ESD-(Electric Shock Drowning)-no swimming

[Pat T]

Have you read the Boat US July issue of Seaworthy? There is a good article on ESD. Under What You Need to Know it states "Never swim within 100 yards of any freshwater marina or boatyard." After reading the article I can understand the possible dangers. But I wonder why this is not enforced. I boat in Lake Michigan. I've been to many a marina on the lower end of the lake. Never seen a sign posted not to swim due to possible ESD. But then again I've never heard of anyone dying from ESD here either. In my marina there are 1000+ boats. On a hot summer afternoon I bet I could easily find 100 people swimming at the same time. Come late July I start cleaning the boat bottom of slime myself. Is this an accident waiting to happen? If it is so dangerous then why does the city of Chicago allow it? What are your thoughts?
At a minimum I am thinking of purchasing a clamp meter to measure if any of the boats nearby are leaking any AC. Do you have a recommendation of a meter and approx. cost?
Thanks all.

 

[Charles Erwin]

Avoidance of ESD risk starts with the marina owner - and their insurer. This assumes the marina insurer (or its risk department if the marina is a port or municipal property) is more dedicated to risk elimination instead of its usual "acceptable risk analysis." It also assumes the marina owner/manager is well motivated to safety.

The city, state, nor any other authority is just not up to being your rule-giving mother nor even an effective rule-enforcer concerning how any given shore-side improvement gets used.

Every marina needs to conduct/measure AC risk at least annually. Assuming your local government authority makes a point of enforcing this national rule - usually this ends up being the fire guys - it is way less often than annually.

In addition, annual inspections are not enough and even responsible insurers rarely press risk elimination as long as premium payments continue to flow in their direction.

Hence while ESD is way easy to eliminate - this happens only if the marina owner makes frequent and serious inspections/measurements.

That is not going to happen unless a group of tenants - like you - make an issue of it. In fact - if you ask - you may find the marina managers are pretty aggressive about sampling and electrical safety.

But - it is simply a fantasy to think your local government can legislate completely safe environs.

Charles

 

[Dalliance]

Read it. Very good article. According to the Rules & Regulations posted on the Chicago Harbors website......

Group Two Rules - Section f
All Persons.
(1) No person shall swim, dive, water ski or windsurf within the Harbor System, except for divers cleaning boats or retrieving lost items after receiving the approval of the Harbor Manager.

For whatever reason, it seems that this is not being enforced. At least not very effectively.

Like you, I've also been in the water cleaning slime off the hull in past years, but after reading this article and also becoming aware of some of the nasty things going into the harbor, I have pretty much resolved that my bottom cleaning swims will be done on calm days out on the lake this year.

 

[Charles Erwin]

The rule literally says the swimmer, diver, water-biker, water skier, windsurfer who suffers an ESD death is guilty of rule transgression thus a self inflicted result. But the rule says nothing whatsoever about the ESD risk.

Why?

The reason is obvious. The rule has NOTHING to do with ESD. The rule is designed exclusively to address collisions involving tenants' vessel movements and those who are engaged in these (unexpected) harbor activities.

Charles

p.s. The source of AC in the water is more likely to be a tenant doing something silly - like bonding AC white and AC safety green on the boat or using a non-marine battery charger - than marina wiring. Either way frequent marina measurements are the only way to discover errant AC current in the water. CE

 

[rgranger]

Have you read the Boat US July issue of Seaworthy? There is a good article on ESD. Under What You Need to Know it states "Never swim within 100 yards of any freshwater marina or boatyard."

Do you have a link?

 

[Sailm8]

http://www.boatus.com/seaworthy/magazine/2013/july/electric-shock-drowning-explained.asp

 

[Sailm8]

I just came back from visiting friends at Lake of the Ozark's and we were discussing the deaths of last year. The Lake required every dock to be illuminated so consequently many were wired by bubba the electrician for $50 and a case of beer. The only inspection is whether or not the light works. IMO Lake of the Ozark's is probably the most dangerous lake in America. It has too many go fast boats and too many dockside bars and party spots. Few know the rules of the road. Strangely enough they are trying to attract sailboaters.

This Lake of the Ozark's
http://youtu.be/FtUUNPrRHKc

 

[tkanzler]

There is also the ABYC webinar created by Kevin Ritz, who lost his son Lucas to electric shock drowning - 1 hour 23 minutes long, but worth it. This is the webinar I sent to my FIL in Florida, and he ran it at his Power Squadron meeting, where a bunch of old-timers stated that it was common practice 'back in the day' to remove the equipment ground (aka 'safety ground') from their shore power connections to prevent other folks' leaky electric systems from eating up their props and shafts.

 

[rgranger]

http://www.boatus.com/seaworthy/magazine/2013/july/electric-shock-drowning-explained.asp

Thnx Sailm8

 

[tkanzler]

This is the article by Kevin Ritz that's linked to in the BoatUS article, but the other link I posted is to the ABYC webinar.

http://www.boatus.com/seaworthy/tragedy/default.asp

 

[Ross]

When there is an electrical current in the water a swimmer entering the water is connected in parallel to the conductor(water) carrying a current. If the voltage drop from one end of the swimmer to the other is large enough then the swimmer will help the carry the load and will receive a continuous shock.

 

[walt]

Also about water and resistance..

We are used to the “scalar” resistance in a known volume for the ohms law we know (V=IR) but resistance to current flow from an energized node immersed in water near the surface with charge flowing outwards to a near infinite sized ground has to account for the three dimensional current flow.

Say some object with voltage on it was placed in water just below the surface. The resistance at any point away from the object depends on a number of constants but also the number of charge carriers. More charge carriers equals lower resistance. The number of charge carriers is proportional to the volume of the water. Since we essentially have a half sphere (air above, water below), the volume of charge carriers varies as the size of the half sphere increases – which happens at the cube of the radius of the sphere.

Since the incremental resistance as you move away from the voltage source is decreasing with the cube of distance, the incremental electric field will also decrease with the cube of distance.

In other words, the electric field produced by a charged node in the water falls of quickly with distance because the volume of water expands rapidly as you move away from the source. Also, since salt water has a lot more charge carriers to begin with, the electric fields drop off much more rapidly in salt water than in fresh water.

This all just means that to encounter those dangerous electric fields even in fresh water, you have to get close to the source. And, the bad news is that once you encounter those dangerous fields, they would also increase very rapidly as you got closer to the source.

The dangerous distance would depend on how fresh the water is. The fresher the water, the larger the distance that becomes dangerous. I think those drowning accident victims all had to get fairly close to the source before it was dangerous and it also goes from not dangerous to dangerous in likely a short distance.

 

[Ross S]

I am confused. I thought salt water was a better conductor of electricity than fresh water.

 

[walt]

Is is. Its not the amont of current that flows in the water bulk, its the electric field gradient that causes the danger to a swimmer. If you put the same chunk of metal that was hot with AC in salt water compared to fresh water, much higher current would flow in salt water - but the voltage gradient moving away from the conductor would attenuate in a much shorter distance in salt water compared to fresh water.

 

[transmitterdan]

High resistance is not enough to prevent current flow. The dielectric constant of water is 80 times more than air thus all water (fresh or salt) conducts electricity. The problem with fresh water is that the resistance is much higher than saltwater. The low resistance of salt water effectively lowers the voltage a swimmer experiences. But in fresh water it can be much higher and therefore lethal. Pure water has nearly infinite resistance yet a swimmer in pure water can be easily electrocuted due to the displacement current in the dielectric.

The electric field problem in water is non-trivial. So non-trivial that there is no fool proof way to electronically protect a swimmer 100% of the time. Even a GFI device will not suffice in all cases.

 

[Ross S]

Walt/transmitterdan,

Thanks for the explanation. I never thought of it in those terms but you are right and it makes perfect sense.

 

[walt]

transmitterdan, I’m curious what the condition would be where a GFI or even an ELCI would not protect a swimmer in the conditions where you get the danger? I think a properly wired ELCI even at 30 ma trip would always disconnect the AC the second any conductor on the hot side made contact with the water - even in fresh water.

I hadn’t thought about the displacement current.. water has an amazingly high dielectric constant..

 

[Ross S]

I am not transmitterdan, but I suspect that there may be cases that due to the poor conductivity of water (especially fresh eater) that it won't conduct 30mA (or 5 mA) to trip the devices. For a fixed voltage, high resistance equals low current.

 

[kenn]

Another article, via the ABYC

...This all just means that to encounter those dangerous electric fields even in fresh water, you have to get close to the source. ...

The dangerous distance would depend on how fresh the water is. The fresher the water, the larger the distance that becomes dangerous. I think those drowning accident victims all had to get fairly close to the source before it was dangerous and it also goes from not dangerous to dangerous in likely a short distance.

The danger zone in freshwater around a fault easily encompasses the area someone would be in if they were swimming near, or diving from a dock or boat, or climbing out.

transmitterdan, I’m curious what the condition would be where a GFI or even an ELCI would not protect a swimmer in the conditions where you get the danger? I think a properly wired ELCI even at 30 ma trip would always disconnect the AC the second any conductor on the hot side made contact with the water - even in fresh water.

In North America, there's no code requirement for shore power to have such protection... and most don't. In Europe, where it's been the law for 30+ years, ESD is virtually unknown.

 

[walt]

Regarding the ELCI trip point of 30ma working in fresh water where ESD is most important.. I don’t know how the 30 ma number was arrived at but it would be hard to believe it doesn’t account for fresh water.

Purely out of curiousity (and I do somewhat related type of stuff at work – am aware of the safety risk), I recently did an experiment to see just how much current would flow if an object about the size of an outboard zinc was to be immersed in fresh water (in this case, Denver CO tap water). Would the current be high enough to trip ELCI?

In the pictures below, you can see the hot energized electrode (110VAC) and also the electrode used to ground the water bulk. It is not exactly the same as dropping the same electrode in a lake but I would argue its similar enough. In a lake, there is a much larger volume of water to provide charge carriers compared to my test with the very limited water volume which would tend to say the current in the lake would be larger. Also, the ground plane in the lake is much larger also saying the current in the lake would be larger. But the distance between the hot object and ground is much smaller in my test compared to a lake which would tend to over estimate the current. I think its likely at least somewhat in the ballpark but cant say for sure.

I measured the current flow by putting an 11 ohm resistor in the line and measuring the AC RMS voltage across the resistor. One of the pictures below shows the circuit diagram.

Results: the RMS voltage across the resistor was 1.6 volts so the RMS current is 145 ma. The peak current would then be about 332 ma.

The Bluesea ELCI trips at 30 ma and we don’t know if this is peak or RMS. But even using RMS, the current I measured was more than 4.8 times higher than what would be required to trip the ELCI.

So, I would be comfortable believing that this particular ELCI would trip for a fault resulting in energizing some underwater metal structure in fresh water. Note that this is also Denver tap water from mountain reservoirs that is likely fresher than average.

Also out of curiousity, I did a swag at how much of the current I measured was due to “displacement” current. This is basically the current that flows across a capacitor. I assumed my capacitor plates were 2x2.5 inch and separated by 8 inch and water has a dielectric of 80. I just used the calculator on the link below to get a capacitance of 11.36 pf.

http://hyperphysics.phy-astr.gsu.edu/hbase/electric/pplate.html

Using a simple model for current flow given 110VAC, 60 hz and the capacitance above, the displacement current is on the order of 0.47 ua (RMS)

The total current I measured is 145 ma which is over three hundred thousand times larger (ie, a huge amount) than the calculated displacement current so almost all the current flowing is resistive (charge carriers in the water). Still, I wouldn’t discount the importance of the displacement current for ESD.