Equipotential Plane and Neutral Bonding in a Milking Parlor
A 200 A, 120/240 V service supplies a dairy milking parlor. The building has a concrete floor with reinforcing mesh, metal stall partitions, metal water piping, and a 50 A hardwired livestock waterer. The farmer reports that cows sometimes flinch and the milker operators feel a slight tingle when touching the metal stalls and the waterer at the same time. The service has a single main bonding jumper at the panel. A subpanel in the parlor has its neutral bar bonded to its enclosure, and the equipment grounding conductor is run with the feeders. The equipotential plane was omitted during a renovation. The waterer is on a 50 A breaker with 6 AWG copper and no GFCI. The metal water pipe is bonded to the service with a 6 AWG conductor.
1. What is an equipotential plane, and why is its absence a likely cause of the tingling? What does the Code require?
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An equipotential plane is a bonded grid in the floor that keeps the floor, stalls, waterer, and other touchable metal at nearly the same voltage. Without it, stray neutral current or a small fault creates a voltage difference between the concrete and the metal stalls, which the cows and workers feel as a tingle. NEC 547.10 requires an equipotential plane in animal confinement areas and requires it to be bonded to the grounding electrode system with a copper conductor not smaller than 6 AWG. Metal stall partitions, the waterer frame, and metal water piping must be bonded to the plane. The plane must be connected to the service grounding electrode system, not to a separate isolated ground rod.
2. The subpanel has its neutral bar bonded to the enclosure. Is that correct? How would you correct it, and how does it relate to the shock?
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No. The neutral bar in a subpanel must be isolated from the enclosure and from the equipment grounding conductor; the neutral-ground bond is permitted only at the service, under 250.24(A)(5) and 250.32(B). Bonding it in the parlor creates a parallel path for neutral current on the feeder equipment grounding conductor, the panel enclosure, the metal stalls, and the equipotential plane. That current produces the voltage difference causing the tingle. Remove the bonding screw or strap, install a separate equipment grounding bar, and confirm the feeder includes an equipment grounding conductor. Then verify the service neutral-ground bond is intact and that no other subpanel is incorrectly bonded.
3. The 50 A livestock waterer is hardwired with no GFCI. Is GFCI required? What other bonding would you check?
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A hardwired 240 V waterer is not a 125 V, 15 or 20 A receptacle, so the general GFCI receptacle rule in 547.5(C) does not automatically require a GFCI. The manufacturer instructions may require one, and a GFCI breaker is good practice in a wet livestock area. The more likely problem is bonding: the waterer frame and metal water pipe must be bonded to the equipotential plane and the equipment grounding conductor. Check for a neutral-to-ground fault inside the waterer, verify the 6 AWG water-pipe bond goes to the grounding electrode system, and confirm the 50 A breaker and 6 AWG copper conductor are correctly protected.