There was a medical emergency yesterday morning at our marina. A radio call for help was received from one of two small pocket cruisers rafted together about a mile away. The lady said her husband had received an electrical shock. A delivery captain based at the marina jumped in his dinghy and headed for the boats while the dockmaster called for an ambulance. When delivered to the launch ramp, the man, in his late 40s or early 50s was unconscious. He was taken to a local hospital then later airlifted to Nashville. The latest word is that he's in a coma. He was attempting to put a new belt on the engine in the cramped engine room. Apparently, his own body, which their cruising friends speculated was one of his bare legs, accidently shorted out the terminals on the house battery. After it was over, we at the marina had no answers to several questions such as why the battery terminals were exposed. Also, the man may have had a preexisting health condition. But all of that aside, it was a sobering reminder that large batteries, although only 12 volts, can pack a powerful punch with their amperage. I thought this incident was worth sharing because it's so easy for all of us to forget safety concerns as we work on our boats. Fred Myers CruiseGuide Author & Publisher

> But all of that aside, it was a sobering reminder that large > batteries, although only 12 volts, can pack a powerful punch > with their amperage. > It doesn't actually work that way. If it were the case that a 12V battery could send a heart into ventricular fibrillation, it wouldn't matter how large the battery bank was. It's all explained by Ohm's Law: amperage = volts / ohms. It is the amperage that kills but the current that flows is dependent on the voltage of the battery and the resistance of human tissue involved. Going further with this... It takes between 6 and 200 milliamps flowing through the heart for a couple of seconds to cause problems. Let's underrate that and say that 1 milliamp is dangerous. Plugging this back into Ohm's Law and solving for the resistance at 12.7V: R = Volts / Amps R = 12.7 / .001 R = 12700 ohms This means that if the resistance of the human tissue is 12700 ohms or less, enough amperage (well, 6 times less than necessary but who's counting) will pass through the tissue. DC current (as opposed to AC current) has only 20 to 30 percent of the ability to cause a cardiac arrhythmia but I won't consider that either. Here are some general resistance measurements in Ohms: Dry calloused hands: 1-2 million Dry "office" hands: 500 K - 1 million Sweaty hands: 20 K - 40 K I just took out my ohm meter and tested myself - place an electrode in each hand and squeeze tightly. This is showing the resistance across my chest area. I get about 1 million ohms when my fingers are dry and about 500 K ohms when my fingers are wet. If I were sweating a lot, the resistance would be a lot less although much of the current would also travel across the surface of the skin and not through the heart muscle. Still, estimates for sweaty hands are 20 K - 40 K ohms. Even at this extreme case, 20 K ohms isn't enough to cause harm. It doesn't matter if the battery bank is a single 12V car battery or 4-8D's in parallel as a house bank for a trawler. The resistance of the human tissue is to high to allow the current to pass through the body. It is even less likely that an arrhythmia was generated by legs touching the positive and negative posts of a 12V bank (one to each leg). The current will follow the path of least resistance and would be shunted across the pelvis. Very little current would actually make it to the chest area. Of course, if one leg touches a positive post and the opposite hand were touching a large negative ground (like the engine), there would be a nice path across the chest cavity. Even in this case, the resistance of the human tissue isn't enough to allow the current to pass though. >From Ohm's Law the way to increase the chance of ventricular fibrillation is to increase the voltage. If the voltage were 120V (and remember that AC is more deadly than DC) then a 1 milliamp dangerous current can be caused with this resistance: R = Volts / Amps R = 120 / .001 R = 120 K ohms 120 K is a bad number. It is getting close to the resistance between dry hands and is well above the resistance of sweaty hands. 120V AC current will easily kill a human if allowed to travel across the chest (thoracic) area. This coincides with most of our experiences - we've all been shocked by a 120 V socket - although probably not across the chest area. One more interesting "human" resistance thought... If two IV's are inserted into each arm of a patient, the measured resistance between the fluids of the IV and bloodstream are only about 50 ohms. Plugging this back into Ohm's Law: Volts = Amps * Ohms Volts = .001 * 50 Volts = .05 This implies that a small AAA battery (1.5V) connected across 2 IV's is way more than enough to cause a very dangerous situation. In all situations, electrical current should be respected. At the same time, a 12.7 volt supply is really pretty safe for direct human contact. ================ Jeffrey Siegel M/V aCappella DeFever 53PH W1ACA/WDB4350 Castine, Maine

Fred, I would suggest they need to look at a "hot" box, such as a battery charger that has a hot chassis or exposed wire. Hot, as in 120 V AC. Take care and be safe. Take care and be safe. Wayne M/V Celestial Albin43 Sundeck