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Old 15-09-2010, 03:46   #16
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Just to reiterate,

The notion that currents circulating in the seawater are entering through one underwater metal and leaving through another are not possible,

The problems are , assuming your traffo is wired right on your boat alone. Since AC stray curent corrosion is prevented by the transformer, (unless you have a hot fault.) and ground fault currents via other boats and the marina is also so protected, that leaves DC stay current. ( this is assuming the problem is stray current corrosion, but its seems likely given the speed at which teh zincs are going)

Get a good multimeter and see if there are currents flowing in the DC system when there shouldnt be. DC stray current is more destructive that AC stray current as well.

Dave
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Old 15-09-2010, 08:35   #17
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Originally Posted by goboatingnow View Post
Just to reiterate,

The notion that currents circulating in the seawater are entering through one underwater metal and leaving through another are not possible,
That's good to know since it means that it is "not possible" to get electrocuted by falling in the water in the vicinity of a shore power cord that someone has dropped in the drink.



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Old 15-09-2010, 08:57   #18
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This thread is getting very confusing.

Galvanic corrosion is caused by an electrical circuit between 2 dissimilar metals immersed in acid. On boats this happens 2 ways:

1) When your boat ground is connected to the commercial (shorepower) ground AND a neighboring boat is connected the same way, you have a circuit between the boats via the commercial ground and each boat has metal hanging in the acid (water). This creates a circuit (the shorepower ground) between the metal(s) and current then flows.

2) When all the metal that hangs in the water on YOUR boat is bonded together, i.e., a grounding system, the dissimilar metals on your boat now form a battery and current will flow causing galvanic corrosion.

Ways to alleviate the problems.

A) An isolation transformer allows a complete disconnection of the shorepower ground from the boat ground, it's not needed. Yes, your boat ground is still the same potential as shorepower ground, but you no longer have a circuit between your boat ground and the shorepower ground.

B) A galvanic isolator places diodes in series with the shorepower ground which stops the low voltage DC current flow between the metals on your boat and the metals on your neighbors boat. The capacitor in the galvanic isolator allows low voltage AC current to flow, thus maintaining the safety factor afforded by the ground.

C) Normally, #2 doesn't cause big problems, normal zinc maintenance handles #2. Although controversial, removing the bonding system within your boat that connects to all metal hanging in the water will stop #2 above.
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Old 15-09-2010, 17:19   #19
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You can take the following to the bank:
1. Galvanic corrosion is a slow process.
2. DC stray current corrosion is a very, very fast process. Shafts, props and other underwater metal have literally dissolved in just a few days in the presence of DC stray current corrosion.
3. AC stray current is dangerous to people in the water near the faulted vessel. It is downright lethal in freshwater.
4. AC stray current is generally not a factor in anode loss.
5. A galvanic isolator will block a significant amount of galvanic current either leaving your vessel (bad) or coming aboard your vessel (not so bad) over the shore power safety green wire. (Note: Galvanic currents can also come aboard/go ashore over cable TV and telephone lines.)


Hope this helps clarify.
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Old 15-09-2010, 17:34   #20
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Typically, boaters are mis-informed and under-educated on this subject. Personally, I have 35 years experience as a marine electronics service technician and have had formal training on this subject and have done extensive reading on it as well. A good technical library is your best friend here. Nimitz Library at the Naval Academy for me. Anyway, here's a couple links to get you started.

http://www.kastenmarine.com/_pdf/mbqCref.pdf

http://qualitymarineservices.net/Tra...%2010-2006.pdf

Eric
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Old 15-09-2010, 17:39   #21
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Talking

I bought the Dairyland unit. Thank you to all that answered my question. I didn't know this would evolve into an "anchor" thread!
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Old 15-09-2010, 18:32   #22
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(Note: Galvanic currents can also come aboard/go ashore over cable TV and telephone lines.)
This is only a problem if these are connected to the boat ground. By design, no equipment you would connect to the telephone or cable TV would connect either of these to boat ground.
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Old 15-09-2010, 18:46   #23
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This thread is getting very confusing.

C) Normally, #2 doesn't cause big problems, normal zinc maintenance handles #2. Although controversial, removing the bonding system within your boat that connects to all metal hanging in the water will stop #2 above.
It sure is getting confusing. OR, at least I am now confused (course, it doesn't take much...). Here I am, embarking on a total "re-do" of the rats nest that passes for an electrical "system" on my (new, old) boat and I am mapping out how to elegantly tie things together in one big unified bonding/grounding system as per a tidy ABYS diagram shown in one of Calder's books..... and now I read hear that doing exactly the opposite is the key to non-stray current nirvana. Help! What's a simple ("always wanted to work on an automatic transmission") man supposed to do here?
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Old 15-09-2010, 18:56   #24
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B) The capacitor in the galvanic isolator allows low voltage AC current to flow, thus maintaining the safety factor afforded by the ground.
While the purpose of the capacitor IS to allow low voltage AC current to flow, it has nothing to do with any safety factor. It is there to prevent the low voltage AC from causing the diodes to go into conduction and thus passing any superimposed DC current. The capacitor provides a path for the AC current while still blocking the DC. It takes a rather large capacitor to be of any good in this regard and the size's used in many isolators are just too small to be effective. If you have AC leakage, you should fix the problem, not try to prevent it from making your isolator useless by using isolator bypass capacitors.

Eric
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Old 15-09-2010, 19:32   #25
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While the purpose of the capacitor IS to allow low voltage AC current to flow, it has nothing to do with any safety factor. It is there to prevent the low voltage AC from causing the diodes to go into conduction and thus passing any superimposed DC current. The capacitor provides a path for the AC current while still blocking the DC. It takes a rather large capacitor to be of any good in this regard and the size's used in many isolators are just too small to be effective. If you have AC leakage, you should fix the problem, not try to prevent it from making your isolator useless by using isolator bypass capacitors.

Eric
Indeed, you are correct. Thanks for the correction!
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Old 15-09-2010, 19:51   #26
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Originally Posted by Anomaly View Post
It sure is getting confusing. OR, at least I am now confused (course, it doesn't take much...). Here I am, embarking on a total "re-do" of the rats nest that passes for an electrical "system" on my (new, old) boat and I am mapping out how to elegantly tie things together in one big unified bonding/grounding system as per a tidy ABYS diagram shown in one of Calder's books..... and now I read hear that doing exactly the opposite is the key to non-stray current nirvana. Help! What's a simple ("always wanted to work on an automatic transmission") man supposed to do here?
Read the referenced documents in post #20.

Everything is a trade-off. Bond everything and invite same boat galvanic corrosion and stray current corrosion. Don't bond and invite catastrophic damage from a lightening strike as it blazes a path to the thru hull via the fiberglass (vs the bonding system).
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Old 15-09-2010, 20:19   #27
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Originally Posted by fairbank56 View Post
Typically, boaters are mis-informed and under-educated on this subject. Personally, I have 35 years experience as a marine electronics service technician and have had formal training on this subject and have done extensive reading on it as well. A good technical library is your best friend here. Nimitz Library at the Naval Academy for me. Anyway, here's a couple links to get you started.

http://www.kastenmarine.com/_pdf/mbqCref.pdf

http://qualitymarineservices.net/Tra...%2010-2006.pdf

Eric
Eric,

Thanks for the references, indeed good reading.

I choose the below method for connecting my Isolation Transformer, which the authors of the referenced documents seem to disagree with but it is supported by ABYC. The key to this method is the ground fault breaker in front of the IT as it won't allow a primary winding fault to the shield from seeking a ground.
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Old 15-09-2010, 20:34   #28
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DotDon-
Referring to your #22...you are correct...these are not designed paths but they are paths that must be investigated during a corrosion survey. Sorry if I wasn't clear.

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Old 16-09-2010, 04:15   #29
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I choose the below method for connecting my Isolation Transformer, which the authors of the referenced documents seem to disagree with but it is supported by ABYC.
That drawing is not in accordance with ABYC rules. Shore power ground should be connected to the transformer shield.

11.17.5
Isolation Transformer System with Single Phase 240 Volt Input, 120/240-Volt Output with Boat Grounded Secondary. Transformer Shield Grounded on the Shore. Transformer Metal Case Grounded on the Boat.
11.17.5.1
Each ungrounded shore current carrying conductor is connected from the shore power inlet to the primary winding of the isolation transformer through an overcurrent protection device that simultaneously opens both current carrying shore conductors. Fuses shall not be used instead of simultaneous trip devices.
11.17.5.2
The shore grounded (white) terminal of the shore power inlet is not connected on the boat.
11.17.5.3
The shore grounding (green) conductor is connected, without interposing switches or overcurrent protection devices , from the shore power inlet to the transformer shield.
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Old 16-09-2010, 04:58   #30
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It gets worse

This is one of most misleading articles. I have ever read on the subject. The author dimisses isolaton transformers, simply on the basis that a hot fault on the orimary could render the system dangeous. ( a simple RCB fixes that failure mode anyway) He then goes on to reccomend a "fail safe"galvanic isolator. Total and absolute and even dangeous advice.

Secondly the incoming ground to the transformer shield, is only a ABYC requirement, which is not a mandatory requirement, its interesting to not that teh EU's RCD specs, which are mandatory do not require any such thing. It spurely a protective thing and in my view far better protection is served by fitting a 30ma whole boat RCB just after the shorepower inlet ( as is used by millions of EU boats).

Remember the ABYC is an organisation that just discovered RCB's!!.

An isolation transformer is not installated anywhere to primarily protect against impressed corrosion. Its installed to protect humans. It does this by removing the ground return path from the supply. Hence you can only get a shock by inserting yourself into the circuit.

Its a side effect of them that allows the protective earth to be discontinued so as to eliminate fault currents that cause impressed corrosion. Arguably with the preponderance of double isulated devices and the the use of RCB's the protective earth , role is greatly deminished anyway ,as its historical role was to blow a fuse on a hot fault.

to back to DockHeads problem. If he has a proper functioning isolation tranformer, with no connection betwwen his boats mains circuits and the shorepower ( live neutral or earth etc). Then the only other issue that is causing such corrosion is DC stray current from his own boat. As has been pointed out normal Galvanic corrosion ( which is what the zincs are there for) is a slow process and one that is not stopped by isolators or traffos.


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That's good to know since it means that it is "not possible" to get electrocuted by falling in the water in the vicinity of a shore power cord that someone has dropped in the drink.
Not by currents entering and EXITING your underwater fittings no. Only by you , in the water , coming in contact with a lower earth path then the water. IN europe this is is a non stater as the RCB is used everywhere and trips immediately.
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