What is the REAL resistance of a Dairy Cow?

Many hours have been spent trying to unsuccessfully explain why 500 ohms was selected by some of the authors of the "Red Book" as the resistance of the pathway for current through a cow. Let's go through it step by step and you will plainly see that 500 ohms SHOULD NOT be the standard used in the industry for testing for stray voltage.

First, the pathway for current that goes through a cow to complete a circuit is from a source, goes along a pathway to a contact point, and from a cow contact point (for example, 4 hooves contacting concrete in a free-stall barn) to the cow itself, a second contact point (nose/mouth with a waterer) from the cow to the pathway back to source. When you add all of those resistances together, you get a total number. The authors of the Red Book agreed with the science that said the source and pathway resistance can be assumed to be zero (See page 7-6 of the Red Book shown below). So that leaves the resistance number used in the equation (V = I x R) and in farm testing to be the combination of the cow and the cow contact resistances. The authors of this chapter of the Red Book, claim that 500 ohms is the "worst case circuit impedance*" for cow plus contact measurements.

It is clear in all of the research cited in the Red Book and all research done since the publishing of the Red Book, 500 ohms is not the worst-case circuit impedance of a dairy cow. See table 7-1 below, which lists all the research available at the time of the white paper's publication. There is also a list of additional research done after its publication and testing results from hundreds of cows done on individual farms that also does not support 500 ohms as being the worst-case scenario. 

So how did the authors come up with 500 ohms to be the lowest resistance that should represent the "worst case" scenario? That's a great question that has yet to be answered. The research confirms that cow resistance/impedance without contact resistance is commonly in the range of 150-250 ohms taking the pathway of nose/mouth to all four feet. Using 500 ohms would mean you would have to add 250 to 350 ohms for contact resistance.  However, that doesn't square with science. Dr. Robert Gustafson, one of the authors of the Red Book, testified that contact resistance of a hoof on wet concrete and a nose on a wet surface will be 1-5 Ohms. So according to one of the two electrical/Ag engineers that were authors of the Red Book, the cow contact impedance will only add 5 ohms to the cow's resistance. That is a far cry from 250-350 ohms needed to get the cow plus cow contact to 500 ohms. 

Below, you will find a deposition done with Dr. Gustafson, where he verifies on page 69 that the cow contact resistance is 1-5 ohms for a cow standing in manure/urine/ water mixture, like would be found around a waterer in a barn. There is also a sworn deposition of Dr. Aneshansley, who is another author/electrical engineer of the Red Book. He verifies under oath that he would trust the conclusions made by Dr. Gustafson in regard to his work on cow contact resistance being between 1-5 ohms. That is found on page 80 of his deposition. Dr. Aneshansley also testifies that since the papers cited in the Red Book show the lowest cow resistances below 244 ohms and the cow contact resistances well below 200 ohms, that then the worst-case scenario for resistance should NOT be 500 ohms. This is on page 89 of his deposition. Dr. Gustafson, on page 67 of his deposition, acknowledged his own research showed resistance of a dairy cow to range 250-350 ohms. His research was done many years ago in a very different cow environment then what is found on modern farms. 

The Red Book quickly became the handbook referenced around the world by utilities when conducting testing. Legislative hearings, state testing procedures and even course studies were developed using the much overstated 500 ohms resistance when testing dairy farms for stray voltage. Even 30 years after its publication, it continues to be used in testing despite there being NO SCIENTIFIC EVIDENCE to back that number - EVER.

To sum this discussion up, Drs. Aneshansley and Gustafson are both engineer/scientists who have worked extensively in litigation cases on behalf of utility companies. Both have acknowledged facts in sworn depositions that proves 500-ohms does not represent the worst-case circuit impedance for cow plus cow contact. So how low is the worst-case scenario for cow resistance? Scientific research done in 2020 with randomly selected cows shows minimum body resistances of 141-ohms as found in Dr. Richard Norell's abstract found above.  Since every cow in every environment has not been tested in a scientific study, the full range, and therefore lowest cow measured, is still unknown at this point. Thankfully, we have field investigations which give us additional data to pull from. The data shown in the table below, with testing conducted by Concept Electric, shows the lowest cow tested at 109-ohms. Therefore, based on both peer-reviewed and scientifically conducted field studies, the worst-case scenario for cow plus contact resistance should be the combination of the lowest tested cow plus cow contact (5-ohms). NOT 500 ohms. Many of the top Ag organizations are promoting the use of a 125-ohm resistor for stray voltage testing on farms.

*The Red Book uses resistance and impedance interchangeably. There is a technical difference between them, however, the Red Book largely ignores the difference for this purpose.