Municipal safety centers typically combine police, fire, 9-1-1 and other emergency services and must remain operational at all times. If communications systems fail, dispatchers are helpless, emergency workers can't find each other and the public could be at risk.
COMTEC, the county's impressively rebuilt, 25,000 square feet safety center in Mt. Clemens, Michigan is in the up and running since they're often dealing with life-and-death southeastern part of the state. It's critical that all communications services remain situations in their 9-1-1 dispatch center. All communications systems need antenna towers, which naturally invite lightning strikes that can cause damage to a center's sensitive equipment.
The COMTEC center’s transmission tower was grounded when built, but age and corrosion had deteriorated the connections. Upgrades during restoration of the center included exothermically
welded connections, new grounding electrodes and a buried copper ring ground.
The solution: equip the towers with robust, correctly designed and properly installed lightning protection and grounding systems to direct lightning energy away from sensitive equipment and harmlessly into the earth before it damages equipment.
Victoria Wolber, Macomb County's director of emergency management and communications based at COMTEC, said that they knew they had to call in experts who understood lightning protection and grounding fields, knew their systems and how to protect their critical assets. Since then, she said that COMTEC has never had any lightning-caused downtime.
Wolber and her electrical contractor hired Greg Fair, a project manager and field superintendent with Guardian Equipment Company, a Michigan-based firm that specializes in lightning protection. Since COMTEC was relocating to an existing structure with a pre-existing tower, the challenge was to formulate a system that met the applicable codes and accommodate the site's components.
Tower-grounding cables were originally bonded with mechanical clamps such as the one shown at the far left foreground, but age and corrosion had loosened the connections, raising ground resistance. The facility upgrade included replacing those connections with exothermically welded bonds and 28R Class II lightning conductor cable, center.During reconstruction, the building's open walls helped Guardian Equipment advise the engineers on designing and concealing the new grounding system. The building's antenna tower had previously been grounded, but the existing mechanical connection method was found to be inadequate. Existing connections to the tower showed signs of aging. They were mechanical, in one case using a simple water pipe connector, and had become loose over time. There was also some corrosion, which increases ground resistance.
To aid in the new grounding system design, Fair and his team referenced several codes that dealt specifically with grounding for lightning protection: Underwriters Laboratories' UL96A: "Installation Requirements for Lightning Protection Systems," the National Fire Protection Association's NFPA 780: "Standard for the Installation of Lightning Protection Systems" and the Lightning Protection Institute's LPI 175: "Standard for the Design – Installation – Inspection of Lightning Protection Systems". The well-known and often used Motorola 56A, which also deals with towers, did not fall within the scope of work.
Grounding of the center’s water and gas service entrances is provided by the heavy-gage copper cable. The tag shown alerts technicians that grounding to gas and other services is code-mandated.
Fair and his team began with a site survey to guide their engineers in designing the work. Afterward, the team installed air terminals at 20 foot intervals along the roof's perimeter and elsewhere on the roof. The perimeter cable was used as a ring ground from which to drop down-conductors to driven 10 feet x ¾-inch copper-clad electrodes in the ground below. Approximately 15 driven electrodes were spaced about 100 feet apart surrounding the structure. Two of them specifically bonded to the ring-ground surrounding the tower.
The tower was grounded with 28R cable, a special Class II lightning conductor consisting of 28 strands of bare, braided AWG 14-gauge wire. Class II cable is used for structures taller than 75 feet. It is also good when conductors have to be exothermically welded to ground rods, building steel, or in this case, to the tower itself.
All services, such as electric, water and gas were bonded together at a common ground potential and bonded to the building's grounding system, in accordance with code requirement. Fair tags his installations where appropriate to inform workers of the code-sensitivity of those connections.
When lightning strikes one of the nine auxiliary sites that the county operates, energy is bled off the coax on the vertical run, the coax where it penetrates the firewall, and the interior side of the firewall. Fair said they also bonded to exposed building steel. Bonding the grounding system to building steel is also a code-mandatory practice. However, while building steel might provide a de facto ground/earth connection, a continuous electrical path cannot always be verified and grounding connections should properly be made via adequately sized copper down-conductors bonded to appropriately spaced, driven electrodes.
A rooftop level ring-ground surrounding the structure might also be employed, as was done at the COMTEC facility. The ring simplified installation of the lightning protection/ grounding system, and its copper down-conductors, blended in well with the center's brick walls.
Macomb County operates several auxiliary transmission facilities that are located such that they provide emergency communications to the county's 850,000 residents throughout the large 479 square miles jurisdiction. The auxiliary facilities are each equipped with a tower. The towers, as well as the facilities themselves, are protected from lightning strikes by robust, all copper grounding systems.
The photo on the right shows the reliable, exothermically welded grounding connection to the center cable, which connects the tower’s steel structure to a buried
bare-copper ring ground surrounding the tower and to the interior grounding bar.
Notably, neither the COMTEC facility nor any of the county's properly grounded auxiliary transmission stations have ever experienced a lightning-induced service disruption. For that, Macomb County residents can thank smart decisions by COMTEC management, proper design and installation, regular maintenance, and of course, robust, all-copper grounding and lightning protection systems.
The full article with more details and images may be found at www.copper.org.
Principals
Victoria Wolber
Emergency Management Coordinator
Macomb County, MI
Greg Fair
Project Manager/Superintendent
Guardian Equipment
Greater Detroit Area
