ENSURE PROPER GROUNDING
The third and equally important part of a lightning protection system is the grounding. The MAWSS standard specifies that there should be one single ground point that is common to all electrical equipment at each site. That interconnection should include lightning protection, surge suppressors, electrical service, telephone, antenna, motor control center, generator, fuel tanks, and any other electrical equipment at each site.
The contractor is encouraged to test the grounding for all electrical equipment at existing locations to verify they are common. If they are not, he should run copper wire underground between ground rods and equipment to ensure that they are common. All connections for ground wires to ground rods shall be cad-welded, according to standard 4014 of the NFPA 780.
Noncurrent-carrying metal parts of electrical equipment such as cabinets, enclosures, frames, and the neutral conductor should be grounded in accordance with the National Electrical Code. The metal backplane of all enclosures should be grounded.
Ground rods should be copper-coated steel, ¾ inches in diameter, and 20 feet long. The top of the ground rod should be driven to 1 foot below finished grade and cad-welded to the ground wire. Resistance to the ground of each rod should not exceed 25 ohm when measured in dry weather.
— May is a consulting engineer with Mobile, Ala.-based engineering firm McCrory & Williams Inc.In the heart of darkness
The potential for damage justifies the costs of a lightning protection system.
Implementing a lightning protection system isn't cheap. It may cost between $2,000 and $20,000 per site, but it is worth the investment.
From Houston to Tampa, the central Gulf Coast is home to some of the highest frequencies of lightning strikes in the nation: An average of 20 million lightning strikes occur each year in the United States, most of which hit the Gulf Coast. Located in the heart of lightning country is Mobile, Ala.
“We have a large number of thunderstorms with a lot of lightning — especially during the summer,” says Charles Hyland, water and sewer administrator for the Mobile Board of Water and Sewer Commissioners (MAWSS). So it made sense for Hyland and his colleagues to implement a project designed to protect city buildings and equipment from lightning strikes in this city of half a million.
A lightning strike may cause cascading failures on lift stations: The strike first takes out the SCADA antenna, which is connected to a radio, whose capabilities are then destroyed. The radio is connected to a remote terminal unit (RTU), which also likely is ruined. The RTU is the brain of a lift station since it controls and monitors each site. The RTU itself costs approximately $6,000. “Stopping the lightning before it knocks out the RTU point not only saves the RTU but undoubtedly numerous other devices that have connections to the RTU,” Hyland says.