Electricity in water / diver drownings

We have had two divers in our area drown in the last few months while working underwater on boats. As a diver myself, I can't imagine that they drowned in 5' of water by running out of air, for whatever reason. Neither was tangled up in anything. Is there a quick test these guys could use to test for stray currents before they go in the water? I'm only vaguely familiar with the voltmeter tests and have never done one. Any suggestions would be appreciated, as well as links that would help with this kind of measurement / accident. I have considered CO poisoning as well, but don't know enough about either incident to know if that could have been a factor.

Keith,
One diver was Jonas Al-Trabulsi of Kemah. The accident was at Waterford. Do you remember who the other diver was?

No, never heard. It's just bothering me that two of them have "drowned" in the same situation which doesn't make any sense to me.

I believe it only takes a very small amount of 'stray' current (a few mA) to cause muscles to freeze. Happens, for example, between the boat's underwater metal and the pontoon and is due to incorrect boat earthing. Didn't Steve D'Antonio write an article on this in PM some time ago?

There's an interesting article here - http://boatingsailing.suite101.com/article.cfm/electric_shock_drowning

BoatUS ran an article about a boy who drowned while swimming around a group of boats docked with dockside power. The solution is a Equipment Leakage Circuit Interrupter (ELCI). More info can be found at: http://www.boatus.com/seaworthy/kritz.asp

There seems to be a lot of chatter on all the forums on electricity /drowning and for a good reason. I never gave it too much thought until I dove on a friends commercial passenger ferry one night to do a "emergency " removal of a line that had got wrapped around one of the prop shafts. I was in my drysuit, with "wet" gloves on when I got bit through the glove. Needless to say I survived and had him shut down the power and disconnect the shore power cable before finishing the task. As I plan to move my boat into fresh water this summer for the first time, the issue of electricity/water is even more important than in salt water. All the precautions of not swimming when in a marina, connected to shore power, or with the genset running are great... but no one ever mentions the inverter running??. We rely on our inverter for most ac needs... the laptop is always running on the bridge etc. Common sense tells me that if it only takes a milliamp to induce paralysis the inverter needs to be off also. I know my kids will be in the water around the boat so I am trying to plan for all scenarios. Any additional thoughts?
LD

-- Edited by hollywood8118 on Tuesday 13th of April 2010 08:18:20 AM

-- Edited by hollywood8118 on Tuesday 13th of April 2010 08:19:37 AM

When I had some commercial divers look under Play d'eau, they wanted me to confirm all power was isolated. No shore power, all batteries off, and inverter off. Then when they dived, they stayed close to Play d'eau - they wouldn't go close to any other boat in case.

Try this link for an interesting review of the issue.<a href="http://www.qualitymarineservices.net/Electric%20Shock%20Drowning%20Incident%20List.doc">



http://www.qualitymarineservices.net/Electric%20Shock%20Drowning%20Incident%20List.doc</a>

Wow! Something I wasn't aware of. How many times have I been working on*or swimming around boats without a thought. Mostly in salt water which apparently is not as dangerous except it could lead to carelessness when the boat is moved into fresh water.

Thanks for the info. I wonder if BoatUS Insurance has any info on this?

My thinking from 45 year ago Chem 101 is -- salt water is a better electrolyte than fresh water. Big zap in salt. Distilled water is not*very good electrolyte but dirty (laden with dissolved solids*- TDS) *fresh lake water ok to kill you.

Is this why many docks have wooden rather than metal ladders?

This topic plays a key role in a wonderful novel by Martin Cruz Smith (of Gorky Park fame) called Havana Bay. If you've not read it it's worthwhile, I think, if for no other reason than his spot-on description of life in the Cuban capitol.

Makes me happy I have a 12 v DC only boat in all modes, and we stay real close to our boat if swimming off the platform. It would seem to make a case mandating all shore power having to be through an isolating transformer? I know these are expensive, but this would then bring the costs down, as so many more would be sold. However, would there still be a risk swimming round a boat using an inverter or generator if poorly wired? Rick, what do you think. If the wiring was that poor, you would think it would warn of that by shorting out fuses etc, would it not?

I must admit I have never heard of this before.
I dive around my own boat all the time (it is timber and so very wet and conductive) and mostly have the generator* or inverter running.
Never had a stray current either there or around my mates steely.
Had divers doing a hull check on our rig tender the other day whilst at work( we hit a rig and holed her) they were not concerned about us having our generators running or isolating any power source.

I think there would have to be some pretty serious faults or earths for these currents to affect people in the water and one would suspect also those on board.

Benn

Peter B wrote:

*If the wiring was that poor, you would think it would warn of that by shorting out fuses etc, would it not?

Click to expand...

What generally happens is that the AC safety ground (not neutral) is not connected to the boat's common DC ground.

If there is an AC electrical fault that connects a "live" wire to some piece of hardware like an engine*(or a metal hull) that allows either the hull or the propeller shaft to be energized, those parts become part of the "live" circuit. From then on, any conductor that bridges the ground and the energized parts will complete the electrical circuit.

There is not usually enough current flow in these cases to trip a breaker. But, there is a high enough potential to establish an electrical field around a metal hull or in the vicinity of through hulls or shafts and rudders.

Picture a*hemisphere radiating out from a metal hull underwater, the closer you get to the hull, the stronger the voltage, sort of like the power radiating from a radio transmitting antenna.*The*gradient of the field surrounding the boat can be measured as volts per centimter, amps per centimeter squared, and watts per cubic centimeter. The conductivity of the water is measured in Siemens per cm that is what determines the risk of electrocution.

In salt water the conductivity of the water is enough to minimize the gradient of the field, the*leakage current is dissipated over a large area and volume of water with the result that a swimmer is unlikely to get zapped with 120V at his head while*his feet are at or near ground potential so little current will flow.

In fresh water however, the low conductivity creates a strong gradient, it keeps high voltage near the*hull.*Picture the boat hull*as a wire and the water*as the insulation, if you puch through the insulation, at some point it will not be enough to stop you from getting zapped. A swimmer could have his arms or head near the hull in a high voltage gradient while the feet are near ground potential and a current will flow through the comparitively good conductors inside his body (including the heart and muscles) and out the feet. Remember it only takes about 50 milliamps to stop the heart and much less to paralyze the muscles and drown the unfortunate swimmer.

All that was probably more than you really wanted to read but the bottom line is to make sure that*all AC safety grounds*are connected to the common DC ground and if you are worried about galvanic corrosion, use a galvanic isolator, make sure your shore power ground is intact and in good condition, and do not disconnect the AC ground to the dock. All this applies as well to boats on the hard. You could touch your prop and get killed if you have a ground fault on the AC system.

Rick, that was a great teaching. Thank you. Now I understand what an electrician meant when he called on my mobile when working in Play d'eau's Lazarette to correct another electrician's poor wiring. He said he'd found the metalwork live and that he wanted me on the phone while he moved out of the area in case he became electrocuted. He found the bonding had been bypassed on the work the previous guy had done.

Does this make sense?

Piers wrote:Does this make sense?

Click to expand...

*Yes. He discovered that the metalwork in that area was not electrically connected to the DC ground*and the AC safety ground and somewhere there was an AC "leak" or ground fault that energized those parts. That is exactly the scenario that I was describing in my last post. That is also an example of what I meant when I said you could touch your prop while on the hard and get zapped.

I always use a meter and check between the metal bits and ground when I am working on my boat out of water. Mostly because I do my own electrical work and know how easy it is to get complacent and make a mistake.

Your electrician*is a wise man, he put a meter on the parts he was about to work on before he put his hands on them.

Coo. Thank you. A very sobering thought.

-- Edited by Piers on Thursday 15th of April 2010 09:56:27 AM

Yep. I'm sticking with 12v DC only, and my trusty air turbine generator and solar panels.

Our marina allows I think less than .06 total stray amps AC over the AC neutral and/or water.* I check about every 3 months especially if there is a new boat closes by with a multi meter and amp meter.* Our dock had very high stray amps over the neutral in December so the marina checked every boat on our dock.* Most of it was coming from a steel trawler they are converting to a live aboard.*


*They also tagged my boat as acceptable but a little high which was mostly coming from the charger and a combination of high amps heaters/water heaters.* I was told battery chargers are the biggest producer.** Most direct current like chargers/heaters/water heater/driers/stoves produce some stray electricity so they can add up.* I had a marine electrician come check our boat that day. *When my diver comes, we turn off the AC power and he checks the shore power cord and test the water himself before he gets in the water.
Anyway ask your marina what is the average/acceptable stray current allowed?* Also ask how long the average zincs last in your marina.* If your zincs a going fast then the norm that may an indication.


-- Edited by Phil Fill on Friday 16th of April 2010 09:22:57 AM

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.

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.

RickB - Great explanation. What is the "latest" on shaft grounding vs hull grounding on larger yachts -*FRP vs metal hulls?

sunchaser wrote:*What is the "latest" on shaft grounding vs hull grounding on larger yachts -*FRP vs metal hulls?

Click to expand...

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

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.

Click to expand...

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

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.

Steve W.

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.

Click to expand...

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.

*

Rick,

Can the problem occur if an isolating transformer is used?

Piers wrote:Can the problem occur if an isolating transformer is used?

Click to expand...

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

No need to apologise for the analogy. It makes the point very clear!

Click to expand...

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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