Several years ago, we published an article called “Is that a tingle she feels.” It was about a dairy farmer who had to learn all about electrical measurements in order to remove stray voltages affecting the cows in his dairy. We told of his successes at the time. Now, he and his wife have continued to find sources of stray voltage and remove them one-by-one, further improving animal health and increasing milk production even more in the process. Chuck Untiedt still insists he’s just a farmer, though.
It came to my attention recently that Chuck has now progressed to being somewhat of an electrical expert in his own right - in addition to farming, and is now serving other farmers as a consultant in proper measurement techniques to identify and track down electrical problems.
He recently asked me to review an explanation he wrote for a farm equipment supplier. As I did so, I began to realize that he has a clearer understanding of the importance of grounding and bonding issues and effects than many of us. Here is some of what he wrote recently for other members of the farming business.
“We must be aware of the difference between grounding and bonding - this is critical. Most of the utilities use a multi-grounded neutral wye primary distribution system which uses earth as a conductor in parallel with the primary neutral, this fact allows primary neutral current to enter earth at any point of grounding of the primary neutral to earth, which creates earth currents that seek any and/or all conductive paths back to the source, which is ultimately the substation of the utility primary distribution that serves the dairy farm.
Please be aware that electricity takes all paths in inverse proportion to the resistance of the path, not just the path of least resistance. With the intentional grounding to earth, you create “Earth Currents”; this simply means that the electricity is free to choose its own paths and is not confined to the wire (metallic neutral conductor of the electrical circuit).”
It is my understanding that the NEC has changed the code for dairy farms and no longer allows the use of a 3-wire (single phase) or 4-wire (three phase) service drop or service conductors (in some instances these may be feeders) from the source (utility transformer) for the dairy facility. It is my understanding, that the code now requires the 4-wire single phase (figure 1) or 5-wire three phase installation. This is a crucial, and to this dairy farmer, an essential compliance issue as well as good wiring practice.
Prior to this it was common for electricians to use the 3-wire single phase or 4-wire three phase which used a single conductor (N/GRD) for the neutral (grounded conductor) and the ground wire (grounding conductor), for the electrical circuit conductors between the source and the Service Entrance Panel (SEP) for the dairy facility.
Because the neutral and ground were bonded together in the SEP and because the code requires the SEP to be grounded to earth, for lightning protection, this created a secondary neutral voltage drop due to the secondary neutral current being allowed to “divide” and/or “use” all available paths that are bonded to the ground in the panel as well as an earth return which is the same as the utility usage. This allows the secondary neutral current to divide due to the resistance of the secondary neutral back to the transformer, using all paths in inverse proportion to the resistance of the path back to the source (the transformer) serving the dairy farm.
It is my understanding that by keeping the neutrals and grounds completely separate back to the farm service transformer, that we have eliminated the use of earth and the grounding conductors from normal secondary neutral current, on the secondary distribution system of the dairy farm. I want to caution you on the following. It is my understanding that under “normal” conditions at the transformer the secondary neutral (farm electrical system) is bonded to the primary neutral system (utility electrical system.) This allows the primary neutral current to access the secondary distribution and impose a voltage/current on all conductive paths connected to the secondary distribution grounding system. As a dairyman and owner of the secondary distribution on my dairy farm, I feel this is an unacceptable connection that allows electrical current to access the dairy animals. Fortunately, the NESC provides an exemption to the connection of the secondary distribution system and the primary distribution system. This is commonly known as “Isolating or Isolation”, of the dairy farm, from the primary neutral system. If you have highly effective isolation you prevent or severely limit any access of earth currents that may access the secondary system.”
In the industrial world, where the electrical system is generally contained in a building, what Chuck describes as the preferred wiring, is called a Separately Derived System (SDS.)
The green wire safety ground connection from equipment is kept separate from the neutral at all points except the neutral tap of the distribution transformer secondary. It is a requirement of the NEC that this be done.
Separately Derived System Grounding and Bonding
When multiple bonds between neutrals and grounds are created away from the transformer, the usual problems that surface are related to interrupted network communications, or electromagnetic fields that cause CRT screens to wiggle, but human safety may also be seriously compromised. In the case of a dairy farm, the problems often result in poor milk production and ill health for the animals.
Chuck has used several Fluke logging dmms, similar to the new model 289, and a couple of Fluke Scopemeter test tools to track down the ground loops created on his own farm, and now other farms, showing that they can be effectively used by someone who is “just a dairy farmer.”
A more detailed discussion of the National Electrical Code provisions for SDS grounding and bonding can be found in this recent ECM article.
The 5-wire 3 phase system is similar in that it also adds a separate grounding conductor to the 3 phase conductors and the neutral to direct normal return currents through the neutral, and leakage and fault currents to the grounding conductor.