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Old 30-08-2013, 06:38   #1
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Floating Ground on Aluminum Boat: Voltage Reading?

Hi Everyone,

For a DC floating ground on an aluminum boat, should there be a voltage reading between the hull and the ground? Or should there be no reading at all, as if one is waving the meter's leads in the air? The boat is on the hard.

I'm new to 'floating ground', and this boat, and a little confused by what I see-- when the DC master switch is off, there's about 2 volts between the hull and ground, with about 800k ohms resistance between the two. When the DC master switch is on, there's 0 volts difference between the hull and ground, and about 100 milliamps of current flowing between the two (with the volt meter bridging them).

I need a better idea of what success is like in this project. So far I've increased the resistance between the hull and ground from 0k to 800k ohms by disconnecting the VHF radio. I think there's some other issue to isolate, but am not sure what I am going for:

a) no voltage reading between the hull and ground
b) no resistance reading on a 2000k ohm meter
c) zero milliamps of current when the DC switch is on

or:
d) any arbitrary and low voltage reading between the hull and ground
e) greater than xxxx ohms of resistance between the hull and ground (1000k?)
with c (zero milliamps of current)

I'm about a third of the way through all of the circuits on the boat, and have cleared a few as 'OK', even though their grounds had a high resistance to hull (notably, the stereo system ground has 800k ohms resistance to the hull, instead of the usual 'no reading' of other circuits). If success is no reading at all, then I need to add those circuits to my 'bad list' to fix.

Thanks!

Matt (on the hard in Grenada, but hopefully in the water next week)
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Old 30-08-2013, 06:51   #2
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Re: Floating ground on aluminum boat: voltage reading?

without a circuit diagram its hard to fully understand

(A) I assume you have isolated starters and alternators , so that there is no unexpected paths to ground , This would also include engine senders.

(B) Remember a voltage meter is a very high impedance load, hence you can get "voltage" readings off almost anything. Whats matters is whether there is current flowing

so the 100ma would be a concern.


Quote:
When the DC master switch is on, there's 0 volts difference between the hull and ground, and about 100 milliamps of current flowing between the two (with the volt meter bridging them).
I suspect you have a ground return path which includes the hull ( well not suspect ,you do actually have a return path). You need to look at all the negative return paths from all DC devices.

you will be unlikely to eliminate all leakage current , but it should be possible to get the isolation reading into the mega ohm range which on a 12V system gets the leakage current into the microamps.


Areas to look at

Radio gear,
Devices with EMI filters with earth bonds to AC
AC devices with earth and DC - connections
Engine electrics
poor wiring causing leakage to hull


Its just a case of painstakingly going over the wiring and the devices.

Dave
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Old 30-08-2013, 07:14   #3
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Some tend to treat a metal boat just like a car and add stuff wrong. If it works it must be wired right. Not! While you check things keep notes and label wires.
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Old 30-08-2013, 08:01   #4
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Whoops, I meant there is 100 microamps, not milliamps, of current between ground and the hull when the master breaker is flipped on. Not sure if that is significant or can be ignored.

Thanks, I'm keeping copious notes, since its so much work to test each circuit with typical boat access issues.
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Old 30-08-2013, 08:17   #5
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Re: Floating ground on aluminum boat: voltage reading?

at 100 microamps, you are getting to the point where it can( just ) be ignored , tracking this last bit down could take a while.

preferably Id like to see it down to about 10-15 microamps. But you will have to examine every DC return path in the boat.


dave
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Old 30-08-2013, 08:38   #6
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Re: Floating ground on aluminum boat: voltage reading?

I just measured it, and on Hawk right now I have no resistance reading between the negative and hull.

Previous times I have found a resistance measurement I could usually eliminate it by cleaning the battery top surfaces. Apparently a 'dirt/salt' path builds slowly up from the neg terminals to the hull.
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Old 30-08-2013, 09:40   #7
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Re: Floating ground on aluminum boat: voltage reading?

Quote:
Originally Posted by estarzinger View Post
I just measured it, and on Hawk right now I have no resistance reading between the negative and hull.

Previous times I have found a resistance measurement I could usually eliminate it by cleaning the battery top surfaces. Apparently a 'dirt/salt' path builds slowly up from the neg terminals to the hull.

a ground path from the battery to the hull isn't all the story, like to takes two to tango , you need a dc positive leakage to hull or dc negative leakage to hull somewhere else to induce current.
( or a dirt path from batt+ to hull to batt - )
dave
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Old 30-08-2013, 09:56   #8
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Re: Floating Ground on Aluminum Boat: Voltage Reading?

With a digital meter's impedance of around 11 megohms and a dc resistance measured around 800k ohms, I suspect you have just created a voltage divider.

If my suspicions are correct, you are measuring the collective leakage path from all your negative connections to the hull.

In other words, it's nothing to worry about unless you feel the need to scrub every molecule of dirt off every bit of wiring connections even close to anything even slightly conductive inside the boat including mast conduit etc...

Oh, and don't forget the collective resistance in all the boat's switches...

Steve B.
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Old 30-08-2013, 10:09   #9
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Re: Floating ground on aluminum boat: voltage reading?

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Originally Posted by goboatingnow View Post
a ground path from the battery to the hull isn't all the story, like to takes two to tango , you need a dc positive leakage to hull or dc negative leakage to hull somewhere else to induce current.
( or a dirt path from batt+ to hull to batt - )
dave
Sure but as 'preventive maintenance' if you have eliminated all paths from battery ground to hull, you are then 'protected' if another short to hull should happen accidentally somewhere.

Edit: I do the same checks from battery positive to hull. And many aluminum boats have a 'two way' led 'check light' (or meter) on their breaker panel. Flip the switch one way and it will light if there is a negative short and flip the other way and it will light if there is a positive short - you want it to be dark both ways all the time.
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Old 30-08-2013, 14:33   #10
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Re: Floating ground on aluminum boat: voltage reading?

Quote:
Originally Posted by estarzinger View Post
I just measured it, and on Hawk right now I have no resistance reading between the negative and hull.

Previous times I have found a resistance measurement I could usually eliminate it by cleaning the battery top surfaces. Apparently a 'dirt/salt' path builds slowly up from the neg terminals to the hull.
Does "no resistance reading" mean no resistance (0 ohm on meter) or no continuity (dashes or similar on meter)?

For aluminum hulls, don't you want no continuity to the electrical system at all?

Mark
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Old 30-08-2013, 14:51   #11
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Re: Floating Ground on Aluminum Boat: Voltage Reading?

with pos and neg battery terminals disconnected from your batteries.

you can connect to your negative bus bar,and then to the hull with the multi meter on a resistance setting.,then turn on one switch at a time.
this will indicate any leakage.

then do the same on the positive side bus bar.

humidity is enough to give low readings,but higher readings should be investigated.

i found my vhf via the antenna was a culprit on the neg side.
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Old 30-08-2013, 15:27   #12
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Re: Floating Ground on Aluminum Boat: Voltage Reading?

Thank you all, so much, for your help in understanding this.

The plot thickens! So I have 100 microamps of current (and ~800k ohms resistance) between the hull and ground. But, from what I learned here today, I also checked the current between the hull and the positive side of the battery: 100 milliamps.

This means that there's a 'positive leak to the hull', right? Or is this some kind of inverse relationship where of course, if there's 800k ohms between the hull and ground, then some current will naturally flow from the positive side?

Is the goal to also have this at 0 (or only microamps of) current (or no reading for resistance) between the positive terminal and the hull?

Thanks again, everyone.
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Old 30-08-2013, 15:44   #13
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Re: Floating Ground on Aluminum Boat: Voltage Reading?

air humidity will give a "false" current reading,best is to disconnect the battery to get a true reading of resistance then get worried if you can locate a direct leak to ground from each individual circuit
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Old 30-08-2013, 16:45   #14
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Quote:
Originally Posted by msponer View Post
Thank you all, so much, for your help in understanding this.

The plot thickens! So I have 100 microamps of current (and ~800k ohms resistance) between the hull and ground. But, from what I learned here today, I also checked the current between the hull and the positive side of the battery: 100 milliamps.

Thanks again, everyone.
It's not clear to me how one can measure the leakage current. Can you describe exactly how you are doing that?
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Old 30-08-2013, 22:49   #15
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Re: Floating ground on aluminum boat: voltage reading?

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Originally Posted by colemj View Post
Does "no resistance reading" mean no resistance (0 ohm on meter) or no continuity (dashes or similar on meter)?

For aluminum hulls, don't you want no continuity to the electrical system at all?

Mark
I was just replying back using the OP's language. By "no reading" we both meant "---" not "0" . . . . as you say.

I have never tried to read current flow to the hull. I figure if there is no continuity there can be no current flow. No continuity is preventative.

On the topic of aluminum hulls . . . anyone used the sliver chloride half cells, you drop in the water and measure voltage potential to the hull? I have one, and the reading has always been in what the instruction suggests is the 'good' zone, but have never really figured out what it meant.
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