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Old 04-06-2007, 05:11   #1
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A C units and errant current

I moved marinas this weekend and make a habit to put on new zincs at the same time, no better way to check out the new digs. The diver said he was getting a "tickle" from the zinc located on the rudder strut so after he left I got out the meter and dropped one leg over the side and clipped the other to a bonded chainplate. I started measuring as I turned on each 120v breaker on the distribution panel. The results showed < 0.2v on every item until I got to the A C units, they both showed .6 and .7v with a combined 1.4 v. I put the meter on mA and repeated the same inspection and the results showed <2.1 mA on all combined equipment with the A C units putting 14.0 and 13.4 mA on the system, 27mA combined. Where do I go from here? Your input in always appreciated.
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Old 06-06-2007, 13:48   #2
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AC DC Isolation

I have been inspecting the grounding connections on the boat systems and so far have not found any high resistance at any particular location, cleaning and crimping new lug connectors as I go. I have looked to see if any common or white wires have been put on any of the DC ground busses and have found none so far. The boat is not equipped with a galvanic isolator and I wonder if putting one in the system will correct the problem. The only equipment on the boat that loads the bonding circuit are the air conditioning units and they add 13-14 ma each, 27ma combined. The only time the current shows is when the compressors are running, when the fans run the current drops to 2.0ma. I'm going to shut everything down and take the supply end of the shore power cable in near the distribution panel and inspect the ground resistance through the cable next. If anyone has been through the process before and could contribute some wisdom many thanks
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Old 06-06-2007, 14:14   #3
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With the shorepower cord unplugged from the boat and every AC load turned off (preferably unplugged or disconnected, using a good quality meter, measure resistance between the white and ground leads of the shorepower socket You should have infinity, no continuity at all. Anything less could be a problem. If you get measurable resistance, start looking for salt, corrosion, etc. as the probable cause. If the resistance is zero or very low, you have a neutral/ground connection which shouldn't be there. If that measures ok, measure between the neutral and bonding system. Measure both AC and DC volts and write it down.

Hook up the shorepower again. This time, one at a time turn on your equipment. Using the meter on the voltage scale measure between the neutral and ground. Measure both dc and ac. Write down your findings and post them.

IMHO, this should be enough to find the culprit.

Steve B.

former electronic tech and former full time cruiser
BTW I got my moniker from a Mexican villager when I helped them fix lots of electronics in the middle of nowhere in Baja Sur.
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Old 11-06-2007, 05:56   #4
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Bonding circuit current

Senior, I followed your troubleshooting instructions and found the following. With the shorepower cable disconnected and the main AC breaker off the socket gave infinity between the ground lug and nuetral. The resistance on the panel was low also, .001 ohms. In the distribution panel one lead was clipped to the nearest bonding lug, a chainplate with a good clean connection, and voltage measurements taken at all nuetral leads on the bus. AC voltage Was highest on the hot water and charger at 0.05, all others were less than 0.002 or less. the DC were al 0.007 or less. I also disconnected the waterpumps on the Air conditioning units and cycled both with a 0.5ma reduction at both units. later in the day with both units running at the same time and all breakers an the panel on I saw 24.0ma current on the meter. I have decided that I do have cont. between the nuetral buss and the ground buss and will have to disconnect all leads at the same time to find out which one has the most effect. How much current is allowable?
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Old 11-06-2007, 11:00   #5
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Your post contains an anomaly. First you said infinity between ground and neutral. Then you said the resistance on the panel was low also, .001 ohms.
Which is it? With the shore power DISconnected it should be an open circuit.
With the shorepower connected, there would be a very low resistance between the white and green as they are connnected at the shore main breaker box. They SHOULD NOT be connected on your boat (imho).

If your AC ground AND AC neutral wires are both connected to the bonding system on the boat end, I suspect what's happening is that the load you are imposing on the shorepower is causing enough voltage drop on the shorepower cord to cause a slight (but just enough) AC voltage to be impressed on your bonding system to give the diver the heebeejeebies.
For example, say you have 120 volts at the dock end. If your total current draw on the boat is enough to drop the line voltage to 114 volts at the boat (assuming the shorepower wires are all the same gauge), the neutral and ground wires are parallel connected at the shorepower AND boat ends, they will drop 1/3 of the total voltage drop while the black or hot lead drops 2/3 of the voltage. This means that at the ground connection to your boat you have 2 volts AC on your bonding system. This is measured between the water (actual ground) and bonding system. That doesn't sound like a lot, but if you're immersed in salt water it is enough to give you a tickle. It's also not doing you zincs any good.

If that's the case, make sure the Green wire (ground) is the only AC wire connected to your bonding system (if even that).

There is disagreement about whether or not to connect ANY part of an AC system to your bonding system, but I'll leave that to another poster. On my boat which is relatively lightly equipped as far as AC goes, The only thing connected to my bonding system is thruhulls and a couple of zincs.
The battery negative is separate as well.

I live on a sandy point with canals and docks for each house. Our grounding system at the house is typical with a ground rod outside of each house connected to the ground/neutral inside the house ac panel. The problem arises when there's a big load at the house and the weather's dry.
The ground rod is trying to keep the panel's neutral at ground potential, but the sand is DRY DRY DRY. Consequently, my ground wire to my dock wasn't exactly ground potential. I had about 1.5 volts AC+ about 0.5 vDC as well. I fixed that by driving a couple of bronze rods into my beach at the low tide line. I connected them together with # 6 copper wire with bronze clamps, and tied this to the green shorepower feed to the dock.
Just before I connected the rods to the house ground wiring, I measured what kind of current I was bypassing. At the above voltage (1.5vac) I measured 0.5 AMPS AC. Not ma. Half an amp AC.
Needless to say, my zincs last a lot longer than they used to.

Sorry for the long post,

Steve B.
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Old 11-06-2007, 11:34   #6
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Go here, and check out the article on shore power safety tests.

Professional BoatBuilder - April 2006/May 2006
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Old 11-06-2007, 15:26   #7
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Figure 4 in that link shows what I was saying, except the resistive point in my explanation was not a "point" on the wiring, but the overall resistance of the wiring between the source's ground rod and the boat's bonding system.
With the heavy load of 2 air conditioning units, the voltage drop can be enough to cause the tickle described.

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Old 11-06-2007, 16:56   #8
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Steve, your post was a good one. I just wanted to point out this article to stuffinbox. Sometimes (at least for me) a diagram can simplify the situation and make it much more understandable. You are correct that the resistance could be distributed along the marina wiring circuit. It could also be (as Nigel points out) in the shore power cord contacts, or the shore power cord itself, or any single point along that circuit.

One bit of your post that I would disagree with, however, is that the AC and DC grounds should be connected (in one and only one spot).

Cheers...
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Old 15-06-2007, 05:31   #9
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Thanks for the help. My DIY is DOA and after a week of Calder, internet searches, and your input no hard fault has been found. I scheduled a tech to come out and do a survey, I do my own maint and repairs because part of this lifestyle is independance and on the bright side I know the layout and routing after the time spent with the fluke meter. I have learned that no matter how well installed or how old a boat is this problem can occur at any time and should be inspected regularly, Tara is 20 years old and passed a survey easily but I'm sure this problem was going on when I bought her. I am going to add a galvanic isolater even if the fault is identified. This board has helped in the past and I hope to contribute in the future. I'll let you know what the survey turns up.
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Old 15-06-2007, 06:18   #10
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Nigel Calder on Galvanic Isolators (Yachting Monthly, 13 November 2006)
Yachting Monthly - Any Questions: Galvanic isolators and stray currents
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Old 10-07-2007, 09:04   #11
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Follow up

I thought I would let anyone interested know the fault found on the bonding system. It turned out that the inverter was improperly wired, I was impressed that it took the contracter a full day to find the problem, but the inverter and the sockets are now isolated from the rest of the system. I read some information on this subject in one of Calder's books and now understand a little more about the issue. Live and learn and go to the next problem. I want to thank everyone for your input and I hope I can return the favor in the future.
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