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 ~ National Lightning Safety Institute ~

Section 5.1.1

The Lightning Attachment Process and Risk Management of the Hazard

By Richard Kithil Jr., Founder & CEO, and Richard Hasbouck, Consultant

Introduction

When lightning strikes at or near a critical or high-value facility, stroke currents will divide up among all parallel conductive paths between the attachment point(s) and earth. Division of current will be inversely proportional to the path impedance Z (Z = R + XL, resistance plus inductive reactance). The resistance term will be very low, assuming effectively bonded metallic conductors. The inductance and corresponding related inductive reactance presented to the total return current will be determined by the combination of all the individual inductive paths in parallel—the more parallel paths, the lower the total impedance.

Transfer Impedance and the Earth Electrode Subsystem (EES)

Lightning can be considered as a current source, i.e., output current is independent of load impedance. A given stroke will contain a certain amount of charge (coulombs = amps x seconds) that must be neutralized during the discharge process. If the return stroke is 50 kA, then that is the magnitude of current that will flow, whether it flows through one ohm or 1,000 ohms. Therefore, achieving the lowest possible path impedance serves to minimize the transient voltage developed across the path through which the current is flowing [e(t) = I(t)R + L di/dt)]. Path impedance is directly related to lightning frequency. Efficiency, in part, is a function of EES volume and direction (see IEEE 1100-2005, section 4.8 for further details). Field experience, verified by Finite Difference Time Domain analysis, shows the application of ring electrodes augmented by radial electrodes to have significant advantage over ordinary rod electrodes.

Conclusion

A risk management approach to lightning safety must assume the facility will be struck by lightning. Now what? By adopting a judicious combination of defenses, the lightning safety engineer can attempt to mitigate lightningfs consequences. Since each facility is unique, as is each lightning flash, site-specific designs must be applied. Application of integrated approaches for air terminals, EES grounding electrodes, conductors, bonding, shielding, surge protection devices, etc. will depend on the geographic location and the perceived risk to the facility.

References

  • FAA STD-019e (2005), Lightning and Surge Protection, Grounding, Bonding and Shielding Requirements for Facilities and Electronic Equipment, Federal Aviation Administration: Washington, DC, 2005.
  • IEC 62305, Protection of Structures against Lightning, International Electrotechnical Commission: Geneva.
  • IEEE 1100-2005, Recommended Practice for Powering and Grounding Electronic Equipment, IEEE: New York, 2005.
  • National Lightning Safety Institute, Lightning Protection for Engineers, NLSI: Louisville, Colo., 2006.
  • Yasuda, Y. and Yoshioka, T., FDTD Analysis of Wind Turbine Earthing, Proc. 28th ICLP 2006.

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