Electrical

backflash

Backflash: When Lightning Strikes Back

In the world of electrical engineering, the term "backflash" refers to a dangerous phenomenon that can occur during a lightning strike. It's not the lightning strike itself, but rather a secondary arc that forms along a structure, typically a transmission tower, due to high impedance in the tower or its grounding system.

Imagine lightning striking a tower. The electricity wants to flow to the ground as quickly as possible. However, if the tower or its grounding system has high impedance (resistance to electrical flow), the lightning current can't travel directly to the ground. Instead, it can find another path - a path of least resistance.

This is where backflash comes in. The lightning current can jump back up the tower, creating a secondary arc, known as a backflash. This arc can travel along the tower structure, potentially causing significant damage and posing a serious hazard to personnel and equipment.

Why Does Backflash Occur?

High impedance can arise from various factors:

  • Poor Grounding: The most common cause of backflash is inadequate grounding of the tower. If the grounding system has high resistance, the lightning current can't dissipate quickly enough, leading to a backflash.
  • Corrosion: Over time, corrosion on the tower structure or grounding conductors can increase impedance, creating a pathway for backflash.
  • Tower Design: The tower's design itself can influence impedance. Certain designs may create points of high concentration for electrical current, leading to a backflash.
  • Environmental Factors: Factors like soil conditions and humidity can influence the effectiveness of the grounding system, contributing to backflash.

The Dangers of Backflash

Backflash is a serious threat for several reasons:

  • Fire Hazard: The intense heat generated by the backflash arc can easily ignite flammable materials near the tower.
  • Electrical Shock: The arc can be extremely dangerous to anyone in the vicinity, potentially causing severe burns or even death.
  • Equipment Damage: Backflash can damage the tower itself, as well as associated equipment like insulators, transformers, and conductors.

Mitigating Backflash

Preventing backflash requires a comprehensive approach:

  • Proper Grounding: Ensuring a low-impedance grounding system is crucial. This includes using thick, conductive grounding conductors and ensuring good contact between the conductors and the ground.
  • Regular Inspections: Regularly inspect towers and grounding systems for corrosion, damage, or any other factors that could increase impedance.
  • Tower Design: Consider designing towers with features that minimize the risk of backflash, such as minimizing high-impedance points and ensuring good grounding at all attachment points.
  • Lightning Protection Systems: Implementing lightning protection systems like surge arrestors and lightning rods can divert lightning current safely to the ground, reducing the risk of backflash.

In Conclusion

Backflash is a serious hazard that can occur during a lightning strike on a tower. It arises from high impedance in the tower or its grounding system, leading to a secondary arc that travels along the tower structure. Understanding the causes and dangers of backflash is critical for ensuring the safety of personnel and equipment. Implementing proper grounding, regular inspections, and lightning protection systems can significantly mitigate the risk of backflash, protecting lives and valuable infrastructure.

Test Your Knowledge

Backflash Quiz

Instructions: Choose the best answer for each question.

1. What is "backflash" in the context of lightning strikes? a) The initial lightning strike itself. b) A secondary arc that forms along a structure due to high impedance. c) The sound of thunder following a lightning strike. d) The visual flash of lightning.

Answer

b) A secondary arc that forms along a structure due to high impedance.

2. Which of the following is NOT a common cause of backflash? a) Poor grounding b) Corrosion c) Strong wind gusts d) Tower design

Answer

c) Strong wind gusts

3. What is the primary hazard posed by backflash? a) Damage to surrounding vegetation b) Radio interference c) Fire hazard, electrical shock, and equipment damage d) Reduced visibility

Answer

c) Fire hazard, electrical shock, and equipment damage

4. Which of the following is NOT a recommended strategy for mitigating backflash? a) Using thick, conductive grounding conductors b) Regular inspection of towers and grounding systems c) Using thicker insulators on the tower d) Implementing lightning protection systems

Answer

c) Using thicker insulators on the tower

5. Why is a low-impedance grounding system crucial in preventing backflash? a) It attracts lightning strikes to the tower. b) It helps to quickly dissipate the lightning current to the ground. c) It enhances the insulation properties of the tower. d) It reduces the electromagnetic interference caused by lightning.

Answer

b) It helps to quickly dissipate the lightning current to the ground.

Backflash Exercise

Scenario:

You are inspecting a transmission tower that has experienced a recent lightning strike. You notice some signs of damage on the tower structure and a section of the grounding system appears corroded.

Task:

Based on your knowledge of backflash, identify at least three potential problems that could have contributed to the backflash event. Explain how these problems could have led to a secondary arc forming along the tower.

Exercice Correction

Here are some potential problems that could have contributed to the backflash event:

  • Corrosion on the grounding system: The corrosion on the grounding system would have increased the impedance, hindering the rapid flow of lightning current to the ground. This increased impedance could have forced the lightning current to find an alternative path, leading to a backflash along the tower structure.
  • Damaged tower structure: The damage to the tower structure might have created a high-impedance point, forcing the lightning current to deviate from its intended path through the grounding system. This deviation could have triggered a backflash along the tower.
  • Inadequate grounding conductor size: The grounding conductors might have been insufficiently thick or conductive, increasing resistance and contributing to the impedance problem. This, in turn, could have facilitated the formation of a backflash.

In summary, the combination of corrosion, potential damage to the tower structure, and possibly inadequate grounding conductor size likely created high impedance, leading to the lightning current seeking an alternative path and causing a backflash along the tower.

Books

  • Electrical Power Systems Quality by H. L. Willis, J. D. McDonald, and T. E. McDermott - This book provides a comprehensive overview of power system quality, including chapters on lightning protection and grounding.
  • Lightning Protection by R. H. Golde - A classic textbook that covers various aspects of lightning protection, including the phenomenon of backflash.
  • Transmission Line Design Handbook by J. P. Reilly - This handbook discusses the design of transmission lines and includes sections on lightning protection and grounding.

Articles

  • "Backflashover: A Critical Issue in Transmission Line Design" by IEEE Transactions on Power Delivery - A technical article that delves into the intricacies of backflash in transmission lines.
  • "Lightning Protection of Power Systems" by Power Engineering Journal - This article discusses various aspects of lightning protection, including the role of grounding and backflash prevention.
  • "The Importance of Grounding for Lightning Protection" by Electrical Safety Journal - This article highlights the critical role of effective grounding in mitigating backflash and ensuring electrical safety.

Online Resources

  • IEEE Power & Energy Society (PES): This organization provides a vast collection of technical resources, including articles, standards, and guidelines related to electrical power systems and lightning protection.
  • National Electrical Code (NEC): The NEC is an important reference for electrical safety regulations, including provisions for lightning protection and grounding.
  • Power Engineering Magazine: This online magazine publishes articles on various aspects of the power industry, including those related to lightning protection and backflash.

Search Tips

  • "Backflash in power systems": This search term will yield relevant articles and resources about backflash in electrical power systems.
  • "Lightning protection grounding": This search term will provide information on grounding practices for lightning protection, which are essential for preventing backflash.
  • "Backflashover transmission line": This more specific term will lead you to resources focused on backflash in transmission lines.
  • "Lightning strike tower damage": This search term will help you find information on the damage caused by lightning strikes, including backflash.

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