Summer brings a significant increase in thunderstorms, which can range from quickly passing single-cell thunderstorms to large tornado-producing supercells. Every thunderstorm produces lightning. There are three primary types of lightning, with cloud-to-ground lightning posing the greatest threat to the electric power system. Lightning has been a factor in wildfires, electric system equipment failures, personal injuries and even a wide-scale blackout.  

Florida experiences more lightning than any other area in the U.S. because it is surrounded by large bodies of water which have an abundance of available energy that storms use to develop. The South has significant lightning activity each summer, especially along the Gulf of Mexico, but any region of the country that experiences lightning will benefit from the following mitigation techniques.  

Electric System Lightning Mitigation Practices

Lightning is a significant cause of transmission and distribution line trips and power outages. Exposed overhead lines cover long distances and therefore have a high probability of being struck by lightning.

Typically, for transmission and distribution lines, overhead ground wires, shield wires, lightning arresters and the systematic application of lightning protection equipment are used to mitigate the risk of strikes. Still, the random nature of lightning makes it impossible to eliminate the risk. Shielding failures, direct strikes to phase conductors and induced over-voltages are the principal mechanisms of flashovers on higher-voltage transmission lines.

Cables and transformers are almost always protected by surge arresters. Inadequate lightning protection on a distribution system can result in faults that may cause through-fault failures of substation transformers and pole-mounted distribution transformer failures. Additionally, it may result in burn-through of wires that break and become a serious public safety hazard.

If your service territory is prone to thunderstorms, it is essential to conduct a lightning strike study and implement an effective mitigation strategy, which can include measures such as these:

• Consider designing distribution lines with grounded conductors placed above the phase conductors, which intercept lightning strikes that would otherwise terminate on the phases. There should be an inspection program for overhead ground wires to ensure they are intact and properly installed.
• Inspect and install pole grounds, as needed, which is required to maintain the effectiveness of overhead ground wires. These grounds are required at every equipment pole or at regular intervals based on careful study of earth resistivity and lightning activity, and in accordance with the National Electrical Safety Code (NESC). 
• Maintain low pole ground resistance, generally measuring less than 10 ohms or in accordance with your company procedures. 
• Use modern-day surge arresters manufactured with polymer housings, which appear to have become the product of choice for protecting distribution lines and equipment. 
• Make sure transmission structures also maintain low ground resistance to be effective and prevent line flashover. The amount of resistance to be maintained should be in accordance with your company procedures. Periodic inspection and testing of the grounds should be included as part of your company standards.  
• Be aware that transmission lines, which have uninsulated conductors and are at higher voltage levels, are more susceptible due to their higher elevation and are generally protected with the installation of continuous shield (static) wires. These shield wires should be inspected for proper installation. 
• Test protective relays periodically in accordance with your company standards.  

Power transformers, breakers and switchgear in substations are also susceptible to the direct and indirect effects of lightning strikes. Lightning arresters on substation equipment provide protection along with a well-maintained station ground grid to discharge power surges safely.

Wind turbines are highly susceptible to lightning strikes due to their height and spinning blades. These structures can be a magnet for lightning, and direct strikes usually result in significant damage to the blades along with damage to the generators and control system electronics.  

If you would like to read about the lessons learned from lightning strikes associated with major closed electric claims, visit the AEGIS website. Log into My AEGIS > Review of Major Liability Losses > Major Closed Electric Claims and enter “lightning” in the search field. If you don’t have access to My AEGIS content, you can request it on the My AEGIS login page.

For more information or electric system support, please contact Scot Macomber by e-mail or 201.508.2739, or your designated AEGIS Loss Control Senior Professional.

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