Resistance is Futile
(1999) A US Navy safety publication describes injuries incurred while doing don't's. One page described the fate of a sailor playing with a multimeter in an unauthorized manner. He was curious about the resistance level of the human body. He had a Simpson 260 multimeter, a small unit powered by a 9-volt battery. That may not seem powerful enough to be dangerous… but it can be deadly in the wrong hands.
The sailor took a probe in each hand to measure his bodily resistance from thumb to thumb. But the probes had sharp tips, and in his excitement he pressed his thumbs hard enough against the probes to break the skin. Once the salty conducting fluid known as blood was available, the current from the multimeter travelled right across the sailor's heart, disrupting the electrical regulation of his heartbeat. He died before he could record his Ohms.
The lesson? The Navy issues very few objects which are designed to be stuck into the human body.
August 2000 Dan Wilson elaborates:
I'm a former Navy petty officer, enlisted for six years as an electrician aboard a US Submarine. I got a lot of training. This story was used frequently during my training in the US Navy as an example of what can happen when procedures and safety measures are not followed. I considered the story an urban legend until I found the incident report referenced in the official Navy electrical safety guidelines. I now know it is true.
The actual event is slightly different than described above, and even more deserving of a Darwin award. This sailor stuck the sharpened ends of the probes through his thumbs intentionally. You see, he had just taken a course that taught a critical concept called "internal resistance."
Internal resistance is resistance to electrical power flow that exists inside any power source. It causes the terminal voltage to drop when load (current) increases. You can demonstrate this concept, if you're careful, by monitoring your car battery's terminal voltage, while someone starts up the engine. The reading will be ~13 volts while the engine is off, but during the period where the starter is cranking it will drop to 8-9 volts. The voltage drop is due to the internal resistance of the battery.
This sailor, like all other electricians in training, had already been through a safety class in which one of the excercises is to measure your body's resistance by simply holding the probes between your fingertips. (Most people read 500Kohms to 2Mohms.) Evidently, adding information from the internal resistance class, this sailor wanted to determine his own body's "internal resistance.". So he intentionally pushed the sharpened probe tips through the skin to elimate the rather high skin resistance and get only the "internal resistance". This, of course, caused his death.
How, you might ask, with only a 9V battery? Easy. One of the "rules of thumb" that the Navy teaches is the 1-10-100 rule of current. This rule states that 1mA of current through the human body can be felt, 10mA of current is sufficient to make muscles contract to the point where you cannot let go of a power source, and 100mA is sufficient to stop the heart. Let's look at Ohm's law. Ohm's law (for DC systems - I will not discuss AC here) is written as E=IR, where E is voltage in volts, I is current in Amps, and R is resistance in Ohms.
When we did the experiment in the electrical safety class to determine our body's resistance, we found a resistance of 500K Ohms. Using 9V and 500K Ohms in the equation, we come up with a current of 18 microAmps, below the "feel" threshold of 1mA. However, removing the insulation of skin from our curious sailor here, the resistance through the very good conducting electrolytes of the body is sharply lower. Around 100 ohms, in fact, resulting in a current of 90mA - sufficient to stop our sailor's heart and kill him.
As my electrical safety instructor said, "The reason we now have to teach the electrical safety course to all electricians at least twice per year is because some joe was bright enough to be the one person in the world who could figure out how to kill himself with a 9V battery."
No comments:
Post a Comment