Case study: Emergency call center
The Bexar Metro 911 Network District Regional Operations Center in San Antonio was designed by Page starting in 2014 and began operating in 2017. The facility serves as a public safety answering point, or PSAP, for a tricounty area around the Texas city. When 911 calls are made in this area, operators at this facility answer the call and dispatch emergency responders as needed.
Provisions were provided for up to 100 operators to work in the PSAP at once, as well as for support facilities, to allow for training, meetings, emergency operations, and personnel comforts. With a function so critical to public safety, reliability was of great concern to the building’s electrical design. The various radio and computer equipment required to support the operator positions, communication systems, displays, and dispatch systems vital to this mission were required to be backed up by an uninterruptible power supply (UPS) system. Ultimately, two redundant transformerless UPS modules were selected to serve the critical loads during times when utility power fails while the backup generators are starting up.
Grounding was of especially great concern at this facility, as many of the vendors and owners of the radio and dispatch equipment required that any facility using their equipment comply with the Motorola R56 standard, which includes a great deal of special grounding and bonding requirements. These heightened requirements were driven by the need for a very consistent reference ground for proper equipment signaling functions and the need to protect sensitive equipment from elevated voltages in the event of lightning strikes. As a result, the facility’s grounding system is very robust, often exceeding the requirements described in the previous section.
The facility’s grounding-electrode system primarily took the form of a ground ring encircling the building’s perimeter, as permitted by NFPA 70: National Electrical Code (NEC), Section 250.52(A)(4). This section requires the ground ring to be composed of bare copper conductor that is no smaller than #2 AWG. Because of the size of the facility and the importance of its grounding system, the ground ring conductor was sized at #4/0 AWG.
To lower the impedance to ground, 10-ft-long, ¾-in.-diameter copper rod-type electrodes, as described in NEC Section 250.52(A)(5), were bonded to the ground ring at a regular spacing of 20 ft around the perimeter, driven straight down. Per the NEC’s requirements, rod-type electrodes must be at least 5/8 in. in diameter and at least 8 ft in length. As per NEC requirements, the ring and rod-type electrodes were buried 30 in. below the surface of the soil. For a large facility with soil conditions that allow for it, this type of ground ring with rod electrode system forms an excellent basis for a robust building grounding system. See Figure 4 for an installation detail depicting this type of grounding-electrode system.
The other applicable grounding electrodes present in the building, as defined in NEC 250.52, were connected to the ground ring, all through #4/0 grounding electrode conductors. These other grounding electrodes included metal piping and structural steel. The lightning-protection system and outdoor equipment, such as generators and utility transformers, were also connected directly to the ground ring. With this grounding-electrode system in place, ground bus bars were installed throughout the building wherever equipment would need grounding connections, such as information technology spaces and electrical rooms, and each of these ground bars was connected directly to the ground ring with #4/0 grounding electrode conductors.
See Figure 5 for an applicable installation detail for these grounding bus bars. Through this type of installation, any equipment or conductor, such as the neutral of a transformer’s secondary connections, installed in any of these spaces could be connected to the grounding-electrode system through a direct and low-impedance path.
The building’s UPS modules were installed in the main electrical rooms. Both the UPS enclosure and the enclosure of the battery racks were connected directly to the ground bar installed in the main electrical room using #4/0 grounding electrode conductors. Together with ground-fault detection capabilities built into the UPS module, this ensured that the equipment grounding provisions of the NEC would be fulfilled during power transfer when the system downstream of the UPS is operating ungrounded. Refer to Figure 6 for an installation detail showing the connections between the UPS, battery racks, and other equipment to the building’s grounding system.
Providing a robust grounding-electrode system and a direct, low-impedance connection to ground from all electrical equipment enclosures through a local ground bar is good practice in any system, and this example should illustrate that following good grounding practice will go far toward ensuring that a system will remain compliant even if it must operate ungrounded during certain conditions. An ungrounded system may seem unfamiliar to many engineers, but its requirements are easily met by any well-designed grounding system.
Ben Stevens is an associate electrical engineer at Page. He has worked for Page for 3 years and specializes in science and technology projects.