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Old 02-08-2009, 05:41   #16
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Thanks to all for the great information!
From John's description, if the leak was AC, a Galvanic Isoltaor would be effective. However, since I measure it as 73ma DC, then I guess my next step would be to measure the voltage of the leak, to see if it is above or below 1.4 volts. If its is below, I could install a Galvanic Isolator, which should block it.
But, if it is higher than 1.4V, then an Isolator would be ineffective.
I can't see going through the expense of an Isolation Transformer, for my limited use, so I would probablyjuts continue to plug in for a couple of hours a week, to top up the house bank (2 group 27s), with my Ioata 45a charger. They are never too low anyway, as the 65a alternator is wired to my battery switch, so I can charge them while running the engine.

No reefer, so my charging needs are modest. Biggest user is chartplotter, radar, and fresh water pump.

Now, for my dumb question: I know how to measure voltage, for example, across a battery, or across a lightbulb, etc. (using meter in parallel, red on pos, black on neg). But how do I measure the voltage on the AC ground connector?? I measured the DC amps, by unplugging the shore power cord and putting my meter in series with the ground.

Thanks again for the help!!
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Old 02-08-2009, 08:44   #17
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Northeaster
Do you have some sun in your part of the world if so you could consider a solar battery charger and some battery with low self discharge.
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Old 02-08-2009, 09:03   #18
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For lots of additional information, see some of our earlier discussions at:

Stray Current Corrosion . . .

The Galvanic Series and Corrosion

Zincs and the 'Hot' Marina

Corroding Engine mounts

Electrical Study Hall:

Reverse Polarity (AC)

And these useful tutorials at:

http://www.fluke.com/Application_Not...r/B0269b_u.pdf

Galvanic Isolator Explained

Seaguard - Corrosion monitors and corrosion loggers

MAS Note-Galvanic Corrosion
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Old 02-08-2009, 09:33   #19
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Originally Posted by Northeaster View Post
Hi folks,

When I connected my cheap multimeter in series between the gound on the cord, and the ground in the dock AC outlet
Can you tell me what do you have connected on the end of your cord, only a battery charger or what else.
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Old 02-08-2009, 10:54   #20
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Thanks to all for the great information!
From John's description, if the leak was AC, a Galvanic Isoltaor would be effective. However, since I measure it as 73ma DC, then I guess my next step would be to measure the voltage of the leak, to see if it is above or below 1.4 volts. If its is below, I could install a Galvanic Isolator, which should block it.
But, if it is higher than 1.4V, then an Isolator would be ineffective.
I can't see going through the expense of an Isolation Transformer, for my limited use, so I would probablyjuts continue to plug in for a couple of hours a week, to top up the house bank (2 group 27s), with my Ioata 45a charger. They are never too low anyway, as the 65a alternator is wired to my battery switch, so I can charge them while running the engine.

No reefer, so my charging needs are modest. Biggest user is chartplotter, radar, and fresh water pump.

Now, for my dumb question: I know how to measure voltage, for example, across a battery, or across a lightbulb, etc. (using meter in parallel, red on pos, black on neg). But how do I measure the voltage on the AC ground connector?? I measured the DC amps, by unplugging the shore power cord and putting my meter in series with the ground.

Thanks again for the help!!
Disconnect the ground wire and put the meter leads on each wire with it set to read volts. With the ground wire in place the voltage drops to zero because the wire is a short. Disconnecting the wire stops the current, but the voltage that drives it will still be there.

Picture a battery with only a light bulb connected to it. Break open the wire and insert the DVM set as a voltmeter. The voltmeter will read 12 volts because the voltmeter looks like an infinite resistance, so there is no current flow, and there will be no voltage drop across the bulb.


Look at further tests on the page Gord supplied:
Electrolysis
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Old 03-08-2009, 01:15   #21
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John,

You are correct, although I think that the capacitor is a bad idea and they dropped that in the current models. Galvanic isolators have many problems and I consider them useless unless you use it in your home marina, where the boat is most of the year and where the shore power is good enough that they will do. Even that wouldn't convince me for using one, I would always go for an isolation transformer because it also provides safety for the crew, in addition to 100% fool proof safety for the boat, no matter the local situation.

About GI blocking a 73 mA DC current. Yes, the difference in potential between shore ground and boat ground must be at least 1.4V before it becomes a problem. However, you don't need 1.4V DC for that. Any DC voltage gets added to any AC voltage. If you would pull out the shore power plug aboard and measure both AC and DC voltage between boat-ground and shore-ground (and you actually have a boat-ground system with dynaplate) you will probably find that these two added are more than 1.4V, rendering the GI useless before you even install it.

In reality, you will have enough voltage when the DC current is well under 10 mA so the 73 mA will be a problem.

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Old 03-08-2009, 09:52   #22
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Chala - we do get sun, but not like Florida / tropics, and I would consider a solar panel more, if I were on a mooring. But, as I will likely be on a dock for the next couple of years anyway, I would like to be able to utilize the shore power safely, as I am paying for it anyway!

Gord - thanks for the great links!

John - thanks for the how-to. I figured I would have to open up the AC ground circuit, while leaving the pos and neutral connected, so I will do this at the boat panel, and measure there.

Nick - thanks again for all of the info as well.
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Old 03-08-2009, 10:31   #23
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Can you tell me what do you have connected on the end of your cord, only a battery charger or what else.
Northeaster may be you whish to find out why you have this current flowing into your earth wire.
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Old 03-08-2009, 11:28   #24
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Chala - Sorry, I missed your question earlier. Well, I only have my AC outlets on one breaker, the battery charger on another, and the hot water on one as well.

I do want to find out where this current is coming from, but I don't believe it is coming from my boat. Per the Don Casey book, when metering the open shore AC ground connector, with my red lead toward the dock power (black toward my boat) the 73ma was a positive reading, indicating that the current ios coming from the shore to my boat, as opposed to the other way around. I believe it is stray DC current, coming from another (1 or more ) boats, whose DC wiring is leaking into their boat ground, and then back to their AC ground (and everyone's) shore power cords. Even if I turn all of my AC breakers off, I would get the samne reading.

If the value had been negative, it says this woulc indicate my boat was passing DC current on to other's boats and i would have to find and correct the problem in my DC wiring / appliances.

Feel free to correct me if I am wrong, as I am just learning here!!
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Old 03-08-2009, 11:31   #25
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John, (or others) Instead of opening up my AC ground (at my boats AC panel), to meter the DC voltage across the ground to hot, and ground to neutral - Can I just do this at the shore power receptacle, or does the hot and neutral have to remain connected to my boat to test this?
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Old 03-08-2009, 21:35   #26
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Yes, only disconnect the ground wire.

I think that Chala wants to know what you have connected to the ground wire aboard. Some items are a no-brainer, like the outlets, water heater and battery charger. But is it connected to your 12V negative??? or to a bonding plate/thru-hull fitting under the hull???

For example: if you have a bonding plate, you could try to disconnect it to see what happens with the DC current on the shore power ground wire. If you have it connected to a thru-hull and the DC current disappears when you disconnect the ground wire from the thru hull, this means that the thru-hull is being eaten away rather quickly! In other words: some of us are a bit worried... So disconnect the shore power cable at all time except may be to charge the batteries, until you know exactly what is happening.

cheers,
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Old 04-08-2009, 05:10   #27
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Nick - thanks for the explanation!

I have 2 bronze seacocks which are not bonded (not connected to anything else other than through rubber hose).

All grounding is done via the prop shaft (there is NO rubber drive saver, etc). The engine is, of course, connected to the trannie / prop shaft, and the engine ground is connected to the Neg DC battery terminal.

The AC ground is also connected to the Engine ground (to prevent AC shocks to swimmers, possibly leaked through charger, etc, into DC ground)

I do have a very small inverter (300 watt_ wired directly to the house bank with adequate sized wiring.

I have 2 prop zincs on the shaft, as well as 2 engine zincs in my raw water cooled Yanmar 2GM.

Hope that answers most questions. Will be testing in next few days!
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Old 04-08-2009, 09:20   #28
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NE'er,

Okay, clear. Your worry is the prop & shaft but as long as the zincs are on the shaft you should be fine. Check the zincs, also the ones in the engine. A bad problem can eat them up in days rather than weeks. But as long as you're disconnected from shore power you should be fine too.

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Old 10-08-2009, 10:52   #29
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Hi Guys,

I didn't have time to disconnect my AC ground wire inside the boat (busy taking family / friends sailing).
However, I did quickly measure the AC and DC voltage on the AC ground wire, simply by unplugging the shore cord and measuring between the cord ground and the shore AC receptacle ground.
Measured approx: 1 V AC, 1.3 V DC, and 63 ma DC

Will do further tests when time permits. Blew the little fuse in my meter (wrong setting likely) s oI have to replace that first!
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Old 10-08-2009, 11:36   #30
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In the boat, my AC gound is tied to the DC neg buss bar. Will temporarily removing the AC ground from the DC ground buss stop the problem until an iso trans can be fitted. You guys have me now worried about my new expensive MaxProp. I haven't measured yet, but some guys in the Marina have been complaining about their zincs not lasting. I keep my boat plugged in all the time and I normally keep the boat in the water with two years between haul outs for bottom paint.
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