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Old 04-16-2010, 09:24 AM   #21
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RE: Electricity in water / diver drownings

If you have an inverter or a generator running you could have the same problem. A fault in the grounding in the boat could cause a similar situation.
If you have a so called 12V only system, but that includes an inverter don't get careless.
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Old 04-16-2010, 11:53 AM   #22
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RE: Electricity in water / diver drownings

RickB - Great explanation. What is the "latest" on shaft grounding vs hull grounding on larger yachts -*FRP vs metal hulls?
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Old 04-16-2010, 01:26 PM   #23
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Electricity in water / diver drownings

Quote:
sunchaser wrote:*What is the "latest" on shaft grounding vs hull grounding on larger yachts -*FRP vs metal hulls?
The shaft should always have a grounding brush so that there can not be any difference in electrical potential between the bearings or gears in the transmission or crankshaft.
We just finished a*$300K overhaul of a 3000 hour engine because the crank was damaged by arcing in way of the main and rod bearings. This was on a steel hull yacht* and the root cause is believed to be from welding with the ground clamp too far from the weld location.

Steel or plastic hull, the shaft should be*connected via the common ground wire since if the shaft and prop were at a different electrical potential than the hull, rudder, or a through hull, galvanic corrosion would*be much*more likely.


-- Edited by RickB on Friday 16th of April 2010 01:27:22 PM
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Old 04-16-2010, 02:00 PM   #24
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Electricity in water / diver drownings

Quote:
C lectric wrote:If you have an inverter or a generator running you could have the same problem. A fault in the grounding in the boat could cause a similar situation.
Not really the same as there is no path between the boat and the world for a person to provide a better conductor.

If a boat is floating along at anchor or just floating and the safety ground on a microwave (for example) becomes disconnected and the same meteor strike that cut the ground wire also nicked the insulation on the hot wire and forced it against the housing, the next person to lean on the microwave might get zapped if he or she was touching something else that still had a ground connection to the source but none of that power would leave the boat to put a swimmer at risk.

If the ground wasn't broken but the meteor strike connected the hot wire with the housing, the current flow would be great enough to pop the protective fuse or breaker, assuming the system used a grounded neutral.

The difference between that scenario and the electrified water hazard is that there is no return path through the water since the source is in the hull itself, it is not referenced to earth as is utility power.


-- Edited by RickB on Friday 16th of April 2010 02:04:00 PM
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Old 04-16-2010, 07:26 PM   #25
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RE: Electricity in water / diver drownings

I have been traveling around Florida for the last couple of months, at just about every marina I have been to I have seen divers cleaning hulls or doing other tasks in the water. Right or wrong they don't seem concerned, if I get a chance I'll try to ask the next ones I see if they are aware of the topic.

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Old 04-18-2010, 03:00 AM   #26
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RE: Electricity in water / diver drownings

Quote:
sunchaser wrote:

Peter:

Do you ever plug in at the dock to charge batteries? That is when/where many of these accidents occur, tied into shore power.
Tom I do, but I use a special marine type C-tec 8 step charger, desgned for the purpose, and it is in effect an isolating transformer - there is no ground terminal on the plug even.* Also I do not use an inverter.* I use no AC devices.

*
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Old 04-18-2010, 03:02 AM   #27
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RE: Electricity in water / diver drownings

Rick,

Can the problem occur if an isolating transformer is used?
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Old 04-18-2010, 05:07 AM   #28
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Electricity in water / diver drownings

Quote:
Piers wrote:Can the problem occur if an isolating transformer is used?
If you mean the swimmer zapping thing, yes, an isolation transformer will all but eliminate the risk. The only source of an earth referenced fault then would be in the shore power connection to the boat itself and the wiring leading to the (primary) input side of the isolation transformer.

The output (secondary) side of the isolation transformer is only associated with the earth referenced utility (shore power) through a magnetic field.

Pardon the crude analogy but using an isolation transformer is like putting a condom on your shore power plug.


-- Edited by RickB on Sunday 18th of April 2010 05:11:19 AM
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Old 04-18-2010, 07:35 AM   #29
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RE: Electricity in water / diver drownings

No need to apologise for the analogy. It makes the point very clear!
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Old 04-18-2010, 09:48 AM   #30
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RE: Electricity in water / diver drownings

Quote:

Pardon the crude analogy but using an isolation transformer is like putting a condom on your shore power plug.


-- Edited by RickB on Sunday 18th of April 2010 05:11:19 AM
*

So.....
If I carried this condom in my wallet for a long time and it got a hole in it would she.... I mean the shore power know it.....! Would brand or shape effect performance??
I couldn't help it....


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Old 04-18-2010, 10:02 AM   #31
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Electricity in water / diver drownings

So the short version of all the electric hazard is this?

1. connect to shore power, unless isolated with a isolator or better yet isolation transformer current may leak back to ground of create a galvanic corrrosion issue with the marina or surrounding boats.

2. on board sources of ac power ( ac generator or inverter ) do not pose a risk to swimmers as they ground back to them self and not the earth through water and or swimmers.

3. the great risk is only when connected to the dock, or the very rare occasion when two rafted boats shared power and a stray current connects through the grounding side of boat boats....

Is this correct???

I am so interested in this because I did get shocked from my boat during my last haul out.
I do have a isolation transformer, I was kneeling on the ground, touched a through hull, felt the tingle. Used a meter and had 117v between a screw driver stuck in the ground and the through hull. I found some moron had reversed the polarity in the boat yard outlet. Needless to say it got my attention

-- Edited by hollywood8118 on Sunday 18th of April 2010 10:03:35 AM
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Old 04-18-2010, 10:31 AM   #32
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RE: Electricity in water / diver drownings

Got it.*
Thanks
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Old 04-18-2010, 11:38 AM   #33
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RE: Electricity in water / diver drownings

Quote:
hollywood8118 wrote:

I am so interested in this because I did get shocked from my boat during my last haul out.
I do have a isolation transformer, I was kneeling on the ground, touched a through hull, felt the tingle. Used a meter and had 117v between a screw driver stuck in the ground and the through hull. I found some moron had reversed the polarity in the boat yard outlet. Needless to say it got my attention

-- Edited by hollywood8118 on Sunday 18th of April 2010 10:03:35 AM
This is why every boat should have a reverse polarity indicator. They're cheap and pretty easy to install. My '86 Krogen didn't have one, and the last survey recommended one.

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Old 04-18-2010, 11:51 AM   #34
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RE: Electricity in water / diver drownings

Quote:
hollywood8118 wrote:I do have a isolation transformer, I was kneeling on the ground, touched a through hull, felt the tingle. Used a meter and had 117v between a screw driver stuck in the ground and the through hull. I found some moron had reversed the polarity in the boat yard outlet. Needless to say it got my attention
That shouldn't have happened even if the dock outlet was wired wrong. It had to have been the result of mis wiring on the boat as well as the dock.

The "polarity" of the single phase power going into the transformer is irrelevant, so switching the "hot" and "neutral" makes no difference. If the shield of a real isolation transformer was connected to a hot or neutral, it still wouldn't make any difference, the thing just wouldn't work.

But, if your shore power socket has the safety ground connected to the hull common ground and someone wired the dock outlet so that a hot wire fed the ground wire of the shore power cable then you would get zapped just like you described.

When you install an isolation transformer, the shore power socket must be electrically insulated from the metal hull and isolated from the hull common ground. Here is a link to the manual for a Charles Marine isolation transformer that shows two ways to handle the shore power ground and transformer case grounding.

http://www.charlesindustries.com/mar...R36N-1_PR2.pdf

Look very carefully at how the safety ground is handled. The shield in a true isolation transformer is isolated electrically and its electrical connection is brought out to the terminal strip so that it can be connected as the user desires. A normal industrial transformer may or may not have a safety shield and the case may be internally connected to the ground or neutral, effectively canceling the safety feature of a true isolation transformer.

I don't doubt for a moment that you got a tingle and measured 117 volts between the fitting and the dirt ground. What I do suspect though is that unless the conditions I described above existed, you measured a small (and very common on boats) leakage current with a high impedance digital meter. If you put a load across the fitting to earth it probably would not have indicated anything. That is where the good old fashioned analog meters are valuable for troubleshooting.

In the meantime, take a good look at how your isolation transformer is wired. Check out your shore power socket and see where the ground goes.
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