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Tracking down a ground fault

My boat sits on a mooring in Boothbay Harbor Maine. No shorepower nearby. The boats moored next to me have zincs that last forever. My closest neighbor has the same zinc now for 4 years and still looks new. The zinc on my shaft (bronze shaft & prop) won't last the season.

All the wiring is new, I rewired everything this past winter. All marine wiring, heat shrink crimp connectors, carefully done ect. However, I could easily have done a great job wiring something incorrectly.

Testing: Switch off, bilge pump disconnected, I get 12volt reading between disconnected positive wire and positive post. So I must have a leak right?

Next I tested for ohms between the positive wire and the negative post, and got what I believe is 60k ohms. According to what I read, anything over 10k ohms is negligible..so I have a very negligible leak ..correct? Can 60,000 ohms be correct? If so small a leak, then why the rapid zinc decay?

I began to test for amps in each circuit and quickly blew the fuse in the meter due to the wrong settings. After hunting down some .5amp fuses I'll go back and try to look more.

Do I have a problem? 60k ohms would suggest otherwise, but it doesn't explain the zinc decay. I suppose I could be floating over a leaking underwater cable, but none of my neighbors are effected by this. Could I be measuring this incorrectly?

Preserved Killick


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post #2 of 11 Old 08-30-2009 Thread Starter
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Thinking more.

I installed a Blue Sea ACR SI, could that draw some current? Product documentation does not indicate that it would when not in use. I don't have the external led attached.

Before I blew my fuse, I tested the amps between the positive wire and the positive post and I think I remember seeing less than 200 milliamps. *This is from memory, I was figuring how how to use the meter so I didn't record this result.

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post #3 of 11 Old 08-30-2009
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Quote:
Originally Posted by preservedkillick View Post
Testing: Switch off, bilge pump disconnected, I get 12volt reading between disconnected positive wire and positive post. So I must have a leak right?
That's right. The ends of the positive cable should be dangling in the air, so to speak, at both ends. So you shouldn't read anything.

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Next I tested for ohms between the positive wire and the negative post, and got what I believe is 60k ohms. According to what I read, anything over 10k ohms is negligible..so I have a very negligible leak ..correct?
I've never heard that 10K figure and I would think that is a real problem. As I said before, both ends of your positive circuit should be pictured as dangling in the air, not attached to anything. Some how, some way the current from your ohmeter is finding its way from one lead back to the other lead. And that shouldn't happen unless maybe you have a digital clock or something like that attached to both positive and negative leads.

Just an idea:
10K sounds like it could be water. Do you have a wet connection somewhere that provides a route to ground? A pin-***** in the insulation of a wire laying in water somewhere would do it. Bilge pump lead?

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I began to test for amps in each circuit
I've never gotten anywhere by doing that, but if you measured 200 ma, then your meter is sensitive enough so that it may provide useful information. You're talking 200 ma with a device turned on, right?

With solid state devices, Ohm's law doesn't always seem to apply. Hit a circuit with your teeny 1.5 volt meter, you get one reading, turn on the battery at 12V and it does something altogether different. So you measure resistance with your meter, and then - Holy Cow! - the circuit draws a lot more current than you figured it would.

Keep fiddling around though, and you'll figure out what your problem is. (Or else you will finally figure out what the definition of is, is )

When you come to a "Y" in the road, take it....Yogi Berra
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post #4 of 11 Old 08-30-2009 Thread Starter
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Quote:
Originally Posted by Selkirk View Post
I've never heard that 10K figure and I would think that is a real problem. As I said before, both ends of your positive circuit should be pictured as dangling in the air, not attached to anything. Some how, some way the current from your ohmeter is finding its way from one lead back to the other lead. And that shouldn't happen unless maybe you have a digital clock or something like that attached to both positive and negative leads.
I didn't explain myself with that one. I got that number from Don Casey's book. See:
Sailboat electrics simplified - Google Books

I don't want to sound like I know what I'm talking about (no chance of that anyway). I understand this to be a fairly high level of resistance. What that means to my leaking current I don't know.

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post #5 of 11 Old 08-30-2009
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It's good to remember Ohm's law here. E=IR where E is voltage, I is current, and R is resistance. Move the terms around and you get I=E/R so 60,000 ohms resistance at 12 volts will flow only 0.2 milliamps. However if you showed a leakage current of 200 milliamps from the battery, then R=E/I or 60 ohms! A 60K ohm leak would be what you would expect from bad wiring insulation, while 60 ohms would look like a device under power. A 200 milliamp leak would run down your battery pretty quickly if you weren't charging it regularly. A 60K ohm leak wouldn't even be noticed.

Just some info to help you understand what your meter shows.

Gary H. Lucas
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post #6 of 11 Old 08-30-2009
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One point I haven't seen mentioned here: your neighbor's boat whose zinc has lasted for four years and still looks new. That's unusual. If it isn't a shaft zinc, it may not be grounded.

An ungrounded zinc will last a long time. It's useless, but it will last.

We've seen this many times when we haul power boats - one of the transom zincs will be decayed beyond recognition - the other looks like new. When they're checked, inevitably the one which looks new isn't grounded.

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post #7 of 11 Old 08-30-2009 Thread Starter
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Quote:
Originally Posted by GaryHLucas View Post
It's good to remember Ohm's law here. E=IR where E is voltage, I is current, and R is resistance. Move the terms around and you get I=E/R so 60,000 ohms resistance at 12 volts will flow only 0.2 milliamps. However if you showed a leakage current of 200 milliamps from the battery, then R=E/I or 60 ohms! A 60K ohm leak would be what you would expect from bad wiring insulation, while 60 ohms would look like a device under power. A 200 milliamp leak would run down your battery pretty quickly if you weren't charging it regularly. A 60K ohm leak wouldn't even be noticed.

Just some info to help you understand what your meter shows.

Gary H. Lucas
Thanks Gary,

I'm pretty sure the meter read 60k ohms. Didn't actually show 60,000 but 60 with the little "k". This I was able to test and retest a few times and was confident enough to record the result. The 200 milliamps was from memory when I was trying to figure out how to use the multimeter, just before I blew the meter's fuse while testing the VHF while the meter was set to test milliamps.

I've got a bunch of fuses for the meter, I'll go back and see what I'm getting for amps. I'm wondering if it was .2 milliamps. If so, that might be difficult to track down.

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Last edited by preservedkillick; 08-30-2009 at 09:04 PM.
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post #8 of 11 Old 08-31-2009 Thread Starter
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Quote:
Originally Posted by GaryHLucas View Post
It's good to remember Ohm's law here. E=IR where E is voltage, I is current, and R is resistance. Move the terms around and you get I=E/R so 60,000 ohms resistance at 12 volts will flow only 0.2 milliamps. However if you showed a leakage current of 200 milliamps from the battery, then R=E/I or 60 ohms! A 60K ohm leak would be what you would expect from bad wiring insulation, while 60 ohms would look like a device under power. A 200 milliamp leak would run down your battery pretty quickly if you weren't charging it regularly. A 60K ohm leak wouldn't even be noticed.

Just some info to help you understand what your meter shows.

Gary H. Lucas
Thanks Gary,

I'm pretty sure the meter read 60k ohms. Didn't actually show 60,000 but 60 with the little "k". This I was able to test and retest a few times and was confident enough to record the result. The 200 milliamps was from memory when I was trying to figure out how to use the multimeter, just before I blew the meter's fuse while testing the VHF while the meter was set to test milliamps.

My neighbors zinc may indeed not be properly connected. I'll poll a few more neighbors.

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post #9 of 11 Old 08-31-2009
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I like Larry and Susan's post. Having anodes wearing to fast can be a pain but at least it's not your prop. Mine only last about 8 months. I'm not clever enough to track down a problem but have found a great way to change the in water. It's always tricky to handle nuts and bolts and anodes and spanners underwater. I have started to use a galvanised bucket tied under the prop shaft on a short lanyard. I have somewhere to put tools and when I drop a nut ( and you know you will) It falls in the bucket instead of the mud.

It is easier on land but it saves slipping the boat more than necessary.
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post #10 of 11 Old 09-16-2009 Thread Starter
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I have more information on this. Now that I know what I'm doing, and with a new fuse, I was able to measure 12 milliamps of current at the battery. I tracked this down to the Blue Sea ACR. Spoke to a Blue Sea tech guy and he confirmed that the device consumes 12 to 15 milliamps just waiting for something to happen. He also instructed me to just disconnect the ground to the ACR to turn the device off completely when away from the boat. I did this, and measure zero volts, zero amps. No measurable current flowing. After doing this, I removed the shaft zinc, which was very corroded (after 2 months). Without the shaft zinc, my drive system is so much smoother.

Now, what I don't understand is the rate of corrosion on the zinc. I can't believe that the 12 milliamps would be enough to chew through a zinc after 8 weeks. I'm wondering if there is some source of electricity in the water. Other moored boats near me don't have any problems. I'd love to test the water, although that requires a silver electrode, and I'm not curious enough to buy one.

I'll go the remainder of the season without the shaft zinc, then see if I can't fit a zinc to the prop nut or something else for next season.

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