Corrosion and Shocking

[Rkofe]

I've been asked by a friend to look at a situation on an aluminum hull boat which has been "eating up" zinc plates and is now causing a slight shock when you touch the hull while in the water.

A little bit of background. This never happened until after the following (coincidence maybe??): (1) a new dock was made out of steel piles driven in the water with a steel structure welded to the piles and then clad with wood to form the dock. Previously there was a steel structured dock, but not quite as large and mostly made close to the shore. I might add that this is in sea/salt water. (2) A new electrical supply (120V, three wire) was run in pvc conduit out to a weather proof box on the dock (the conduit is not in contact with the water). This is used for the shore power. There was a similar setup on the old dock for shore power. The electrical distribution box is sub-fed from a main distribution panel and is about 150 feet away from the shore power. The grounding on the electrical system for the shore power was not the best and it was made better with the addition of some ground rods recently.

The boat owner has a galvanic isolator installed and apparently there is a meter on board which measures the potential of the hull. When the shore power is plugged in it reads higher than when the shore power is unplugged. The boat owner tells me that the DC system on the boat is -ve ground but the AC system is reverse to conventional AC in that the hot wire goes directly to the device/appliance and the neutral wire goes through the breakers and switches.

Any suggestions of what can be happening/what to look for would be greatly appreciated.

 

[JustJason]

Re: Corrosion and Shocking

you need to get a reference annode and check the electricity in the water.

 

[bruceb58]

Re: Corrosion and Shocking

When the shore power is plugged in it reads higher than when the shore power is unplugged. The boat owner tells me that the DC system on the boat is -ve ground but the AC system is reverse to conventional AC in that the hot wire goes directly to the device/appliance and the neutral wire goes through the breakers and switches.

Very weird! Is he sure that the AC is wired that way. He does know that the BLACK wire is the hot side right?

 

[Rkofe]

Re: Corrosion and Shocking

JustJason, how do I go about checking the electricity in the water?

Bruceb58, the owner says that he found it out the hard way. He was installing a watermaker, had all of the sub-breakers in the panel off and went to cut a cable...ended up taking a notch out of his pliers. When he checked, he realized that the hot wire went directly to the devices and the neutral through the breakers and switches. He said that he had done some investigation and it appears that it is done that way because it's an aluminum hull boat...don't know if that makes sense.

The boat is a Striker sports fisher built in 1981.

 

[bruceb58]

Re: Corrosion and Shocking

JustJason, how do I go about checking the electricity in the water?

Bruceb58, the owner says that he found it out the hard way. He was installing a watermaker, had all of the sub-breakers in the panel off and went to cut a cable...ended up taking a notch out of his pliers. When he checked, he realized that the hot wire went directly to the devices and the neutral through the breakers and switches. He said that he had done some investigation and it appears that it is done that way because it's an aluminum hull boat...don't know if that makes sense.

The boat is a Striker sports fisher built in 1981.

That is just wrong and dangerous. That means you can't turn off the power with the circuit breakers!

 

[captainbirdseye]

Re: Corrosion and Shocking

If you are using AC on a boat you should be using a double pole breakers to supply the equipment and GFCI's if they are run to sockets, DC only needs a single pole breaker and should be on the Positive wire. It doesn't matter if your boat is made from steel, alu, Glass fiber, or wood.

If you are getting a small tingling shock and the zincs are going fast then it could well be a ground fault from a piece equipment inside the boat that is causing it.

 

[JustJason]

Re: Corrosion and Shocking

There is a Mercury service bulletin about testing the water, I don't have it but it's been posted many times in the forums. If you ask the question in the sterndrive section i'm pretty sure Don and Rodbolt have copies of the bulletin.

Now as to how your boat is wired. As Bruce said, wrong and dangerous.

If 1 of your black wires was to rub and expose itself to the aluminum hull, it wouldn't short and pop the breaker as intended, instead, the entire hull would now have 120VAC potential wether your in it or not. If your in it, and depending on what your touching, it could kill you.

You have 3 wires, Black, White, and Green/Bare Copper. Black is hot. White is Neutral, Copper is ground.

Black 120VAC+, it is the hot wire, it has all the current on it, Black is the kill wire, Black is death
White 120VAC also, But because carries no current, this wire is neutral, If this were a DC system think of white as ground, or the return path. White is grounded at the power station, and tied also to ground locally. If your black were to short and If your local ground were to fail, in a perfect system white would take over as the ground. But it doesn't always work that way.
Green/Copper, Your local ground. Since the boat can't be grounded, this wire should be grounded at the closest possible convienent spot. It's probably grounded where the entire shore powers main panel is grounded at the marina. If this where your house, Copper would be grounded via an 8ft metal spike that runs through the foundation into the ground in your basement.

Now that's the way it's supposed to be. But if somebody didn't know what they were doing, as it sounds like they didn't, you can't rely on colors to tell the truth.

If you don't aren't 100% confident on AC electricy, how to use a meter, and how to protect yourself when working with it, then please please please don't screw with it, hire a pro. The electricity here isn't the old 9 volt on the tounge zap, this stuff will kill you dead where you stand.

 

[bruceb58]

Re: Corrosion and Shocking

If you are using AC on a boat you should be using a double pole breakers to supply the equipment and GFCI's if they are run to sockets, DC only needs a single pole breaker and should be on the Positive wire. It doesn't matter if your boat is made from steel, alu, Glass fiber, or wood.

If you are getting a small tingling shock and the zincs are going fast then it could well be a ground fault from a piece equipment inside the boat that is causing it.

Double pole breakers are only required on the Input to the panel if it is 220V and any other 220V load that is on the boat.

 

[bruceb58]

Re: Corrosion and Shocking

You have 3 wires, Black, White, and Green/Bare Copper. Black is hot. White is Neutral, Copper is ground.

Black 120VAC+, it is the hot wire, it has all the current on it, Black is the kill wire, Black is death
White 120VAC also, But because carries no current, this wire is neutral,

That is just wrong. Neutral DOES carry current on any 110V load.

Black is the hot wire because it is the wire that comes from the transformer that feeds your house. It is also never called 120VAC+. There is no positive polarity on AC.

 

[JustJason]

Re: Corrosion and Shocking

That is just wrong. Neutral DOES carry current on any 110V load.

You are correct, maybe I mis-worded what I was trying to get across, i'll try again

In a complete AC circuit, voltage and current comes in on the black wire, and leaves back to the panel (and back to the power station) on the white wire.

If you touch a live black wire, you do not even have to touch another wire because you are grouned just by standing on dirt, you get zapped something good.
If you touch a live black, + touch either a neutral or ground, you get lit up like a christmas tree.
You can touch ground all day long.
You can touch a neutral if there is no complete circuit, because the neutral is grounded.
If the neutral is not (were not) grounded, or if for some reason your body provides a better ground than earth, you get zapped.
If you grab neutral in a live complete circuit, as long as the neutral is grounded better than you are, your fine. But if the neutral is loose, corroded, or broken, you get lit up, so it's best to assume the wire is hot all the time (even though it's not)

It's all about hot going to ground/neutral. Hot is supposed to go to neutral. If hot doesn't see a neutral, then it wants to go to ground. If hot finds a path to ground through you, your screwed.

It is also never called 120VAC+. There is no positive polarity on AC.

Yep that was a typo. A case of using the old + as an "and" and throwing in a bad comma