Equipotential Plane and Voltage Gradients in Agricultural Settings and Raised Floors in IT Rooms

This month’s column addresses two rather different, recurring subjects on Electrical Contractor magazine’s “Online Code Question of the Day.” QUESTION: When building a barn for horses, I was required to establish an equipotential ground plane at the entrance, from the concrete floor of the barn to the grass, by using 8-foot ground rods driven in a fan pattern at a 45 degree angle every 12 inches! I used 65 ground rods. Is this a National Electrical Code (NEC) requirement? I was informed that the NEC refers to some agricultural standard that requires all this extra grounding. ANSWER: You didn’t say whether the 65 ground rods were required by the project specifications, the inspector ostensibly enforcing the provisions of the NEC, or the inspector enforcing a local requirement. Assuming it is not a condition of the specifications you contractually agreed to, let’s look at the NEC for its requirements. Section 547-9(b) is very explicit in regard to the establishment of an equipotential plane, which is defined in (a) as “An area accessible to livestock where a wire mesh or other conductive elements are imbedded in concrete, are bonded to all metal structures and fixed nonelectrical metal equipment that may become energized and are connected to the electrical grounding system to prevent a difference in voltage from developing within the plane. For this section, livestock does not include poultry.” (However, it does include horses.) Paragraph (b), however, is not very explicit where it says “to provide an equipotential plane that may have voltage gradient ramps at entrances and exits that are traversed daily by the same livestock.” Being traversed daily by the same livestock makes you wonder exactly what that strange language means. This phrase was a 1999 Code cycle change that took into account the fact that in some industries the animals go in one door under their own power and when they come out it is in a box. So, in effect, that phrase differentiates between a barn and a slaughterhouse. (For livestock in the latter, it doesn’t matter if there is a voltage gradient ramp at the door.) However, dealing with cows or horses is a different matter. Animals that become conditioned to the fact that they are going to be “tingled” when entering or exiting the barn soon become reluctant to enter or leave the building. It is a particular problem with milk cows, whose productivity may be altered when this tingle voltage causes them to “suck up” their udders and give less milk. Fine Print Note (FPN) No. 1 appears to include a mandatory requirement for a voltage gradient ramp where voltage gradients exceeding 1 volt per foot exist at the edge of the equipotential plane. But since FPNs are informational only, there really is no mandatory requirement for voltage gradient ramps in agricultural buildings. This will become much clearer if a proposal that was accepted for the 2002 NEC survives the rest of the Code revision process. This proposal removes all reference to voltage gradient ramps and clarifies the requirements for an equipotential plane. These issues will be regarded as design requirements and will be left up to the owner and designer based on the particular needs of an installation. All things considered, I feel you are a victim of “overkill” and have more ground rods than dirt at the building doors. The NEC does not require the installation of the voltage gradient ramp and it is up to the designer whether a voltage measurement should be taken or not and what action should be taken if the voltage measurement exceeds 1 volt per foot at the edge of the equipotential plane. You can get more information on the American Society of Agricultural Engineers (ASAE) Standard EP473-1997 referenced in FPN No. 2 by contacting the Society at hq@asae.org or by calling (616) 429-0300. QUESTION: Why isn’t the area beneath a raised floor in a computer room considered to be a plenum? ANSWER: The area under a raised floor in an “information technology (IT) room” is a plenum if it is used as part of an air-handling system. The definition of a plenum is shown in Article 100 and applies to any area that fits that definition. Some of the requirements for wiring in plenums as shown in Section 300-22 have been modified in Section 645-5(d) to relax or make them less severe if the room complies with the special requirements of Section 645-2. One of the special considerations given to areas under raised floors in IT rooms is the use of power and communications cables under the raised floor. Generally, wiring in plenums is restricted to metallic cable and raceway wiring methods specified in Section 300-22. This is intended to minimize the products of combustion and help contain flame spread. Section 645-5(d) permits certain nonmetallic cables such as Type DP to have adequate fire-resistant characteristics suitable for use under the raised floor of an IT room. This requirement for fire-resistant wiring methods is continually under attack by those who wish to have IT equipment furnished with ordinary power cords that don’t meet these stringent requirements. Their argument is if we can’t use power cords beneath the floor, we will have to install tombstone-type power receptacles above the floors and create a tripping hazard. Most jobs are done this way anyway, and since it’s common practice, why not allow it? Or, they reason that the amount of flexible cord is minimal and would probably have minimal effect, even if a fire were to start. However, none of the proposals submitted to permit flexible cords not having fire-resistant characteristics contained any restrictions on maximum length of unapproved cord that would be permitted under the raised floor. Because of the “less stringent” requirements for installing wiring for plenum areas of IT rooms, other criteria must be met. Section 645-5(d)(3) requires that ventilation in the underfloor area be used for the IT room only. This is intended to prevent smoke and flame being spread from an information technology room to other areas of a building by means of a shared heating, ventilating, and air conditioning (HVAC) system. Section 645-2(b) permits an HVAC system that serves other occupancies to also serve the IT equipment room if fire/smoke dampers that operate on activation of smoke detectors are provided at the point of penetration of the room boundary. A proposal was accepted for the 2002 edition of the NEC to require that “upon the detection of fire or products of combustion in the underfloor space the circulation of air will cease.” I’m not sure why this proposal was accepted, since the existing requirements prohibit plenum connections to areas other than the information technology equipment room itself. Again, we will have to wait and see if this proposal survives the rest of the Code revision process. So, all things considered, the underfloor area in an IT equipment room is a plenum but some of the general requirements of Section 300-22 have been modified with the provision that certain other special criteria outlined in Section 645-2 are met. TROUT is a technical consultant for Maron Electric Company of Chicago, and represents NECA as chairman of the National Electrical Code-making panel 12. He is also the principal author of ECMAG.com’s online feature, “Code Question of the Day.”

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