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Man Overboard!

Electrofishing is a primary sampling method for near-shore fish species in lakes and rivers, and for fish species and assemblages in streams. We must admit, however, that it is a sampling technique with inherent risks. People, water and electricity are in close proximity. This blog describes the potential for human injury or death if someone falls into water having an electrical field. It specifically involves boat electrofishing, but the same idea can be used for tow/push barges, shore-based units and backpack electrofishing.

From studies of human safety, it is widely taught and accepted that electrical current is the parameter that causes the damage to internal organs or to the body as a whole. In the US Fish and Wildlife Service electrofishing course, we teach that humans can barely perceive 1 mA of current, 20 mA causes paralysis of respiratory muscles, and 100 mA is the ventricular fibrillation threshold. If dry skin resistance is about 100,000 ohms, then someone exposed to 120 volts receives 120 V / 100,000 ohm = 1.2 mA of current. They would barely perceive a shock. If a person become wet, it is a different story. If wet skin has a resistance of 1000 ohms, then they receive 120 V / 1000 ohms = 120 mA of current. That is enough to cause ventricular fibrillation. Even CPR will not help that person; only a defibrillator could save them.

Now let’s get even more specific as it relates to electrofishing.  One study suggested that the danger level for humans is approximately 50 mA. The resistance of an unprotected, immersed person, such as a normally clothed dipper on the bow of an electrofishing boat if they fell in, is about 100 ohms. Therefore, 100 ohms x 50 mA danger level = only 5 volts through the body!

To what amount of electricity would a person be exposed if they fell off the bow and into the electrical field out from the anodes of a typical electrofishing boat?  A person who is six feet tall is 183 cm from head to feet and about the same between their outstretched hands. And the path from the left hand to the right foot would be even more. The maximum safe voltage gradient through the body would average 5 volts / 183 cm = 0.027 V/cm.  We typically use 0.4 to 0.7 V/cm to capture fish, depending upon water conductivity, and the voltage gradients near the anodes often are 2 or more volts per centimeter.  These voltage gradients greatly exceed the maximum safe limit for humans.

Now, let’s quantify the situation with some real data.  The graph below is the voltage gradient profile lateral to one anode array of a Missouri Department of Conservation electrofishing boat.  The applied voltage was 216 volts for this mapping exercise.  A simple power regression was fit to these data so that the voltage gradient at any distance from the anode array center could be calculated.  VG profile

It is easier to think in terms of the whole body voltage to which a body is subjected rather than to compute some average voltage gradient.  Voltage gradient vs distance is the first derivative of voltage vs distance in an electrical field.  In this case, we used a voltage gradient probe to directly measure the voltage gradients at various distances from the anode array center.  The anti-derivative of the resulting regression equation was used to estimate the volts at any distance from the anode center.  The graph below is the anti-derivative of the first graph.  Note that the new equation was extended beyond the original data so that we could estimate the whole body voltage to which a person is exposed if they were floating some distance from the anode array.

V profile

The graph below includes the second graph with an added rectangle.  The x-axis length of the rectangle represents the 183 cm height, or arm spread, of a person who is six feet tall.  The y-axis height of the rectangle is the voltage to which the person is exposed over the 183 cm of their outstretched body whether their body is in line with the electrical current or is basically at right angles to the current so that it goes from one hand to the other.  Frankly, I don’t know if someone who is shocked extends their arms or legs, but frogs certainly do.  In this case, the body is centered at 200 cm from the anode array center.  In other words, the closest part of the body is about 109 cm from the array, and the farthest part of the body is about 292 cm from the array.

V away

The last graph is the same as the third one plus a second added rectangle.  The x-axis length of the second rectangle is the same 183 cm as the first rectangle.  Note that the y-axis height is much higher. This is because the change in voltage with distance, the average slope of the curve, is much greater near the anode versus farther away. In this case, the nearest part of the body was only 40 cm from the anode array center, and the farthest part was 223 cm from the anode center.

V away and near

By investigating the heights of these rectangles, it is clear that someone immersed in the water near the anode is exposed to over twice as much total body voltage as one who is farther away from the anode.  If we integrate the voltage gradient vs distance equation over the respective distances in the two examples, the far body is exposed to 45 volts and the near body is exposed to 105 volts.

Let me note that something important is missing from the information so far.  It involves the transfer of electrical power from the water to the person.  To better understand the real voltage exposure through the body of a submerged person, we need to know the effective conductivity of an unprotected human, and likely for a range of human subjects.  Any volunteers?  We would have to monitor your response to various levels of voltage and over a wide range of water conductivity with a controlled experiment of you submerged in a homogeneous electrical field.  That would be dangerous and likely painful.

Without the knowledge of human effective conductivity, let’s just be cautious and say that all of the electrical power in the water is transferred to an immersed human in a man overboard situation.  Recall that the danger level was at only 5 volts through the body. Even in the example given, in which the applied voltage for the voltage gradient map was only 216 volts, the total body voltage was 105 volts near the anode and 45 volts farther away.  Either one is considerably more than the 5 volts which may cause serious injury or death.  Note that the 45 and 105 volts represents about 21 and 49%, respectively,  of the 216 volts applied.  If the applied voltage was doubled, for example, the whole body exposure would also be doubled to 90 and 210 volts.  In low conductivity, we may use up to 1000 volts or even more when boat electrofishing.  Total body exposure is the same 21 and 49% of the applied voltage at the same distances and for the same electrode configuration.

I hope this explanation of the risk associated with an electrofishing man overboard situation has been helpful. That is why the US Fish and Wildlife safety policy requires the dippers on the bow of an electrofishing boat to have a safety foot switch.  If the dipper falls overboard with the electrical current on, they are in grave danger.  Even if they don’t go into ventricular fibrillation, or worse, they may lose the ability to swim or to even breathe.  I have personally tested a foot switch which failed in the closed position; it could never turn off!  Please test foot switches and the grounding of pulsators and generators.  This is easily done via a simple continuity test with the power off.  It could save someone’s life.  The “need” to get data may outweigh our caution.  And I have taken such chances.  There may not be a second chance.  Be effective, but first be safe…safe for you, and safe for the fish.  The latter is another topic.

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© 2015 Thread One Page. Imagery: Tom Rayner, Alan temple, Richard Pearson, Paul Godfrey, Roger Scott, John Rayner.