Current Affairs: Suppressing The Surge
In these particular cases, the primary symptoms of the problem were electronic-level damage to low-voltage systems, and attention quickly focused on control and communication circuits running between buildings and from outdoor equipment into the building. Although signal-level TVSS may be needed even inside a facility, these outside circuits present a special risk.
Grounding and bondingAll buildings with electrical service are required by the National Electrical Code (NEC) to have a grounding system in which the electrical service, enclosures of electrical equipment, structural steel, metal piping systems, and lightning protection (if provided) are bonded together and connected to an earth ground.
Bonding prevents harmful potential (voltage) differences between systems within the building during electrical faults or lightning strikes, and the earth ground connection provides a path for lightning and utility fault currents trying to return to a place engineers refer to as "remote earth."
All grounds aren't the sameUnfortunately, even the best ground system has some resistance in the connection to remote earth, so the potential of that system is elevated when transient lightning or utility fault currents pass through it.
If correctly bonded, this does not represent a hazard to occupants, but the structure itself briefly rises to a higher potential than structures and systems located remotely from it. Even without direct lightning strikes, thunderstorms can produce similar potential differences between structures from lightning currents in the ground or the passage of charged clouds overhead.
These potential differences, which can reach thousands of volts, become a problem when installing wiring between structures. These circuits are then directly exposed to the same type of voltage surges that the TVSS on the power system addresses.
In years past, the only low-voltage wiring running between buildings and from buildings to "remote earth" were telephone circuits, and telephone engineers became experts at protecting these circuits and the connected equipment from surges.
Today, data, alarm, communication, and video circuits that were formerly limited to interior use are often extended between buildings in campus settings and between buildings and outlying equipment such as cooling towers or electrical substations. With this practice comes a need to recognize the problems caused by transient ground potential differences and provide protection against them.
Minimize the riskWhenever a circuit extends beyond the perimeter of the building ground system, the risk of potential differences exists. Whenever a low-voltage circuit enters a building from an outside source, or a circuit originating inside the building must be extended beyond the perimeter, one of the following options should be used to minimize the risk of surge-related damage:
- Use fiberoptic cable instead of metallic conductor. This completely eliminates the risk, but is feasible and cost effective only for certain types of circuits.
- Combine the grounding systems at both ends of the circuit together into a single system. This is effective for outside equipment areas and multiple buildings that are not separated by large distances, but must be included in the design and construction to be practical.
- Install appropriate signal-level TVSS protection at the point where the circuit crosses the perimeter of the ground system. This applies to both ends of building-to-building circuits.
Plan for effectivenessThe TVSS option is the one that I see implemented most frequently, but often only after the fact, when the system has been installed without protection and problems have been discovered. Unfortunately, for TVSS to work effectively, it must be installed properly, and this can best be accomplished if it is planned for during design. Providing direct connection to the building ground system at TVSS locations and arranging wiring to separate unprotected circuits from protected circuits are important considerations.
Specifications for systems such as BAS should clearly address responsibility for providing appropriate TVSS protection on the low-voltage side as well as on the AC power supply. Coordination with the electrical design is required to ensure that effective grounding provisions exist at the locations needed by the low-voltage systems. ES