Stray current detector

Discussion in 'Boat Design' started by Katoh, Jul 12, 2011.

  1. Katoh
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    Katoh Senior Member

    Gentlemen
    You are quite correct, part 3,4 have been drilled with a 3/4" 19mm drill to allow access a "bore camera" cant think of the right name at the moment. basically a snake through the hull to view the internals without excavating the deck. The others are as is. Do you want a photo of the latest hole I found, swear it was made by a 10mm (3/8) drill, but is not!
    Yes some of the previous wiring was automotive related, but what really caught my interest was when the fuel tank was removed, it was totally submerged in water, if that did not create a leak in current to the hull, well I dont know what will.
    The question again is can we wire in, or somebody have an idea for an alarm, meter or something along those lines when stray currents happen you are alerted to them. It may never happen but if it does you would like to know.
    I have no doubt this boat was abused! I have owned numerous aluminium boats and never seen this sort of corrosion, and must add never want to again.
    I include a close up of piece 1 typical, from outside and inside. if you like I will show you a photo of Hole No.7 still on the hull which I only just found.
    Again this is what stray currents will do. Or incorrect wiring.
    Katoh
    Just to add that's 5mm pate we are dealing with.
     

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  2. GTS225
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    GTS225 Junior Member

    ****************************************************

    OK....Perhaps I haven't been around boats long enough to have witnessed some of what the rest of you have, but I have seen aluminum corrosion. I will concede that most of what I've seen is from the automotive world.
    From what I've witnessed, holes don't corrode into aluminum plate in such a shape. In my opinion, the holes in parts 1, 2, 4, & 5 are "too round" to have been general corrosion. Aluminum doesn't come out of the mill with such a uniform granular structure that a hole will corrode through it in such a way. What I've seen is irregular-shaped holes, with usually larger areas of pitting around the hole. Those don't seem to be the case. As for pic three, it appears to me to be irregular enough that it could have been corrosion. Katoh's last post proves me wrong in this case. He indicates that hole was drilled. I suspect it was drilled some time ago, and then corroded into it's irregular shape.
    Something I may not be familiar with is the effects of galvanic or electrolytic corrosion. It may be that most of the corrosion I've witnessed was from chemicals applied to the aluminum, (as in road salt or battery acids).

    Katoh; are you saying that only parts 3 & 4 have been drilled, and the rest are holes strictly from corrosion?

    Roger
     
  3. Katoh
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    Katoh Senior Member

    GTS225
    Parts 3 and 4 are the only ones that were drilled, they too were exactly the same as 1 2 and 5. I drilled the 3/4' hole to put in a bore camera to see what the inside of the hull looked like. I will post a photo later today of the latest hole I have found in the hull, this piece is still on the hull and has not been touched by me, it is nearly perfect 10mm dia.
    Have a look at the photo, this is the sort of corrosion that your thinking about, this is part of the fuel tank were the foam was up against it and wet. This form of corrosion has nothing to do with stray currents, and using the hull as a conductor.
    Katoh
     

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  4. viking north
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    viking north VINLAND

    Katoh--I seem to remember all this was covered on a prev. thread some time back. The corrosion in the photo is known as poultice corrosion commonly seen where wet wood is in long contact with alum. In your case wet foam-- it results from the interaction of the chemicals (acids) from wood or foam leaching out and interacting with the aluminium. I also seem to recall it was pointed out that your boat was possibly manufactured from a mis labeled batch of aluminium, something other than the 5000 series. Specifically the most common 5052. Did you rule out that possibility by having the metal analyzed. Most scrap metal delears have an electronic sensing tool that can easily determine not only the metal but the specific catagory.---Geo.
     
  5. Katoh
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    Katoh Senior Member

    Hi Geo
    Yes your on the money again, I was trying to point out the differences in corrosion.
    The aluminium is 5083 confirmed and actually the hull is in relativity good shape. above water line you would be hard pressed to find any problem's with it at all. but below water line so far we have found 7 spots, holes and all quite random.
    Anyway I will add the latest hole photo,this is still on the hull, and no it has not been drilled.
    Katoh
     

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  6. erik818
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    erik818 Senior Member

    Katoh,
    There exist RCDs also for DC but they are rare and probably not very sensitive. I couldn't fine any on the internet, just a text that say they exist. An RCD for AC works like a summation transformer where there will be an AC voltage on the sence winding if the power windings (if one turn count as a winding...) don't sum up to zero current. The difference is of course the stray current. Judging by the performance data for clamp ammeters for DC current, the sensitivity would be some 10 milliamps.

    AC RCDs are cheap, but an RCD for DC would need a sensor for a static magnetic field. I don't believe that any other strategy than a summation transformer will give sufficient accuracy to find stray currents of a few milliamps from the total current of 10 - 100 amps. It would certainly be possible to design a device that measure the static magnetic field from the missing stray current in the power leads, but probably not cheap. A transformer core with an air gap and a fluxgate sensor should be usable, but I havn't (and don't intend to) calculated the magnetic field strength that has to be detected.

    I would suggest another strategy. Arrange the battery groups as one start group and one battery group for the rest. The generator and the start motor are firmly connected to the start battery. The only grounding point from start battery negative to hull should be through the engine, which is difficult to isolate from the hull anyway. The other battery group and all electronics devices in the boat are connected to second battery group. If you break the positive connection between the battery groups any current in the neative connection is stray current. That DC current is easy to measure. All electronics are still energized but the second battery group is not charged when disconnected. Accept that you can't measure the stray current in the starter battery group.

    I have two batteries in my boat, and have arranged relays that automatically disconnects the positive lead when the generator is not charging. The reason is that my boat is old, wood and occasionally leaking. I want two truly redundant bilge pumps when I leave the boat for several weeks. A similar arrangement might be possible for you, and an ammeter that is automatically inserted between the negative battery connections when the positive side is disconnected.

    Erik
     
  7. erik818
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    erik818 Senior Member

    I finally found DC sensitive RCDs available for purchase on the net, at a Swedish company I've bought electrical materiel from before. They're called Type B, and can be bought for about EUR 1500. For reference, I've bought RCDs sensitive to AC and pulsating DC (Type A) for EUR 30. The Swedish norm for personel protection is 30 mA so this is the sensitivity.

    So far I've found two brands of DC sensitive RCDs: CDB463F from Hager and 5SZ3 466-OKG00 from Siemens.

    It seems like a cheaper idea to arrange the electrical system so the stray current can be measured in the negative lead between the second battery group and the start battery group instead.

    Erik
     
  8. Katoh
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    Katoh Senior Member

    erik818
    I thank you so much for the information you have given me.
    I really like the idea of the twin battery set-up with dedicated instrument battery and dedicated motor battery, that actual makes a lot of sense and is very doable.
    The other thing that I was thinking is using a megger (meggerohmmeter) meter to initially test all the wiring for shorts or leakage, the only thing you would have to do is check the wiring periodicity, say ever time you did your motor service. That could also be another option.
    Going back to the two battery system,
    Is it not good and common practise to have all negatives from whatever source battery, or power should always be bonded together through a negative link or bus bar. in doing that will still have a common negative and regardless have a reading from hull to active regardless of battery, still an excellent thought.
    Cheers
    Katoh
     
  9. philSweet
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    philSweet Senior Member

    The usual way is with a reference silver anode. Not cheap, but if you have a suspicious situation, I'd splurge on it. Available at boatzincs.com.
     
  10. MikeJohns
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    MikeJohns Senior Member


    I agree with Poida

    You say the hull is bonded to the battery -ve terminal therefore the damage you have is not from current returning through the hull to the battery negative.
    You would need very large currents to produce enough voltage drop so that the seawater starts becoming an alternate conductor to the alloy. The voltage drop along the immersed hull from the lighting circuits using the hull return would not be measurable and certainly not problematic IMO.

    That level of erosion is from Electrolysis from an external source, not from the electrical system within your boat.

    You are mistaken that any amount of current flowing through the hull itself makes the hull corrode, it's the voltage difference between the hull and the seawater that you are really interested in.
     
  11. viking north
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    viking north VINLAND

    Exactly Mike as per my post #8. If all the electrical circuits within the hull are run two wire (one pos. one neg) and those conductors are of the proper size to handle the load , the current will always take the path of least resistance. Thru the copper, it being a better conductor than alum. To be sure with alum. hulls, I always wire oversize as the neg. bonding does set up a hull metal parallel run with the neg. wiring leads. As you state any flow thru the hull would be so small that I doubt it could be measured and as such would not create problems. If the wiring within the hull is correct--I.E. no floating grounds-- No pos. floating leads sitting in water or fastened to an electrical device sitting in water with that device switched off thru an automatic switch at the device then any electrical action has to be from an outside source. The only protection from such is the proper installation of zincs, an active anti device or move the boat out of the area. Also as stated before if kept on a trailer of a dissimilar metal be sure to take precautions. Geo.
     
  12. MikeJohns
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    MikeJohns Senior Member

    From a metallurgical approach, you set a safe electric field in the water before corrosion could start. eg 20mV per meter. Then as a very rough guide with lots of safety factor take a strip of the hull plate say 6m long and 1m wide. Calculate its resistance :

    R = ρℓ/A
    Rho (ρ)of Alloy is around 27 nΩ m
    A is cross sectional area ( 1X .005) for 5mm plate
    ℓ is 6m in this case

    I make that 32.4 micro ohms

    If the electric field was 20mv/m then you'd need a v drop of 6X 20e-3 or 180e-3v

    Using V=IxR the current through the plate to produce that possible external electric field in the seawater would need to be :

    I=V/R =(180e-3)/(32.4e-6) =5.6 thousand amps !

    Check the maths, I did this on the fly but it gives you an idea why even a lighting circuit return using the hull cannot cause any direct problems. In reality there's oxide layers and paint to overcome too which protect the material considerably.

    I think you could use an alloy hull quite safely for the return conductor and run one copper cable to each electrical device. The way you do in an automobile or the engine circuits on the boat.
    There's a lot of urban myth that creeps into boat building regs. Driven by a misunderstanding of what actually caused the problem in cases like this one. The 'knowledge' compounds and becomes a compelling body of 'fact' based on a lot of miss-observation.

    In the case of metal hulls the hull replaced the bonding system and the rules for the bonding system were simply transferred to the metal hull. A bonding strap is relatively small and shouldn't be used as a negative return, but a metal hull is really immune from the same problem.
     
  13. viking north
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    viking north VINLAND

    In a perfect world I agree but as is common in the automotive field the accumilative effect of even minor corrosion at dis similar metal connections over a period of time effectively breaks the return circuit path shutting down what could be a life saving piece of equiptment. Thus the importance of the two conductor system. This would be compounded of course by the copper to stainless to aluminium connection and where subject to constant dampness or actual emersion the copper is effectively electrically connected to the alum. thru an electrolite. The other factor here is the circuit amperage at full load--Bow trusters, electric windlass, and so on. One wouldn't want 50 to 100 amps regularly returning to the power source thru an alloy hull. While I basically agree with you on the copper penny in the bilge myth overall the two conductor system is by far superior in long term reliance. In the case of Katoh's problem It's a hard one to analize without actually being on site. I have a feeling his corrosion has little to do with wiring but a combination of wet foam in long term contact with the alum., sitting on a dissimilar metal cradle/trailer, some outside stray currents and still suspect some mis labeling alloy got into the fabrication.
     
  14. MikeJohns
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    MikeJohns Senior Member

    Viking
    I agree and there's also a big difference between a sealed decked over ship and the open and often wet conditions in a runabout. I've also seen a few old Dutch and German steel vessels that used the hull as an earth return for many circuits. There could be many ways of creating a reliable connection and we can seal dissimilar metal from oxidation or galvanic action very effectively these days. All I'm really saying is that the current through the hull and framing in itself doesn't actually cause corrosion, but it is a commonly held belief.
     

  15. BATAAN
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    BATAAN Senior Member

    ABYC or insurance surveyor would be horrified at someone using the hull for return ground. If that is the case provide return ground wire for all accessories. Best is rip it all out and wire it right in the first place with jacketed, tinned, two-conductor wire, carefully and completely isolated from ground on the hull except for one ground point very close to the engine, as is standard boatyard practice.
    In electricity, like rigging, remember the old phrase, "If it ain't right, it's wrong". I had a stray current experience once.
    Photo is of my shaft after hooking up an advanced automatic battery charger and blissfully leaving it plugged in continuously like the manual said.
    Now I plug it in to cycle and charge the batteries, then unplug it and go back to my proper ABYC DC system and totally floating, isolated AC system with GFI protection.
    This is a boat kept afloat in salt water.
    I don't think your problem is stray current, and if it is and that's the damage you can see, what's it like in the hidden areas?
     

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