Boat cleaning fatality - Fort Myers, Florida

[JohnN]

GFCIs and other devices of the same ilk (ELCIs, etc) do solve the safety problem.

They are prone to nuisance trips in a marine environment and so haven't seen widespread adoption. Much of the problem is that they trip when the wiring on boat has excessive leakage, which is common, and hard for the average yacht owner (or marina electrician, or most electricians for that matter) to fix. Truly capable electricians who can find and fix grounded neutrals and leakage problems are few and far between, and most of them have well-paying cushy industrial jobs that don't require them to try to fix wiring in some tiny little odoriferous forward compartment on the bottom deck.

Thanks

I found this link: ELCI / GFCI Electrical Shock Protection | West Marine

and summary:

GFCIs are used as branch circuit ground fault protection

at the 5mA threshold in potentially wet environments. GFCIs protect against flaws in devices plugged into them, but offer no protection from the danger of a failing hard-wired appliance, such as a water heater or cooktop.

An ELCI provides additional whole-boat protection. Installed as required within 10' of the shore power inlet, an ELCI provides 30mA ground fault protection for the entire AC shore power system beyond the ELCI. ABYC regulations still require the use of GFCIs in environments described above.

I'm not sure the bit about protecting against "failing hard-wired appliance" is valid since both the ELCI and GFCI "look" for an imbalance in the current entering and exiting the point of connection.

 

[Wookie]

I use GFCI outlets in the heads and galley. They work as designed, and I ground to the common. I do not use GFCI breakers, they do not work at all on an ungrounded system. I use a UPS to plug in the computers and other sensitive electronics, as they isolate the boat's ungrounded electrical system from the computers by changing to DC and back to AC. My shore power riser has a large box of magic including phase protection, phase rotation protection, and ground detect protection. I only run the 3 hot legs from the shore power riser to the boat isolation transformer.

 

[richboslice]

I've been thinking about this and realize that there are some unique things about this situation that amplify the hazards.

It's not unusual for owners of ocean-going yachts to deliberately cut the equipment ground wire in the shore power line. It's a dangerous practice, but it reduces the problems with galvanic corrosion on metal parts of the boat that are continuously immersed, particularly the lower unit(s) (on stern drives) or the prop shafts (on inboards). Stern drives are particularly subject to galvanic corrosion due to their aluminum construction. The corrosion results from dissimilar metals being immersed in seawater while electrically connected, and can be made worse by some kinds of DC wiring faults. Connecting the equipment ground makes this problem worse because it connects the boat electrically to all the other boats and equipment in the marina.

Galvanic corrosion is much less of a problem in fresh water, and so the practice of deliberately cutting the equipment ground wire is less common on inland lakes and rivers.

The fact that the accident took place in a freshwater marina that is connected to the ocean leads me to wonder about the history of the boat. Perhaps its home mooring was previously in salt water, leading its owner at the time to cut the equipment ground.

There are various means of preventing galvanic corrosion without creating a hazard, but they are expensive. Perhaps the most effective (and most expensive) means of doing this is through installation of an isolation transformer, either on board or at the pedestal. This leads to a separately derived service and the equipment ground can safely be cut. There are also means of coupling the equipment ground that block low DC voltages typical of those that cause galvanic corrosion, but allow AC to flow in the event of a fault.

Finally there are GFCI-like devices that can be installed at the pedestal that will prevent an accident like this even if there are wiring faults (including an open equipment ground, deliberate or otherwise) in the boat. It would seem to me that such devices would be of particular importance in brackish water where boat owners may be tempted to follow the dodgy grounding practices common in marine environments despite the water having low enough salinity -- even if only some of the time -- for the hazard to be comparable to a freshwater harbor.

Maybe a disconnect would be helpful. Instead of cutting it a boat owner could just keep the disconnect out and plug it in at the marina. Should be mandatory while docked

 

[richboslice]

Could a marina have a common ground on a dock and have cables to connect boats to while docked?

 

[Wookie]

Could a marina have a common ground on a dock and have cables to connect boats to while docked?

They could, but then you are hooking your sins to every other boat owner's sins. I don't know anyone who uses the marina ground.

 

[EFX]

Stop me if wrong - volts don't kill amps do...

Beats me. I never understood all that electrical stuff, which makes the risk greater. I think our cattle fences were high voltage, but the ones powered by approved chargers wouldn't kill a person - just jolt like hell.

There's two general ways you can get killed by electricity: (1) a current through the heart muscle puts it in fibrillation and (2) shock and/or infection from burns due to a sufficiently high current going through the body or limb. What actually damages tissue or shocks the heart is current. It takes only 45 microamps through the heart to cause fib. Voltage can be thought of as the driving force that causes current to flow through a path of resistance. Given a fixed resistance the higher the votage the higher the current flow. The body has a resistance of about 150 to 250k ohms depending on body part and physiology. You can touch a source with very high voltage and no (or very low) current (ex.Vandegraph generator) or a source with very high current and low voltage (ex. car battery) and not get hurt. For the cattle fence the voltage is high but the current source is low so the result is a shock or tingle. The fence's power source either has a sufficiently high internal resistance or is current regulated which limits or reduces the current to a low value. A good example is the starter motor in your car which is a current device. That is, provided there is a minimum voltage and the source can supply the current the motor turns and your car starts. The starter will draw tens or even a few hundred amps but due to the internal resistance of the battery the voltage will drop to about 8 volts.

Birds sitting on a high tension line (1200 VAC?) don't get hurt. The reason is you need a circuit to get current flow and there is no circuit. The entire birds body from head to toe is charged at 1200 VAC but there is zero current going through their body. When measuring voltage it is always a voltage at a particular point compared with voltage at another point. For our bird example the wire and the bird is at 1200 VAC with respect to ground.

When considering electrical safety for human beings we need to consider the voltage of an object with respect to (physical) ground because people are either standing or touching the ground. What we need to avoid is any circuit, deliberate or accidental, that places people in a path between the source and ground. In DandyDon's drawing in post #21 the ground connection between the boat and the power pedestal is broken (open). Most electrical equipment's chassis will be grounded by connecting a separate cable from the chassis back to the ground plane or buss which is connected to shore ground. In this case a shorted feed to the chassis will cause the current to flow through the ground cable so that a person touching the equipment will not get shocked (in this case the persons hand is at 0 volts compared to ground and their feet are at 0 volts compared to ground resulting in a current of (0-0) / 150k = 0 amps). However, with the open ground, current will flow from the electrified chassis or boat through the water to ground at the shore. A diver will get shocked if they form part or all of the path to ground.

 

[Pinecube]

The body has a resistance of about 150 to 250k ohms depending on body part and physiology.

A good post, but I'd like to clarify that the 150,000-250,000ohm resistance is for dry skin. Having wet skin (rainy day, sweating, etc..) can lower that resistance below 1000ohms. I'm not sure what extent the resistance drops when you're completely submerged for a lenght of time.

 

[2airishuman]

..It takes only 45 microamps through the heart to cause fib..The body has a resistance of about 150 to 250k ohms depending on body part and physiology.

The situation is far more complex than that. Most of the ground fault accidents involve situations where the electrical resistance of the body is reduced by various factors such as sweat and size of contact area. The path taken is also important. In water the voltage gradient is what matters. The hazard posed also varies based on pulse width and frequency.

Birds sitting on a high tension line (1200 VAC?) don't get hurt. The reason is you need a circuit to get current flow and there is no circuit. The entire birds body from head to toe is charged at 1200 VAC but there is zero current going through their body. When measuring voltage it is always a voltage at a particular point compared with voltage at another point. For our bird example the wire and the bird is at 1200 VAC with respect to ground.

Around here the most common nominal distribution voltage is 13,600-14,400 volts with some systems running at either half that or twice that. There were some 2400 volt systems being built as recently as the 1950s but 1200 volts would have been most unusual even in the early days. AC transmission line voltages around here run from 68,000 up to around 200,000. A few use higher voltages still but most of those are DC.

More to the point your bird can be injured either by the heat dissipated by the cable, as some of the transmission lines run quite hot in a literal sense, or, theoretically at least, by the voltage between its two feet

I would be interested to understand how much conductance makes water most dangerous. Pure water hardly conducts electricity at all, limiting current, and salt water conducts it well enough that the voltage gradient is hard to get dangerously high.

 

[EFX]

Looking up the conductance of salt water I got:

"The average conductivity of sea water at 20degC and a salinity of 35g/kg is: 4.788 S/m (Siemens/meter). The resistance is the reciprocal of the conductance, so: R = 1/(4.788 S/m) = 0.2089 ohm/m <===ANSWER at 20 degC and a salinity of 35g salt per kg water."

Let's assume that the source (an electrified boat part) can supply the necessary current to all parallel paths such that the voltage at the source remains at 120 VAC. Let's also assume the boat is in 4.5 m of water and the diver is 2 m long. We don't know the resistance of the diver (rd) but we can calculate the resistance of the water (rw) and from that the voltage drop across the diver (vd) and water (vw) given a fixed current (I). If the victim touched the boat part a series circuit forms from the source through the diver's body through the water to ground (bottom). Assuming 1 amp of current flowing we get vw = rw(I) = (4.5-2.0 m)(0.21 ohms/m)(1 amp) = 0.53 volts. The voltage drop across the diver is vd = 120 - 0.53 = 119.5 volts.

We can neglect the small resistance of the water and call it a short from the diver's body to ground. This is the worst case scenario. If the current has to travel farther, say to shore, then the voltage drop across the water increases and that across the diver decreases. However, there are many parallel circuits through the water each conducting a different current. The combined effect of all this current will tend to drop the voltage at the source (because the source has some finite internal resistance). Even 24 V can be dangerous if the total resistance is such to cause a small current to pass through the heart to cause fibrillation. That current is just 45 microamps.

 

[Wookie]

The Navy always taught us that 100 mA is enough to stop the heart in the best conditions.