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Old 31-07-2009, 17:54   #1
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Multimeter DC amps Setting, 200m Reads '73' - Decipher please!

Hi folks,

I was following Don Casey's book trying to see if we have " hot docks". It says to measure the DC amps flowing from or toward your boat, through the AC gorund. When I connected my cheap multimeter in series between the gound on the cord, and the ground in the dock AC outlet, it read 1, in the 2000u setting, 1 in the 20m setting and 73 in the 200m setting. Is this milliamps, micro amps or what?

Is this a lareg amount, or a negligible amount?

I have been conservative so far this season, only pluuging in a few hour on the weekend, to top up the batteries, or heat some hot water.

Thanks in advance for the help!
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Old 31-07-2009, 21:36   #2
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That would be 73 milliamps.
"1" means over the range selected.

Any DC is not good, but I've seen a lot worse.
I wouldn't be too concerned with that amount.


Steve B.
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Old 01-08-2009, 05:41   #3
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Steve,

Thanks for the response. I eventually figured out the settings, but still didn't know if 73 ma was enough for concern, as I had left my book at home. Reading it this morning ( and I realize it is not the gospel!) it mentions that anythinbg more than a few milliamps is a cause for concern.

However, I do not have previous experience with this to know how much is common, or how much is too much. I would think that those who are plugged in 24/7 would be losing their zincs quickly (if they are not the ones causing the problems).

This brings me to my second question. What is wired incorrectly, or defective in the other boats DC systems, to be causing this much DC leakage (ie, do they have DC circuits, like a bilge pumps, whose wires are chafing against other wires, etc)??

There are only about 50 boats at our marina, of which amybe 10 are plugged into AC. Should we be unplugging one at a time, to see who is causing this?
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Old 01-08-2009, 10:36   #4
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The solution is not to see who is the cause. The solution is to protect your boat regardless of who is the cause..or whoever might be a cause in the future.

What you need to do regardless of the cause of the problem is to purchase a galvanic isolator or what is also called a "zinc saver". It goes inline with your boats AC ground. A galvanic isolator is an inline capacitor that stops DC current yet allows AC to flow so that the ground can still do its job of protecting you. You will need to purchase one that is rated at or above your shorepower current...either 15, 20, 30 or 50 amps. The isolator is installed onboard your boat and is connected between your shorepower connector and your AC ground bus terminal (where all your AC grounds come together).

For larger boats, isolation transformers are frequently used in order to stop stray DC currents from flowing through you're hot and neutral shorepower connection.
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Old 01-08-2009, 11:01   #5
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Hi David,

Thanks for the reply. I have read about galvanic corrosion, galvanic isolators, and isolation transformers.
However, after reading some old threads, I was under the impression that a galvanic isolator would only stop smaller amounts of current (from dissimilar metals robbing each other, etc) and not the larger, stray currents from other boats DC systems.
I understood that for that I would need the larger, more expensive isolation transformers. I may have misunderstood.

If I were frequently using the dock power, for such things as AC, or keeping the batteries full while running a reefer, etc, I would definitley invest in one of your suggested solutions.

However, as I will likely only be plugged in a few hours a week, to charge the batteries, etc, and then unlpug, do you think it is really necessary??

Not trying to dissagree with your advice, but I am wondering if it is overkill, for the amount I use the dock power.

I do appreciate your, and other's advice and expertise!!!

PS, I was thinking we maybe should find the boat responsible, so they can know they have an issue, as opposed to passing guilt, etc,
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Old 01-08-2009, 11:57   #6
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If all you are doing is plugging in the boat a few hours a week to shorepower then you will probably not notice any excessive corrosion of your anodes caused by that stray current you are seeing with your multimeter, if excessive anode corrosion is what is happening. Personally, I would get one just for the piece of mind if you ever do want to leave your boat connected to the dock full time....so your bilge pump can run for a long time in case a leak ever starts and you are not there?

The source of the stray DC current in your shorepower ground may not be another boat at all. It may be originating from your boat.

Ages ago, I had the same problem in another marina with at least a hundred boats. I have an aluminum boat, so I had a lot more at stake here. I was losing aluminum along the bottom of the keel (not a lead keel) and at the chines. It was not worth the effort to go find the problem. Even if I had found the problem, if another boat came to the marina with the same problem then I would have had to go through the same lengthy process again. It was much easier and certain to protect my own boat than to try to be the good samaritan. Additionally, other boats are not necessarily the cause.

The galvanic isolator stops all these possible sources which is much easier than hunting them down in order to stop them.

Yes...lets hear form others!
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Old 01-08-2009, 14:24   #7
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Galvanic Isolators are typically rated to block up to 3 - 5 mA (0.003 - 0.005 A) DC, at between 0.9 to 1.5 Volts DC.
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Old 01-08-2009, 14:55   #8
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Gord - so juts so I am clear, a galvanic isolator would NOT help me here, correct? I would need an isolation transformer?


David - vaild points about power to run bilge pumps indefinitely, etc.
In my case, I only have 3 thru-hulls (below water), that I close when I am not going to be near. And the stuffing box is a traditional flax one, with new clamps, hose, flax, etc, so I think the chances of watre coming in are low. A lightning hit would definitley be a problem though!!
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Old 01-08-2009, 15:06   #9
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Quote:
Originally Posted by Northeaster View Post
Gord - so juts so I am clear, a galvanic isolator would NOT help me here, correct? I would need an isolation transformer?


David - vaild points about power to run bilge pumps indefinitely, etc.
In my case, I only have 3 thru-hulls (below water), that I close when I am not going to be near. And the stuffing box is a traditional flax one, with new clamps, hose, flax, etc, so I think the chances of watre coming in are low. A lightning hit would definitley be a problem though!!
That depends on if the stray DC current is coming in through the Ground or the Hot or Neutral. If its just the ground then the isolator would be all you need. But if your shorepower is rarely connected, and you are sure your thruhulls will never leak or rainwater will never reach your bilges, nor will any other way of getting water to your bilges happen, then you don't need any of it.
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Old 01-08-2009, 17:19   #10
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Hmmm... I don't agree with David at all! ;-) I think 73 milliamps is a MAJOR problem and I think there exists no galvanic isolator that will protect you at that much current (he measured the DC current through the ground wire). Like Gord wrote: it'll block 4-5 mA; when you go over that, the diodes open and ALL protection is LOST.

I have seen much less current eat away a 10mm stainless anchor chain within 2 weeks (eaten half away, 5mm stainless steel gone).

Also, the GA goes in-line with the shore-power, not the boat ground.

BUT... the first thing you would need to do is measure the current with a decent meter (see the recent thread about meters!). If it really is 76 mA, I think it should be reported because something is very wrong.

We use an isolation transformer (currently replacing it with the Victron unit) plus we hang an extra sacrificial anode overboard (one of those expensive grouper-zincs but you can make them yourself, just pick up old zincs in the yard and melt&cast them in moist sand mold).

cheers,
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Old 01-08-2009, 17:57   #11
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A μA is a micro-Amp, or 1 millionth of an Amp.
Hence, if Iíve got my arithmetic right (I donít always) 73 μA would be 0.000073 Amps, or 0.073 milli-Amps - not 73 mA. In my view, this wouldn't be seriously problematic.

Iíve got cheaper ($50?) Multi-meters with a μA scale, but the very cheapest usually donít.

On the other hand, as much as I hate to , I agree /w Nick that 73 mA would be problematic.

The Galv. Isolator goes in line with the shore power, on the boat (between inlet & Gnd Bus).
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Old 01-08-2009, 19:37   #12
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Gord - It read 73, on the 200m setting, which is up to 200 milli-amps (not micro-amps), is it not? It just read 1 on the 2000u (2000 micro-amp = 2 milli-amps) setting, as well as the 20m (20 milli-amp?) setting, as I believe the amount was too much for those settings.
So, I believe that is was, indeed, 73 milli-amps.

Nick -yes, it was measured on the AC ground wire.
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Old 01-08-2009, 22:54   #13
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Indeed, μA (ha! I needed to copy & paste that character! ;-) is micro-amp and often showed as uA (with lower case "u"). At the 200m (=200mA) range your are measuring and reading mA. Your interpretation of the meter-reading is correct but I doubt the meter. It uses an internal shunt and cheap meters have cheap shunts that are way off in different temperatures plus it needs to measure the voltage drop over that shunt which is small and cheap meters have trouble with that too. I would really make sure using a good meter and measure it on another, neighboring boat too. If it really is 73 mA, a lot of damage is being done somewhere.

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Old 02-08-2009, 00:18   #14
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As I understand it, a galvanic isolator in its simplist form is two diodes connected in parallel, with opposite polarities. A diode is a check valve, it allows current to flow in one direction but not the other. A check valve with a spring to seat the ball requires a "cracking" pressure to open it in the forward direction. A silicon diode has a, cracking pressure, or forward drop of about 0.7 volts. The reason for two diodes is one opens if the current is in one direction, the other diode opens if the current is in the other direction. So the galvanic isolator passes no current, disconnecting the ground from the shore for voltages under the forward drop of 0.7 volts, preventing galvanic corrosion (in reality galvanic isolators have two diodes in series to make a drop of 1.4 volts, so four diodes total). Above that (cracking pressure) voltage the ground is connected to shore, enabling galvanic corrosion and protecting people from being shocked. The diodes should be rated for the current you could draw in the worst case scenario. So if you have shore power on a 30 amp breaker , you're supposed to have 30 amp rated diodes so that the breaker will trip before the diodes burn out.

If the 70 milliamps is at less than 1.4 volts, then a galvanic isolator will block the current flow, if it is over 1.4volts, the diodes will forward bias and pass the current, allowing the galvanic corrosion to continue.

A capacitor was a later add-on to the galvanic isolator. If the current is AC, then the capacitor will pass the current and the voltage across the diodes will remain below the point that the diodes will forward bias. The reasoning is that AC current will cause little or no corrosion, so leave the ground always connected for the type of current that is only back and forth, and also allow the diodes to remain in the off state to prevent any DC current that exists below the forward bias voltage.

BoatUS.com - Seaworthy Magazine

Yachting Monthly - Any Questions: Galvanic isolators and stray currents


So I don't understand some of the earlier information given about isolators.

John
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Old 02-08-2009, 04:39   #15
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I stand corrected, Northeaster did say* it read 73, on the 200mA setting, and as Nick says, this is problematic, and a galvanic isolator would NOT help you there.
Johnís analogous explanation describes the Galv. Isolatorís operation fairly accurately and quite elegantly.

* That's why I don't enjoy puzzles. I miss too many clues, these days.
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