In agricultural facilities, voltage gradients can be present in the earth. Neutral-to-earth stray current can affect farm animals and cause significant behavior problems, loss of production and death.

These stray currents, or “tingle voltages,” can appear between various conductive components, including metal piping systems, conductive flooring, metal rails and feeding troughs. Some common causes include current present in electrical distribution systems, faulty wiring on or in farm buildings, and inappropriate neutral-to-ground connections on the load side of the service grounding point or grounding point of a separately derived system. Ground-fault events can also introduce current into the earth. Where electrical wiring systems and equipment are effectively grounded and bonded, a phase-to-ground fault condition should operate an overcurrent protective device and clear the abnormal condition quickly. A separate equipment grounding conductor (EGC) that is not part of a listed cable assembly—if installed underground—must be insulated as indicated in Section 547.5(F) of the National Electrical Code.

System requirements

Another important requirement for electrical systems in agricultural buildings is to isolate the neutrals, EGCs and other noncurrent-carrying equipment parts for milking parlors, cattle corrals and feeding areas in buildings. The Code requires isolating grounded (neutral) conductors at separate buildings in accordance with 250.32(B).

These general requirements are supplemented by the provisions in 547.9(B)(3). Each agricultural building or structure must meet the provisions in 250.32 and the sizing and connection rules in 547.9(B)(1) through (3). The minimum size required EGC must be no smaller than the largest ungrounded supply conductor for the building if the same conductor material is used. If the EGC is of a different material, it must be adjusted in size in accordance with the equivalent sizes in Table 250.122.

The required connections between the grounded (neutral) conductor, the EGC and the noncurrent-carrying metal parts of equipment must be followed. This connection must only be made at the site-isolation device at the distribution point of the premises.

The site-isolating device provides a disconnecting means to isolate the agricultural premises’ wiring system from the serving utility for emergency or maintenance, usually. It also provides an accessible point for connecting an alternate power source during electrical outages. In accordance with 547.9(A)(2), the site-isolating device must be pole-mounted and an isolating switch, and is not considered the service disconnecting means for the agricultural premises.

A grounding electrode system or electrode is required to be connected to the grounded conductor at a site-isolating device. When feeders are routed, either overhead or underground from the site-isolating device to buildings or structures on the premises, the feeder must provide an insulated grounded conductor and a separate EGC and meet all other applicable requirements in 250.32.

Equipotential planes

An equipotential plane, defined in 547.2, must be installed in livestock areas as prescribed in 547.10(A). Cows, bulls, horses and other four-legged animals are covered by this rule, but not poultry. Animals with four legs present unique (step potential) problems because of their multiple contact points with the ground (earth), which provide multiple paths for current to traverse the body. The equipotential bonding plane is usually installed before the concrete is poured to let it cure and bond simultaneously with the conductive elements contained in it.

Areas required to be equipped with an equipotential plane are indoor animal confinement areas with conductive floors where metallic equipment that could become energized is accessible to the livestock area. It is important to remember that concrete floors are considered conductive.

An equipotential plane is also required for outdoor animal confinement areas with concrete slabs and for metallic equipment that could become energized and is accessible to livestock. The equipotential plane must encompass the entire area where livestock would normally be standing and are subject to contact with metallic equipment that could become energized.

The equipotential plane can be fabricated using wire mesh, reinforcing bars and other steel rods and conductive elements. The grid is created when all conductive elements are bonded together by, at minimum, an 8 AWG solid copper conductor. It must connect all concrete-encased steel and must extend up to any metal stanchions.

Equipotential planes must also be to the building electrical grounding system, which includes the grounding electrodes and the EGCs. The bonding conductor must be a minimum 8 AWG solid copper conductor. All connections between the bonding conductor and the conductive elements or wire mesh must be made using pressure connectors or clamps made of brass, copper alloy or other equivalent and substantial connection means.

More information is provided in the American Society of Agricultural and Biological Engineers EP473.2 JAN2001(R2020), “Equipotential Plane in Animal Containment Areas.”

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