Electrolysis - Grounding

My shaft zincs were gone in 3 months. My boat is not usually connected to shore power. My 12 volt and 110volt systems share a common ground. When I’m connected to shore power I measure 3volts AC being conducted through the ground. It goes away when I turn off the water heater. The DC volts measured through the ground is less than 1 volt, which my isolator should take care of. My Questions – Should the grounding systems be separate? If I’m conducting 3 volts AC though the ground will this eat up my zincs?

 

I can't speak to the grounding issue, but what type of zincs are you using? I have had this same problem when bringing a fresh water boat into saltwater and forgot to replace the magnesium anodes. The change in salinity changes the electrical potential and can quickly eat fresh water annodes.

 

I not using magnesium I'm using zinc and in saltwater.
I like your quote!

 

West Marine has IMHO an excellent article on marine grounding systems written by Stan Honey, an electrical engineer. It explains everything in pretty simple terms and is easy to understand. It would be a good starting point. You can download it from their website.

 

Temporarily remove the ground connection and measure both AC and DC current in milli-amps at that point. Let me know the results.

 

CDR and Chuck,
Thanks for your responses you guys are great. I looked at the west marine article and will also measure the milliamps. How many milliamps is bad?

About the bonding of the AC and DC grounds, it looks like they should be connected as long as DC wires aren’t being used for the AC system and vice a versa. I was told that if something shorted, like the battery charger, that you don’t want to feed 110 volts in the DC system without it being able to find a ground and trip a breaker.

As for the 3 volts AC measured across my ground, a tech guy at Yandina said first fix the problem – maybe loose ground, or buy one of their galvanic capacitors and install it parallel to the galvanic isolator to drain off the AC voltage.

 

There is one fundamental difference between these two.

DC ground is the return path for DC positive; the sum of all currents running through DC+ also runs through DC ground.

AC ground is only protective and should carry no current at all *(see below). The sum of all currents running through AC live returns through AC neutral. A safety device can detect small differences between these two and cut the power if a threshold is exceeded. The presence or absence of AC ground is irrelevant for this function.

Let me know what you measured before buying any gadgets.

* A small AC current can be caused by electronic equipment with RF filtering circuits, but that is limited to a few mA's without a DC component.

 

That is correct. Both AC and DC systems, as per ABYC standard, are to share the same ground.

Also, CDK is correct.

It's a lot of work tracing down a current leak, but keep at it. You are approaching it in a logical, detail oriented manner, so I'm certain you will eventually find the leak.

It's important to make sure you get rid of that leak.

I'm a catamaran guy. I once measured a sizable current on a new boat I bought - the current was between the two saildrives!!! They were eating through props in a couple weeks, never mind the zincs.

 

Electrolysis – grounding

CDK and Catbuilder - Thanks for the correspondence!


I made measurements this morning and now have different information. The parameters are different from my report of having 3 volts AC across my ground. Today my meter was connected securely, nut and bolt, to the 12-volt ground bar in the electric panel.

The readings across the ground for my shore power by itself with everything turned on inside the boat were: AC volts .8, DC volts .047, and milliamps 0.

The reading across the ground for my air conditioner by itself was: AC volts .2,
DC volts .029, and milliamps 0.

I also measured across the galvanic isolator. With shore power only on the AC volts were .8, The DC volts were .33 and the milliamps were 0. With my air conditioner on only the readings was AC volt .5, DC volts .03 and milliamps .019 on the 2m scale, which I believe is 19,000. Is this 19 milliamps?

I’m guessing that these readings are OK and there could be many variables that could cause my zincs to corrode. I’m thinking that every time I power up at a new source that I should take readings across the isolator to see if a problem exists. Can you suggest what should be an acceptable milliamp reading?

 

The readings show that (at least this morning) no outside current is fed to your boat that could cause accelerated zinc loss. The .019 reading on the 2m scale means 19 uA (micro amps); for this case it is the same as 0. Any value in this range is acceptable.
So your electrical system is OK.

The unusual rate of zinc loss can be caused by:

A. paint damage on a steel hull,
B. DC current leakage in a submerged electrical device like a bow thruster,
C. a nearby steel or aluminum boat with shore power and defective AC wiring,
D. a current carrying metal structure at the shore near your boat, like scaffolding, or
E. a transmitter antenna like a cell phone tower in close proximity.

So take your pick!

 

Thanks to all for the postings. I felt a bit helpless looking for help and this forum has been greatly beneficial in steering me to solving my own problem.

Now that I know what to look for I’ll keep a close inspection of my shore connections. I’ll also need to be very careful in making sure I have good contact when using a voltmeter.