Bryan Cole, Technology Research Council, Nichols, New York, USA
Many articles, papers, and standards have been written and/or developed documenting proper application of surge protection devices (SPDs), identification of SPD performance characteristics, proper SPD safety requirements, etc. However, there are minimal articles on describing when an engineer should specify SPDs to be applied to an electrical distribution system.
SPDs are installed to protect against transient overvoltage and overcurrents from affecting the electrical systems and processes within a facility. Transients occur from environmental and human factors. Protection of the facility, the electrical system and the processes contained within the facility are the second most important item to be considered in the power quality pyramid; preceded only by grounding and bonding for the safety of personnel (Figure 1).
There are numerous environmental causes that can disrupt the facility, the electrical system, or the processes within the structure. These factors include hurricanes, tornados, floods, lightning, etc.
Transients from environmental causes include those from direct and indirect lightning strikes. To protect a facility from lightning induced transients, a lightning protection system is needed. When protecting a structure from direct lightning strikes, standards require that SPDs be installed whenever a lightning protection system is installed .
In the design of an optional, legally required standby, or emergency power system, a risk assessment of the interaction between environment and human factors is not mandated by the National Electric Code (NEC) . In the design of a critical operating power system, the NEC requires that a risk assessment be conducted . Even if not required by the NEC, a risk assessment of environment and human factors for all power systems should be considered in the design or redesign of every facility.
There are many factors to be considered in the risk assessment. Lightning risk assessments are described in US and international standards [2,3]. This article will focus on a risk assessment to determine if a lightning protection system and SPDs should be installed using the National Fire Protection Association standard on Lightning Protection Systems, NFPA 780. Annex L of NFPA 780 describes methods for simplistic and complex risk assessment. This article focuses on a simple risk assessment.
LIGHTNING RISK ASSESSMENT
Performing a risk assessment to determine if the facility needs a lightning protection system requires the engineer to compare environmental factors (Nd) to the tolerable risk factors (Nc). Comparison is conducted by a ratio between the environmental factors and the tolerable risk. If the calculated ratio is 1.0 or greater, then a lightning protection system, which includes SPDs, is required. If the calculated ratio is less than 1.0, then a lightning protection system is not required.
The environmental factors are calculated using the equation of Equation 1.
The environmental factors consist of the collective area of the facility (Ae), its surrounding environment (C1) and the lightning flash density (Ng) of the area. There are different equations to determine the collective area of the facility based on the type of structure: standard rectangular structure, rectangular structure with prominent riser, rectangular structure with small riser. The collective area for a standard rectangular structure is calculated using equation Equation 2.
where L is the length of the structure, W is the width of the structure, and H is the height of the structure.
The surrounding environment of the facility has an integral affect on if and how lightning is going to strike a structure. Isolated structures located on a hilltop or mountain top are more vulnerable to lightning strikes than a structure located amongst similar sized structures. Determining the surrounding environment coefficient is done by choosing the appropriate values from Table 1.
The final parameter needed to calculate the environmental factors associated with the facility is the lightning flash density. The lightning flash density is the amount of lightning flashes that occur per year per kilometer. This value can be obtained through a variety of sources. However, it is important to understand that averages can change over time. Therefore, one should obtain not only the average of an extended period, e.g. ten years, but also maximum and minimal values over a short period of time, e.g. three months. A lightning flash density map is shown in Figure 2.
The tolerable risk of the facility (Nc) is determined by equation EQ3 and is dependent on the type of structure (C2), the contents within the structure (C3), the structure occupancy (C4), and the consequence of the loss of operations of the structure (C5).
The type of structure is either metal with a non-metallic roof or metal with a metallic roof. Structures with other construction are not considered in this risk assessment. The coefficients for the type of structure are shown in Table 2.
The content of the structure is the second parameter to be determined. The structure contents range from low value, nonflammable contents to those of exceptional value, irreplaceable cultural items. The coefficients associated with each parameter are denoted in Table 3.
The occupancy of the structure is the third parameter that is determined. The definition of structure occupancies are: unoccupied; normally occupied; or difficult to evacuate. The coefficients associated with each parameter are denoted in Table 4.
The consequence of an interruption of service as a result of lightning is the fourth parameter to be determined. The definitions are: continuity of service is not required, no environmental impact; the continuity of service is required, no environmental impact; or the there are consequences to the environment. The coefficients associated with each parameter are denoted in Table 5.
The result of the lightning risk assessment will provide insight into whether a lighting protection system, which includes SPDs, should be installed. If the calculated value of the environmental factors is equal to or exceeds the calculated value of the tolerable risk, which results in a Nd/Nc ratio of 1.0 or greater, then a lightning protection system, and SPDs, should be installed. If the Nd/Nc ratio is less than 1.0, then a lightning protection system is not required.
LIGHTNING RISK ASSESSMENT EXAMPLE
In this example, we need to determine if a new structure that we are designing should have a lightning protection system based on the following parameters:
1. Structure size – 100 meters long, 60 meters wide, 15 meters tall
2. The structure is the tallest structure in the vicinity
3. The location of the facility is in St. Petersburg, FL
4. The structure is metal with a metallic roof
5. The structure contains and data center for a regional bank
6. The structure is normal occupied with more than 300 people
Based on these conditions, the values and coefficients have been determined and are located in Table 6.
The environmental factor for the structure (Nd) is calculated as 0.42819. The tolerable risk factor (Nc) is calculated as 0.00017. Dividing the environmental factor by the tolerable risk factor returns a value of 2569. Any number of 1.0 or greater indicates that a lightning protection system should be installed, whereas a number less than 1.0 indicates that lightning protection system is not required.
A lightning protection system is an important component in protecting a structure, electrical systems and critical business processes. Surge protective devices (SPDs) are an important component of a lightning protection system and are required by U.S. and international standards to be installed if a lightning protection system is installed.
Knowing when to and when not to apply a lightning protection system is important analysis that an engineer must examine when design a new structure or updating an existing structure. Using a lightning risk assessment is a tool that an engineer can use to determine whether a lightning protection system and associated SPDs are required.
The lightning risk assessment should take into account parameters associated with the structure and its surrounds, the lightning flash density of the location, and the importance of the facility and its processes to the business, the community, and the environment. While the NEC only mandates that critical operating power systems be subjected to a lightning risk assessment, this requirement should be extended to all legally required and emergency power systems.
 National Fire Protection Association (2011), “Lightning Protection System,” NFPA 780, Quincy, MA USA.
 National Fire Protection Association (2011), “National Electric Code,” NFPA-70 Quincy, MA USA.
 International Electrotechnical Commission, “Protection against Lightning – Part 3: Physical Damage to Structure and Life Hazard,” IEC 62305-3.
ABOUT THE AUTHOR
Bryan Cole is the president/owner of Technology Research Council. Cole has more than 20 years experience in the design, development, application, and product safety of power quality equipment, aviation instrumentation, and various low-voltage distribution equipment. He is an IEEE member, a number of UL Standard Technical Panels, and has assisted in the development of more than 30 national and international standards related to Electrical Power Systems.