.

At the AC distribution panel, everything looks normal - the AC ground from the transformer is connected to the AC ground bus, which is bonded with the DC ground bus. Neutral is connected to the neutral pole of the double bole breaker and hot to the hot pole. With the AC input wires disconnected there is no continuity between neutral and ground.

Is this possibly a fault in the isolation transformer?
Certainly sounds like a fault in the isolation transformer. If it needs replacing look at a Victron. Fully adjustable for input and output voltages if you are heading offshore.

Pacific Yacht Systems in Vancouver would be the best to call in my opinion.
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Old 06-08-2018, 18:22   #10
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Re: AC outlet weird voltages

Quote:
Originally Posted by senormechanico View Post
With no load and a meter with a very high input impedance, all it takes to read a bit of voltage is a bit of salty dust across the breakers and wiring.
If you measured the "off" settings with your fingers across the meter leads, you would probably measure zero volts or very close to it.
Don't do that with all the switches on !
This is a great answer. Chances are most likely this is it. You can test it simple enough. Make up a test lead with a small inline fuse, the glass ones will do. Something 1 amp less. Connect it from the hot lead you wish to test to ground. If there is any substantial current the fuse will open immediately. Test the voltage with the lead grounded. It should now be near zero.
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Old 06-08-2018, 19:41   #11
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Re: AC outlet weird voltages

Can you identify this transformer?
Has a discussion towards the end describing just this situation.
If you are not well versed in electrical “stuff” get a qualified MARINE electrician.
Most shore side electricians will never see situations like this.

I like hpeer’s approach.

Enjoy!
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Old 06-08-2018, 21:42   #12
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Re: AC outlet weird voltages

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Originally Posted by codger View Post
Yup, time to call the electrician...

Opened up the isolation transformer panel (Charles ISO-G2). The shore side is as expected (120 VAC on hot-neutral and hot-ground, 0 VAC on neutral-ground), but on the boat side of the transformer, I see hot-neutral at 124 VAC, hot-ground at 71 VAC, and neutral-ground at 54 VAC.

At the AC distribution panel, everything looks normal - the AC ground from the transformer is connected to the AC ground bus, which is bonded with the DC ground bus. Neutral is connected to the neutral pole of the double bole breaker and hot to the hot pole. With the AC input wires disconnected there is no continuity between neutral and ground.

Is this possibly a fault in the isolation transformer?
the transfomer output needs the ground and neutral connected together in the transformer by the installer (any brand iso transformer). sounds like they missed this very important safty issue.

who did your work?
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Old 07-08-2018, 02:30   #13
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Re: AC outlet weird voltages

If the shore ground is connected properly to the shield of the transformer and nothing else then I would not worry about it. Modern volt meters are very high impedance and can also pick up noise from the inverter switching frequency. Plug a lamp or fan into the outlet, then measure the voltage. I'll bet it reads zero on the unused socket.
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Old 07-08-2018, 11:05   #14
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Re: AC outlet weird voltages

Someone on YouTube makes a good point about false readings, and using a ~$30 Fluke? Sperry? "smart" outlet tester, instead of the usual hardware store $5 outlet tester. The more expensive ones (now available online ~$20) actually are built with a different impedance so they will "ignore" ghost voltage levels.

I'd hope the main AC breakers were all DUAL breakers, that disconnect both the hot and neutral lines. That alone could eliminate some potential issues.

But by all means, calling back the original electricians to make sure it is safe, beats rolling the dice and guessing about it.
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Old 07-08-2018, 12:40   #15
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Re: AC outlet weird voltages

There are two main AC breakers: one just after the shore power cable, and one at the distribution panel. Both are double-pole, so no issues there.

I think smac999 has it. I went back and looked at the picture I took of the transformer wiring. The output side has X1 of the secondary winding tied to black, X2 of the secondary winding tied to white, and the transformer case is tied to green, but not also tied to X2 as it should be.

The transformer's install instructions confirm that X2 should be tied to both the case and the ground wire. This means, presumably, that my AC ground is not held at the same potential as neutral, which I imagine would explain the 70 V and 50 V potentials measured between X1 and ground, and X2 and ground, respectively and possibly the rest of the wonky readings? It certainly doesn't sound safe. If fault current can't return to the secondary winding via the green ground, where can it go? Probably out the propeller shaft? This might explain the very fast degradation of my last prop anode...
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