Isolation or not?

[humble]

I was asked to install an isolation transformer on a vessel. It had just returned from the shipyard after extensive repairs. New shafts.
The other shafts were pitted. The vessel was moored next to an old vessel with possibly ailing wiring. I asked for advice as to what to do and I get conflicting stories. One guy said that there is no threat of corrosion from the shore power connection from AC. He said that only DC can cause it. Another guy said that when AC power strays out of a vessel that the zincs act as diodes and so then it is DC stray current that can corrode everyone. I'm sick and frustrated. I get so many stories that I do not know which way to turn. On a topic that seems like it should be possible to get a straightforward answer, all I get is opinions that conflict. So I ask the group. Isolation transformer or not?

 

[roger]

Re: Isolation or not?

Humble, A/C in it's normal state can not cause the Galvanic action. Think about an "Anode" and a "Cathode". In A/C these two components of D/C are changing constantly.

In short what is happening is the salt water, boat hulls, props, anodized finishes, and other contributors are all coming together to "create" a D/C stay voltage. Dissimilar is the word

Roger

[ November 29, 2003, 05:38 PM: Message edited by: roger ]

 

[humble]

Re: Isolation or not?

Roger,

Thanks.
Can the switching of AC how it switches back and forth as you say take metal with it from which ever metal is the anode for the thirty times a second that piece of metal is the anode?

 

[roger]

Re: Isolation or not?

Humble, first of all the + and - peaks of our A/C cycle would occur 60 times each in a second. The anode would be the structure, boat hull, prop etc....

In essence there would be no "anode" or "cathode" for sake of conversation in the part A/C plays here.

The D/C voltage is a product of the stray current on the ground or induced current on the boat hull, shaft, or prop. (metallic)

The items mentioned in my first post are actually acting as a rectifier in a generic sense.

Maybe an engineer or another member with better instructional skills will come forward and translate this better than I.

Roger

[ November 29, 2003, 07:40 PM: Message edited by: roger ]

 

[humble]

Re: Isolation or not?

Roger,

I think I get it now. The AC is rectified by things like the water or zincs or whatever.
Then the DC does the damage. Is that right?

 

[bennie]

Re: Isolation or not?

Take a look at this, you may have to type it in.

http://www.marinesurvey.com/yacht/corrosion_1.html


Bennie R. Palmer

[ November 29, 2003, 11:11 PM: Message edited by: bennie ]

 

[humble]

Re: Isolation or not?

Thank you Bennie!!!

Man it's good to hear from you.
I thought you sailed away to that island.
I went to that site and it has answered my questions.

Dave

 

[bennie]

Re: Isolation or not?

Dave: Good to see your posts again. I am planning to make the trip to the Pacific Islands about January 2004. It all depends on my health. I have my passport and a lot of camera equipment.
I should get some good pictures of corrosion of ships, if there is any left. I won't be able to dive, so that is out. I may hire a diver to take underwater pictures. I have an enclosure for my camera.

You can see by the articles at the website, that corrosion is created by a small amount of common mode voltage and current. Rectifying is much easier at low power. A small amount of oxide makes a MOV.

Catch you later; Bennie

 

[hurk27]

Re: Isolation or not?

The problem Is that not all boaters will have there electrical up to par and even though your wiring can be in good shape the boat sitting in the next slip might not be and will cause damage to your boat. For those who think electrolysis don't happen on a AC circuit think again as any one who has had to replace a aluminin post light pole because of the bottom being gone where the pole meets the ground knows. I have seen ground rods that there was nothing left but a stub when stray current was placed on it. We had a aluminin boat that had a miswired panel (neutral/ground bond) that lost the hull at the waterline the boat didn't even have a batery or engine on board but had a shore power line. In electrolysis with AC current the metal is removed on the positive cycle but not all the metal is returned on the negtive cycle. this is one of the reasones a battery goes bad after many charge/ discharge cycles. the plates are nolonger there. But it is still called electrolysis. The only way to prevent this type of damage is to ether have a alarm on the ground wire that will let you know when there's current being pulled on it. or an isolation transformer on the boat. I have seen a few that tryed the alarm aproch and after a few times of not always being with the boat they would switch to isolation.

[ November 30, 2003, 09:55 AM: Message edited by: hurk27 ]

 

[hurk27]

Re: Isolation or not?

Oh the other arguement is isolating the boat grounding from shore won't provide a fault current path. well it will as the fault current is seaking a path of return. it's not getting sea sick and wanting to get to shore. the return path will be to the isolating transformer. This also removes any shock hazareds when you have swimmers in the water close to a boat. we did have a case where we lost a neutral on the service equipment that was transfered to the boat luckly nobody was hurt. and use alot of dielectric grease on connections.
Ok I'll quit. LOL

 

[humble]

Re: Isolation or not?

Bennie and Hurk,

You guys are great!
You know, I've been here in Olympia crying wolf while everyone else installs galvanic isolators.
The boats that I install these transformers on stop corroding and it has been because of this site and you guys that I have been doing it. The trouble was, I went away from here and all I heard was that you guys at this site were wrong. That isolation transformers won't and cannot work and that there is no such thing as AC corrosion. I kept the faith though and have sold about a dozen so far. I was getting tired of lugging the heavy things on the boats which are considerably heavier than the galvanic isolator so was looking for reasons to quit doing it. I started listening to the people against doing this action and almost quit installing them all together. Then, in frustration, I thought I'd come back here and get some more advice. I'm glad I did. It has restored my faith in using isolation transformers and in you guys, and this site. Check out my website. www.garnermarine.com The product on there is the result of your advice here. We use a toroid because the EI type of transformers induce current flow on the DC circuits and these toroids are magnetically silent

 

[roger]

Re: Isolation or not?

Wayne,

For those who think electrolysis don't happen on a AC circuit think again as any one who has had to replace a aluminin post light pole because of the bottom being gone where the pole meets the ground knows.

I have seen problems of this nature also, and measuring voltage on D/C scale showed small D/C levels from pole to base bolts if wet.

Is this another case of A/C to D/C conversion or strictly chemical generated?

Roger

 

[hurk27]

Re: Isolation or not?

I don't think there is any conversion involved. I would think it to be more of a electro-chemical process as I stated before yes it could be related to DC just by what happens on the half cycle. And this would be the same process that any rechargble battery would go through at a much slower pace. as a battery is drained it looses mass from the anode to the cathode. When it is charged back up it reveres this process by removing the mass back to the anode from the cathode. But it never gains all the removed mass. so this is why a battery fails over time. We have this same process with AC current but at 60 cycles a sec. But with earth the metal mass is distributed evenly in the earth so less is gained back into the anode in the reverse cycle. this is why we see that big clump of white mass around a break in under ground wires. Now when it comes to metal in water that has motion (waves) the removed mass is no longer there to be put back so the effect is much faster, and the loss of orginal mass is faster this is why boats with aluminin hulls have a big problem with this. and even stainless steal will crode.

 

[madmike]

Re: Isolation or not?

The Navy and Coast Guard are having a similar problem. Increased corrosion causes big $$$ expenditures for additional maintenance. However, their problem is due to 480/277 delta-wye source transformers at the pier substation. With XO bonded to ground at the transformer and three wires (phases) feeding the ship, any unbalance in the load will allow net currents to flow via the ship distribution system capacitance to hull, then through the water, back to the XO terminal. This can be as much as 20 amps per MIL-STD-1399-300. The Coast Guard has solved this by trailer mounting a 750 kVA transformer, delta-delta, to eliminate the net currents and hence, the hull corrosion.
r,
Mike

 

[charlie]

Re: Isolation or not?

Mike, did they take the grounded conductor to the vessel? It seems to me that if they had no line to neutral loads, there would be no current in the water. If there were and the neutral was taken, the current in the water would be minimal. I do agree with their solution since any current in the water is bad news.

 

[madmike]

Re: Isolation or not?

Charlie,

No, No ground wire in the service running to the ship.

Even without L-N loads tho, EMI line filters often account for hull currents returning to the pier service.
r,
Mike

 

[humble]

Re: Isolation or not?

Where did you get this information Mike?

 

[madmike]

Re: Isolation or not?

humble,

Not sure what you are asking. The hull current issue has been solved by the Coast Guard, the Navy is still trying to figure it out. Call Al Boudreau, Coast Guard Facilities, Kodiak at 907-487-5320, ext. 231. He designed the isolation transformer picture and can give lots of details.

The hull currents due to EMI filters are discussed in MIL-STD-1399-300, paragraph 5.1.3. This std can be downloaded from http://www.jsc.mil/jsce3/emcslsa/stdlib/LibDisplay.asp?folder=ms

r,
Mike