E32-3 When AC Current Runs Astray

DISCLAIMER: I am not a licensed electrician on either land or sea. Any alterations mentioned in this article are meant to be references and not suggestions. Electricity is dangerous. Always use safety precautions and call a certified electrician when you feel out of your depth.

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"Noticee"​

It began one day when I arrived at the slip and found Rumour's shore power cord disconnected...



Spelling errors aside, I immediately wondered who was playing a prank on me and why they would do it this way. Upon calling the marina office I discovered the maintenance staff had disconnected the cord a week previous. I had received no phone call, email, or letter. They had detected "1.35 AMP of stray AC current" from the boat and I would need to fix whatever appliance is causing it before reconnecting the power. Maintenance "also suggest a galvanic isolator to be installed." I would later find out the diodes from galvanic isolators only protect against low levels of galvanic DC current -- not stray AC current [1].

The office sent me an email briefly describing the situation with a PDF attached with the filename Hioki test sheet.pdf which appeared to be from a local licensed marine electrician. The next day I met with one of the maintenance staff and he walked me through their test procedure, demonstrating that when he placed their clamp on multimeter the shore power cord to my boat it registered 1.35A of current.

When they placed the meter on the neighboring boat's cord it registered 0A. "See? It has to be your boat then." Never mind this litmus test being logically incorrect -- if I had taken the time to read the entire test sheet, I would have noticed Step #6 - "Leave the vessel connected. Turn OFF dock breaker and recheck readings. Any remaining leak current shown with shore power OFF is coming from another vessel thru the safety ground wire."

I also noticed that the meter was reading 130mA of current while the maintenance technician was claiming it was 1.30A. He explained, "You have to take the range bar at the bottom which has a pattern over "1A" and then add that to the numbered reading on the display... See? 1 amp plus .130 amp equals 1.30 amp."

If you think this is ridiculous and confusing, keep reading.

"Hindsight"​

When I look back on how all of this played out, I realize I broke a couple basic tenants of troubleshooting The first: read everything [2]. If I had read the troubleshooting sheet above and noted #6 to the maintenance staff, the conversation would have ended and they would be on their merry way to blame another boat. I could chalk this up to learning curve as well. I've done basic AC electrical work in the past around the house -- replace a receptacle here, run new Romex with a GFCI outlet there. But, I never took the time to learn the underpinnings of safety grounding wires nor had I the previous opportunity to dig into the marine electricity.

Like most things, I'm grateful for the exposure and the time to learn something new. The experience also provided the opportunity to improve a couple things on the boat, namely the shore power inlet receptacle and adding a galvanic isolator.

It was also good to learn how little marinas care about things like this. They seem to inspect for problems like leaking AC current, are quick to assign responsibility to boat owners, and remain completely negligent when an attempt is made to hold them accountable. Good things to keep in mind. The experience also reminds a familiar pattern which I've used in life: provide the feedback as honestly and directly as possible and accept the outcome.

"The Book"​

One of the first calls I made was to my dad. He's owned at least five boats in his lifetime that I know of, four of which were sailboats. The first question he asked was, "do you have Nigel Calder's book?"


If you're reading this site, there's a good chance you already have a copy of this nautical maintenance bible. If you don't, just go grab a copy now. Mr. Calder provides in-depth explanations behind every system on a boat, and while the photos and tips may not apply directly to your vessel, the concepts will. It's an invaluable point reference.

My attention was drawn towards pages 302-303, which details a Comprehensive Grounding and Corrosion Test Procedure. The pages claim the series of tests will allow anyone with a few items to pinpoint and resolve any ground fault or corrosion problem on a marine vessel. Sounds pretty good, right?

"The Ground Fault"​

Within minutes of running the first few tests I found that there was electrical resistance between the neutral bus bar and grounding bus bar on the electrical panel on Rumour. It was around 35 mega ohms as seen on the meter to the right. Measuring resistance between neutral and green ground should be an open loop (i.e. disconnected) with no outlets or appliances in a faulted state.

So, the boat has a ground fault.



A few more tests pointed to the shore power inlet. Upon inspection, things looked pretty bad there. For one, the prongs of the 30A inlet were a green with corrosion. The contacts were corroded as well and when I opened the receptacle corrosion had grown like moss between some of the contacts. Naturally, when I disconnected the receptacle, the neutral and ground bus bars showed 'OL' for open-loop. The tips of the 10/3 cable (3 conductors at 10 AWG) were corroded and as I started trimming the cable back I found more old mid-80s untinned wire with green crusties. I finally decided to just replace the entire cable run from the inlet to the AC panel.

The most difficult part of the replacement was wrestling with the ancient headliner in the aft port quarter-berth. All the vinegar and gentle tugging at the frozen zipper pulls didn't prevent some breakage. There are some articles about replacing the zippers, but back to our electrical issues for now...



Now that the new power inlet and cable to the AC panel was replaced and the shore power inlet was testing OK with no shorts between busses, it was time to buzz the marina maintenance folks for their test. They arrived shortly with their clamp on multimeter.

By the way, they insisted that their meter bared no similarities to my multimeter, even when I would clamp my Klein Tools multimeter on the same cable and show the same readout minus one extra significant digit. They are, in fact, the same tool with different sensitivities. Keep in mind that the clamp on portion of the meter only measures a difference in current passing in two directions. This means if you were to clamp on an AC cable on a device which is operating normally with no faults, the meter should read zero (0.00) or very close to it. The same current would be flowing over the hot (black) wire and returning via the neutral (white) wire.

This is the next basic tenant of troubleshooting I broke: understand the tools. Had I known how the clamp-on meter worked I would have pointed out that with the pedestal breaker OFF on the dock, there should be no current on either black or neutral cables. And, since Rumour didn't have a ground fault any longer there should be no current returning on the green grounding wire. Unless there was current in the water and it was returning through the bonding/grounding system of the boat. I eventually got there on my own, but after the maintenance staff had left.



The test above with the dock breaker turned off is pretty conclusive. Additionally, when I disconnected the engine ground -- that is, the wire connecting the M-25 engine block to the green grounding bus in the AC panel -- the current on the right side disappeared. The engine is in electrical contact with the water after all. I was convinced. The stray current had nothing to do with the boat. It was originating somewhere else in marina.

The maintenance staff was unconvinced. They suggested a galvanic isolator for the shore power inlet.

"The Galvanic Isolator"​

Okay, fine. I'll play along. Besides, all boats should have a galvanic isolator at a minimum. An isolation transformer is even more appropriate, esp. for cruising boats or boats which move frequently between different marinas where stray currents can vary. So, I ordered a ProSafe FS30 and installed it under the captains seat in the aft lazerette. Even left drip loops in the cables to keep any water away from the contacts.



Looks nice, right?

I tested things again myself, not bothering to call the marina maintenance staff. I had been reading a vibe of increasing irritation and I didn't want to make it worse before I had a solution in place. The tests, of course, turned up the same. Pedestal AC power OFF and ~0.13A of current on the green grounding wire at the boat's AC panel. Shouldn't the isolator be causing a break in the green grounding wire circuit? and this should be preventing current flowing?

Yes and no. But, mostly no. Because it really depends on what type of current we're talking about. Galvanic isolators only prevent small amounts of galvanic DC current in the water from being conducted through the green grounding wire/bus and passing into the boat's grounding or bonding systems. I was dealing with stray AC current in the water, which has enough voltage to close the diodes in the isolator and continue to pass the current through the boat. This makes sense. If there was a ground fault on the boat, you would want the AC current to utilize the green ground conductor to avoid electrocuting people and things. That's why the green conductor is carries the moniker of "safety wire".

Let's take a break and talk about different kinds of currents.

"Galvanic Current vs. Electrolytic Current"​

Wait, there's a difference? They are often confused with one another since the effects are similar in appearance, especially in a marine saltwater environment. This experience dealing with stray AC current became a good excuse to learn more about these concepts.

Simply put in the context of our boats: galvanic current is produced by different metals being immersed in seawater (an electrolyte) while electrolytic current is generated elsewhere and introduced into two metals in seawater.

Electrolysis occurs when an electrical current strays from its path due to improper wiring or a defect coming between two metals in the presence of an electrolyte, usually seawater in this case. Metal hull boats are particularly at risk because the hull is conductive and stray wire or connections use the hull as a ground. The two metals can be the same or different [3].

Galvanic corrosion is when two different metals are in contact in the presence of an electrolyte. One metal will be more chemically active than the other, and a reaction occurs. Very pure water will not conduct electricity, so the electrolyte isn’t present. Saltwater however conducts an electrical current and allows for corrosion to occur [3].

If we want to dig into the science a bit more, galvanic corrosion and electrolysis are named after the currents involved. To identify the differences we can look at how galvanic cells and electrolytic cells are formed and how their electrochemical reactions take place. Keep in mind both cell types consist of two electrodes, an anode and a cathode, immersed in an electrolyte.

• Galvanic cells derive energy from spontaneous redox reactions across two different metals in an electrolyte. This results in a flow of electrons producing DC-like current. Oxidation occurs at the anode and reduction occurs at the cathode. The anode is the source of electrons and becomes the negative terminal. A very common example of this is an alkaline battery discharging [4].
• Electrolytic cells involve an external power source to drive non-spontaneous reaction, or apply reverse voltage to a galvanic (or voltaic) cell. The metals can be similar or dissimilar and are still in an electrolyte. The anode remains the site of oxidation, but it becomes the positive terminal and the cathode becomes the negative terminal. A common example of this is charging a battery [4].

"The Effects"​

Now I had an explanation for why the galvanic isolator wasn't helping -- it was the wrong tool to fix the problem. The current in the water had effectively turned the vessel into a giant conductor using it's grounding system to carry the current back to shore. After all, it was the least resistant path. I suspect other boats nearby might be suffering from similar effects.

This means that electrolysis could be harming my boat. I recalled recently that the diver who cleans the bottom of Rumour had reported sacrificial zincs melting at an alarming rate. He had added an additional zinc to the prop shaft and recommended at the next haul out that I drill a location on the strut to hang a scalloped zinc. This could also explain the propeller being significantly eroded when I took delivery a few months previous. Maybe the boat was in a 'hot' marina.

"Just Disconnect the Engine Ground"​

I called the maintenance staff to come by the boat one last time and explained the situation to them. I demonstrated to the best of my ability my findings and declared the boat was not an issue. To further prove my point, I temporarily disconnected the ground from the engine. I was thrown back at their next statement.

"Oh what? Just disconnect the engine ground from the AC panel then! Problem solved."

I pulled out the Caldwell reference and pointed to the page that reads:


I looked up from the page and saw sour faces. With grumpy tones they stood on the dock finger and told me they had disconnected the grounds on their boats and everything was "just fine".

Things will be fine. Until they're not.

"The Letter"​

I resolved myself to write a letter. I wasn't getting through to the maintenance staff. Maybe the office staff would be more willing to listen.

While my boat was hauled out for a bottom job and various other repairs, I went down to the dock with a zinc anode, a length of wire and the multimeter. I turned the pedestal breaker off and connected the wire to the grounding contact on the 30A receptacle. The other end of the wire was clamped into the zinc and dropped in the water. Using the clamp-on meter I measured the current. 160mA.

I walked down the dock and carried out the same test from various other positions. The current readings dropped slightly the farther away from the shore I went. This could be for a variety of reasons, but at least one thing was consistent: stray current everywhere around the dock.

I included photos, readings, excerpts from the Nigel Caldwell book, a copy of the original email from the marina office, and an account of interactions with the maintenance staff, including their insistence to do something "potentially lethal" to the electrical on my boat. I also included a request for the marina to hire a licensed marine electrician to identify and eliminate the source of the stray current.

The letter was sent over five months ago.

I have not heard back.

"The Best Solution"​

The best solution I know of is an isolation transformer. Bulky, heavy, and expensive, these boxes electrically isolate AC shore power from the boat's AC system. Instead, the ground wire is connected to a shield that is wound between transformer windings which conduct power through a magnetic field. This is the best of both worlds: complete galvanic isolation with the green grounding wire still connected to grounding/bonding system of the boat.

I haven't installed one yet, but I may someday.

---

Footnotes​

[1] The diodes on a galvanic isolator control whether current passed through the green grounding wire. Galvanic DC current is blocked as it doesn't have enough voltage to activate/close the diodes. Any fault in the AC system which allows current to flow over the green grounding wire will close the diodes and pass safely through. MaineSail has a great article here: https://marinehowto.com/testing-a-galvanic-isolator/

[2] Unless it's troubleshooting in an emergency.

[3] Difference Between Electrolysis and Galvanic Corrosion - https://www.boataccessoriesaustrali...rence-between-electrolysis-and-galvanic-corro

[4] Electrochemistry - https://www.khanacademy.org/test-pr...ntro-electrochemistry-mcat/a/electrochemistry