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Old 13-04-2019, 20:29   #1
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AC ground -> boat 500mV?!

​In the process of investigating a corrosion issue on a boat I discovered the voltage of all boats in our marina are (positive) +500mV in reference to the AC shore ground and therefore passing conventional current to ground of up to 125mA!

I would expected the boats voltage [Zinc Anode –1000 mv] should be lower in reference to the shore AC ground wire + rod [Copper – 500mv] But this is not the case. ( Picture for clarification inline)

Most boats in NZ have the AC ground and DC negative connected.

What am I missing here? As i don’t understand why the yachts measure positive!! Does anyone know why this is the case? Thanks for any advice and input.

I also checked using an independent reference with similar results.
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Old 13-04-2019, 21:21   #2
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Re: AC ground -> boat 500mV?!

the silver cell goes in the neg of the meter. it is your referance point. so the dock would be -450mv once correctly wired. which isn't ideal. probably some derelict boats pulling the marina down.

measuring between the boat and shore ground I don't think would mean anything. both are tested against the silver cell. with the boat unpluged, and then plugged in. if you don't have a galv isolator. the boat voltage will change once plugged in. if the boat is plugged into the marina without galv isolator and is sitting at -450 that is not good for it. so if say the boat was -800 unplugged, it will drop to -450 plugged in. which means current is flowing to other boats.
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Old 13-04-2019, 21:45   #3
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Re: AC ground -> boat 500mV?!

Isolation transformer <— solution for all shore power related problems.
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Old 14-04-2019, 03:01   #4
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Re: AC ground -> boat 500mV?!

Thanks for the advice and input - i will need to read it through a few times to make sure I understand.

My diagram was unfortunately unclear - the [+] and [-] was just to indicate my meter is seeing the Anode's are positive !! {weirdly] in reference to the AC ground [ Which I am having trouble understanding]

The AC ground seems to be MORE Negative (than the collective anodes) and the same potential all through the marina around - 500 mv more negative

If I measure one boat independently I get around negative -900 mv --to ---> the silver-silver reference (normal)

OK need to do more investigation and more brain activity.

As a by-note my Beneteau does not have this issue as the AC and DC ground are not connected + all AC appliances are double insulated (Plastic case not metal) and I'm protected by a fast double poll RCD = no galvanic issues like this.
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Old 14-04-2019, 10:14   #5
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Re: AC ground -> boat 500mV?!

All measured To the silver cell. With boat unplugged.

Measure to ac ground bus, Dc ground bus. Bonding bus.

All ishould be Exactly the same. And between -.6 to -1 volt If not same, They are not connected together. If out of range. Issue on boat. Not issue with shore power.

Measure dock to silver cell.

Plug boat in. Measure 3 values in boat again. They should not change and remain different then dock value .
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Old 14-04-2019, 16:37   #6
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Re: AC ground -> boat 500mV?!

The convensional way to issolate a boat is to use an issolation transformer but they are heavy and expensive. My solution was to use the inverter to provide all onboard power and just use shorepower to run the charger. This not only avoids corrosion potential as there is now no direct A/C path ashore but also means you have much more stable power onboard. By the time A/C power gets to the end of the dock it is often well under voltage and I don't like relying on an earth link provided by the marina. Most boats already have a big enough inverter and charger but if not upgrading these tends to be cheaper than an issolation transformer that is only useful when hooked to the dock.
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Old 15-04-2019, 09:52   #7
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Re: AC ground -> boat 500mV?!

Quote:
Originally Posted by roland stockham View Post
The convensional way to issolate a boat is to use an issolation transformer but they are heavy and expensive. My solution was to use the inverter to provide all onboard power and just use shorepower to run the charger. This not only avoids corrosion potential as there is now no direct A/C path ashore but also means you have much more stable power onboard. By the time A/C power gets to the end of the dock it is often well under voltage and I don't like relying on an earth link provided by the marina. Most boats already have a big enough inverter and charger but if not upgrading these tends to be cheaper than an issolation transformer that is only useful when hooked to the dock.
So the ground wire on your AC in is not connected to anything on the boat? Should work but many installations use a combined charger+inverter unit with built-in transfer switch and can't run in both modes at same time
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