A lightning strike to an unprotected structure can be catastrophic. One bolt can pack up to 100 million volts of electricity. Imagine that kind of current traveling through house circuitry. The National Lightning Safety Institute estimates there are roughly 15 to 20 million ground strikes per year with a higher ratio in areas where soil resistivity is greater.
A path of least resistance
Damage from lightning strikes to residences and businesses can be reduced with the installation of a lightning protection system. Systems are designed to control or provide a designated path for the lightning current to travel to ground, with the purpose of minimizing the risk of fire or explosion within nonconductive parts of the structure.
Most lightning protection systems are made up of several components, including air terminals (lightning rods), conductors, and surge arrestors and suppressors. Regardless of the type of system, all must link to some type of ground terminals, usually in the form of metal rods driven into the earth.
While grounding is not exclusively intended to prevent lightning damage, it can help ensure electrical safety with its ability to provide reliable electrical connection to the earth. Homes that meet the current National Electrical Code (NEC) typically have a grounding-electrode system connected to their electrical service, and these homes are typically equipped with an electrical distribution system, which includes grounded outlets.
Commercially, good grounding is essential for the power quality of electrical equipment and distribution systems. The IEEE Emerald Book provides additional guidelines for grounding electronic and electrical equipment. Some utility companies develop substation grounding grids to provide electrical protection for items such as power transformers.
Grounding rods can vary from sophisticated electrolytic rod systems with active soil moisture replenishment to metal-coated steel rods. In addition to the copper-clad and stainless-steel ground-rod electrodes currently offered within the metal-coated steel-rod category, the industry has a relatively new alternative with UL-listed hot-dip galvanized (zinc-coated) ground-rod electrodes.
According to David Prior, technical services manager at Galvan Industries-which manufactures all three types of metal-coated rods-Underwriters Laboratories has listed the hot dip galvanized ground rod electrode to UL 467, ensuring the same critical criteria is mandated for the galvanized rods as the copper-coated rods currently listed.
“Since the 5/8-inch nominal diameter copper-clad and hot-dip galvanized rods are produced fundamentally from the same steel core, the only difference is the coating,” said Prior. “The intention of UL 467 is to provide conformity testing of the rod throughout the industry.”
Before the introduction of the listed galvanized ground rod, most manufacturers produced the ground rods to an ANSI C135.30 specification, which expired in 1993 and did not meet the NEC. The listed galvanized ground rod, Prior said, meets the strictest interpretation of the NEC.
All rod-type grounding electrodes or “ground rods” are manufactured from a steel core with a nonferrous coating to guard the steel. To protect the industry against poor-quality steel, Galvan and other NEMA members developed the ANSI-approved NEMA GR-1 in 2001 ground-rod specification, which established minimum acceptable performance criteria.
Eliminating coating controversy
Electrical codes allow users to specify either bare or coated grounding electrodes. UL-approved coatings include copper, zinc or stainless steel. Galvanized rods have a zinc coating thickness of 3.9 mils (.0039 in. or 710 g/m2) and copper rods are coated to a thickness of 10 mils or 0.01 in.
Technical information indicates that hot-dip galvanized coatings are formed through a diffusion reaction between iron and zinc resulting in a metallurgical bond of the two metals. Copper is electrodeposited as a pure copper coating bonded to the steel's surface.
Within the industry a difference of opinion stems from a belief that copper cladding is superior because its thicker coating offers better conductivity of fault than zinc and provides longer life due to better corrosion resistance. The difference in conductivity between copper and zinc coatings is statistically insignificant, according to IEEE-80, said Prior. Either coating is capable of safely conducting fault to ground.
“With regards to life expectancy, state departments of transportation have used buried galvanized steel culvert bridges for decades. If galvanized steel could only be expected to last 15 years, our transportation system along secondary roads would have been closed to traffic permanently long ago,” Prior said.
According to the National Association of Corrosion Engineers, the galvanic or electromotive force (EMF) scale of metals illustrates in ascending order the more-noble metals to less-noble metals. Copper is more noble than steel and zinc.
“Less-noble metals are sacrificial to more-noble metals when they are connected in a corrosion cell. That means a steel core will sacrifice itself to protect the copper coating if it's damaged while being driven into the soil. Zinc is less noble or anodic and will sacrifice itself preferentially to protect the steel core if the coating is damaged,” Prior said.
Prior pointed out that neither galvanizing nor copper cladding of steel ground rods provide the ultimate protection against corrosion in soil. Other environmental considerations such as pH, electrical resistivity, moisture, stray AC or DC current, and dissimilar metals are additional factors that influence corrosion.
Prior added, “Each application must be evaluated by a qualified engineer based upon the local conditions experienced at the job site. There are instances where copper will be the logical choice and others where galvanizing is the most effective. I wouldn't advise using a galvanized rod in a coastal environment with heavy chloride contamination adjacent to it, nor using copper rods adjacent to galvanized steel screw anchors.”
According to Michael Johnston, director of education of the International Association of Electrical Inspectors, all grounding installations, especially those performed for remodels, retrofits, service changes or new construction, are generally subject to electrical inspection for NEC compliance.
“From a Code standpoint, the NEC has been relatively silent on the Code requirements that relate to lightning protection systems; however, the NEC requirements (Sections 250.60 and 250.106) cover the materials and bonding of the electrode of a power distribution system to the ground terminal of a lightning protection system on the same structure,” Johnston said.
The requirements for lightning protection systems are provided in NFPA 780-2004 Standard for the Installation of Lightning Protection Systems.
Johnston added if rods are used as the electrode for a power supply system for a building or structure, there are several primary items that concern electrical inspectors:
_Rod size and depth-Minimum diameters must be met, but the single most important factor in grounding, according to Prior and Johnston, is length, with at least one 8-foot rod driven flush with the earth. Provision in NEC Section 250.53(G) can be referenced where rock bottom is encountered presenting difficulties with a driven perpendicular installation.
_Resistance-The Code as a minimum requires that where a single rod, pipe or plate electrode is used, the connection to the earth not have a resistance greater than 25 ohms (250.56). A single rod-, pipe- or plate-grounding electrode that produces greater resistance should be augmented by an additional electrode of any of the types specified in 250.52(A)(2)-(A)(7), which many local jurisdictions mandate.
_Connections-Connections to the rod-type electrodes generally must be listed and compatible both with the material of the rod and the conductor used (250.70).
“The galvanized rod is certainly recognized by the NEC, UL, NEMA and is ANSI-approved and, as long as compatibility issues are met with environmental, physical-protection and connection concerns, the inspector should have a minimum basis for approval of the installation,” Johnston said. EC
MCCLUNG, owner of Woodland Communications, is a construction writer from Iowa. She can be reached via e-mail at firstname.lastname@example.org.