June 1, 2011
It’s been over ten years since a Minnesota dairy farmer’s call was forwarded to my desk at Fluke in 2000. Chuck Untiedt was considering liquidating his dairy farm because he was losing animals to mysterious causes.He thought that cows coming in contact with voltages from grounded metal fixtures could be the problem.
For the five years after, and before I retired, I assisted him as he learned to use Fluke digital multimeters (DMMs) and a Fluke ScopeMeter® 123 test tool to find and fix the electrical problems on his dairy farm.
The final outcome? When the electrical exposures were removed from the cows’ environment, the dairy herd recovered and flourished, and milk production improved.
I have maintained a continuing dialogue and a friendship with “the udder Chuck” in the years since my retirement six years ago. Recently, I published a column about his work for other dairies suffering similar problems in which he showed his in-depth understanding of electrical grounding and bonding.
Finding the Source of Stray Voltages
Making sense of electrical tests that trace the paths of ground currents across a farm is a daunting task at best, particularly with constantly varying load conditions on the utility primary, the secondary and farm service entrance, on-farm emergency generators, and the electrical equipment that is used in the wells, milking parlors, and other farm facilities. What Chuck discovered a few years ago is the value of recording DMM readings from up to four different sources simultaneously.
Most of the tested points are referenced to a common remote reference ground. If the point being tested is a cow contact point, it is necessary to first read the voltage with a high impedance meter input, and then apply a 500-ohm resistor across the meter input to simulate the resistance of a cow. The difference in readings can be used to determine the source resistance of the detected voltage to estimate the current that will flow through a cow.
Chuck has been using a ScopeMeter® 123 and, more recently, the Fluke Scopemeter® 199C to add waveform information to his investigations. He has used both TrendPlot™ and ScopeRecord™ features to collect data over time. Until now, it has been difficult to align timing of the various recordings because the data was coming from a mix of meters or scopes.
Then, Fluke introduced the 190 Series II four-channel ScopeMeter® test tools.
Now, using the ScopeRecord™ feature, when an event occurs at one point in the system its effects on three other points, including ones where a cow may contact a resulting stray voltage, can be observed on the same screen. The four isolated inputs allow fully independent connections if needed, but in most cases a common remote reference is used.
Chuck has found that when measuring what are primarily 60 Hz signals, the 20 kHz (HF reject) setting is useful. On the other hand, when looking for the presence of repetitive noise or transient signals, either a Full or 20 MHz setting may be needed.
A Real-World Example Using the 190-204
Now it’s time to see how this all works out on a real farm. The case in point is a location where, in addition to the power utility, there are potential stray voltage sources from communications (phone) lines and from a nearby broadcast radio transmitter tower. For most of the 100-screen recording (see the first screen below), the house service and the separate farm service were both active.
Channel A (red) is the reading from the utility primary neutral to the remote reference ground rod. This channel is the only channel operating at full bandwidth, but shows a clean sine wave of 6.6 volts p-p. All other channels have bandwidth limited to 20 kHz.
Channel B (blue) is a cow contact point with 500 ohms load across the input. Note the repeating transients on each cycle, and that the waveform is approximately 90 degrees shifted from the utility neutral signal. The level is 249 mv p-p.
Channel C (black) is the house service panel ground to reference. The p-p voltage registered 2.6 v p-p. There appears to be a high-frequency signal mixed with the 60 Hz.
And finally, Channel D (green) is from the well head ground to the reference. Note the noise spikes that are occurring at the same time as those at cow contact. The voltage is 2.1 v p-p.
The next two screens indicate the change: first, when power to the house is turned off, and second, when power to the farm is switched off.
By analyzing the changes that occur, Chuck and his assistants are able to isolate sources of stray voltage that affect the animals.
Chuck has now gone to new heights, using Fluke and other test tools to document, troubleshoot, and correct dairy farm electrical problems for others in Minnesota and neighboring states. His most recent work incorporates his first use of the new Fluke 190-204 four-channel ScopeMeter® test tool.
He says, “With this new meter, I definitely can find concerns faster and easier and make an even greater difference for the dairy farms I am on. It is so cool to feel that I am making a difference and finding what others may be missing.”