Electricians? Stray current- Conclusion

Okay, maybe I am confused. If I understand (not sure I do) the engine block which should be at ground is reading +0.2V above ground and the shaft is reading +0.6V ? If that's right it sure sounds to me like you have a faulty ground connection somewhere on that engine/shaft side. If the connections were sound it should be at zero volts. There is a 0.4 V difference between engine and shaft, and shaft brushes may help that, but that doesn't explain the +0.2V on the engine.

I am guessing you have more than one grounding bus or bonding strip that tie back to a main or central bonding strip. Have you tried that measurement at different reference points - like a negative battery terminal? Have you checked for voltage differences between the various grounding points / bonding strips?

Check you PM's.
 
I believe that is why the marine electrician uses the half cell to water to use as a ground reference, a starting point since the boat no longer has a reliable ground(if that makes since).
Cause he is then also probing ground wiring in engine bay along with block , shafts etc to find current.
Here is a pic of the shaft brushes I installed before the first of the year. They were relatively cheap, I did both shafts for 80 bucks, again this was an electroguard product I bought locally.
Not sure what the brushes are really gonna do for ya if ya have a half volt floating around in your system. The brushes may just be another transfer point to carry the current. The brushes are either connected to your bonding wires, or engine block.
Just for Grins Bruce, disconnect the bonding wire from you engine block and then test your bonding wires somewhere, do a before and after.
Also, if you are running with twin engines-Are they grounded together, say like with a # 2 battery type cable? if not for another test just take a jump cable that you would use to jumper a battery and connect your two engines so you have them grounded together, then test again.
Try a couple different grounds like DD mentioned also, since you do not know for certain where this stray is comming from. Good Luck
DVMandshaftbrushes-4.jpg
 
Well, I know couple of things. The voltage is not supposed to be there, and there is some resistance between the other stuff and the engines, also the voltage is in the range of what would be produced by freely eroding stuff relative to the zinc bonding system. Shaft brushes will eliminate that voltage and since everything is supposed to be wired together that is a start. Since the paint problem is associated with the parts that have this voltage that’s where I am going to start.
 
nav,

That's pretty much why the reference half cell is used. It provides a fixed point of reference (if it's a good half cell)independent of the boats ground point. With that, you can do exactly as you say - test for potential differences relative to the half cell potential.

One of the better ways to use it when probing for a galvanic corrosion problem is to disconnect everything from a bonding strip with all switches off, measure the potential, then systematically reconnect each bonding wire one at a time and remeasure. The change should be no more than a few millivolts - about 10.

Looking for a stray current is a bit of a different beast. Finding a voltage difference from ground when it should be at ground tells you there is a problem in a connection or a broken ground path somehwere, but it doesn't tell you what the path is. For that you need to start looking for currents. If there is a stray DC current you should be able to find the arm by using a multimeter in current mode placed between the positive of a battery and the battery cable - all breakers and switches off. Current should be zero, unless a bilge pump or stereo or something running on the battery side. If that's okay, then turn breakers on one at a time, so long as there is nothing on that circuit turned on current should remain zero. If it doesn't then you go looking on that circuit for a problem.

The brushes are intended to assure that the shaft has a good ground connection, which is typically difficult to achieve through the transmission. An additional element is a simple grounding strap connected to both sides of the coupler - it doesn't even need to be insulated - just a good copper strap across the coupler.
 
quote:
Originally posted by pdecat

Well, I know couple of things. The voltage is not supposed to be there, and there is some resistance between the other stuff and the engines, also the voltage is in the range of what would be produced by freely eroding stuff relative to the zinc bonding system. Shaft brushes will eliminate that voltage and since everything is supposed to be wired together that is a start. Since the paint problem is associated with the parts that have this voltage that’s where I am going to start.

Click to expand...


I would encourage you to find the reason for the errant ground connection first. If the engine block that reads +0.2V is well connected to the bonding strip you used as a reference, then there should be no voltage difference even if you have a galvanic problem.
 
right, so the connection must be bad. I am just going to add a new connection as the original is in a very difficult location.
 
Installed shaft brushes and more bond wires today everything measures zero. Tomorrow I'll start from scratch, measure everything everywhere again, and see what's up but since the problem was props etc I am a bit more confidant.
 
pdecat, Nice.
When ya get the time can you explain more about adding more bond wires. I would not mind seeing zero myself.
Thanks
 
Extra bond wires were simple. 14GA tinned stranded green wire from HD. they didn’t have 10GA which the boat maker used, oh well. HD also had marine quality ( silver or tinned ?) crimp on eye connectors. I used those twisted edge washers instead of split lock washers for better electrical contact. Bolted new wire to the new brushes and tomorrow I’ll ad another wire to the engine direct to the buss bar.
 
Good work.

I disconnected and sandpapered all of my engine and battery connections. I also coated them with dielectric grease from an automotive store to slow future corrosion. This got my disappearing zincs under control. I highly recommend the Nigel Calder book.....that Dive Daddy mentions. He talks about measuring the voltage between the negative battery post and the disconnected negative cable. Remove direct wired bilge pumps one at a time looking for voltage changes. Then disconnect one terminal at a time on the negative buss until the offending circuit is found. Disconnect shore power and make sure genrator is off before starting.

If you are baffled by a stray current problem also consider taking a measurement away from your slip. Sometimes your neighbor's boat is helping the problem!

Good hunting.
 
I think I am finished, after receiving a lot of information and advice from several folks but especially Divedaddy and GVP and Wayne Alexander I will have to wait for the divers reports on new paint after a few months. The only problems I found were four unbounded sea strainers and no brushes on the shafts. GVP explained that shaft flange couplings can build up resistance where none was previously through rust.
The mystery voltage at the dock power ground is probably the result of using the alligator clamps I was using to hold the test metals in the water. A careful look at the silver chloride cells shows that everything is well insulated except for the test metal. The voltage varied up to .5V but I discovered that by keeping the clip out of the water and trying Zinc, SS and copper as the electrode that the readings did were different for each metal. My conclusion is that the clip was overpowering the metals when everything g was submerged and creating a couple with the copper house ground rod.

The other interesting point is that CorrosionPro stuff that I spray on everything is an excellent insulator. I was puzzled over poor conduction readings on the newly bonded strainers until I filed down the surfaces to bright metal. The CorrosionPro actually prevented the meter probe from making electrical contact despite scraping and heavy pressure. Perhaps a sharper probe would have worked but Corrosion Pro is impressive. Sure wish I could find it again around here but now I have some CRC stuff and Boshield. I hope they are as good.

After all was done I measured .6V across the zinc saver so it must be doing its job of blocking the DC. Today the boat gets hauled and paint repaired then we’ll see.
 
I know this is an old thread, but it's a great one - I'm fighting massive corrosion right now so hopefully you guys won't mind helping a newb...

When checking the reading at the block, what probes your you holding where?... yes, I'm electrically challenged but am very mechanical and hope I can learn this process.

Thanks in advance.

quote:
Originally posted by divedaddy

Okay, maybe I am confused. If I understand (not sure I do) the engine block which should be at ground is reading +0.2V above ground and the shaft is reading +0.6V ? If that's right it sure sounds to me like you have a faulty ground connection somewhere on that engine/shaft side. If the connections were sound it should be at zero volts. There is a 0.4 V difference between engine and shaft, and shaft brushes may help that, but that doesn't explain the +0.2V on the engine.

I am guessing you have more than one grounding bus or bonding strip that tie back to a main or central bonding strip. Have you tried that measurement at different reference points - like a negative battery terminal? Have you checked for voltage differences between the various grounding points / bonding strips?

Check you PM's.

Click to expand...
 
quote:
Originally posted by pdecat

Extra bond wires were simple. 14GA tinned stranded green wire from HD. they didn’t have 10GA which the boat maker used, oh well. HD also had marine quality ( silver or tinned ?) crimp on eye connectors. I used those twisted edge washers instead of split lock washers for better electrical contact. Bolted new wire to the new brushes and tomorrow I’ll ad another wire to the engine direct to the buss bar.

Click to expand...


I hate to say it it those bonding wires should have been at least #8 wire.
 
This thread may be dead, but I'll post anyway and hope it comes alive.

I picked up the Silver Silver/Chloride reference unit from Boatzincs.com and did some testing. In testing the Bonding strip and Engine Block, It appears that my battery charger may be the culprit but I'm a little confused... when the charger is charging I'm getting a reading of just over 30 millivolts more negative. That is, if my base reading is -1.036 with the charger off, it is reading -1.060 with the charger on. Does that make sense in terms of creating a corrosion issue?
 
quote:
Originally posted by GilligansIsland

This thread may be dead, but I'll post anyway and hope it comes alive.

I picked up the Silver Silver/Chloride reference unit from Boatzincs.com and did some testing. In testing the Bonding strip and Engine Block, It appears that my battery charger may be the culprit but I'm a little confused... when the charger is charging I'm getting a reading of just over 30 millivolts more negative. That is, if my base reading is -1.036 with the charger off, it is reading -1.060 with the charger on. Does that make sense in terms of creating a corrosion issue?

Click to expand...


You have two issues. The first is why there is a change when the battery charger is energized and the second is that you may be over protected. Zinc by itself read with a silver-silver half cell should read -1.05 and I am presuming you are using zinc and not magnesium.

Be careful about over protecting, it can cause problems.

I would expect readings around -0.75 volts maximum. For example, if you are reading over -0.8 on your shafts with bronze props attached, those props are over protected. JMHO!
 
Thanks. I have a fiberglass hull with aluminum outdrive and Zinc anodes.

According to the chart the reference anode (silver-silver/chloride) came with a proper reading should be between -.850 and -1.100. A more positive reading indicates under protected and a more negative one indicates over protection.

I'm not showing a more neg reading with the charger on by 7mv more negative - not sure what that means.

I did find a loose wire on the charger which helped my readings.

...still trying to learn

***apparently having to much sacrificial anode cause other problems, like paint be blown off the drive and exposing the aluminum.
 
You must log in or register to reply here.
Back
Top