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How does one choose the lightning protection class for risk assessment calculation, and how do you calculate radius of striking sphere & lightining current?

Hi Sekar,

Thank you for your question regarding lightning protection class, it is our pleasure to help.

There are a number of different and varied methods of providing protection from lightning strikes, and the selection of which methods to use is a critical one.  Clearly, a chemical factory, a facility handling flammable materials, or a computer data center with millions of dollars worth of electronics, should be very  concerned with the damage caused by a lighting strike.  A warehouse holding non-flammable items, a garage, a simple office building, etc., may not be so concerned.

The proper design of a Lightning Protection System is a detailed task, especially when protection is needed in complex 3-dimmensional environments (such as irregular shaped buildings, lattice structures, pipe systems, towers, tanks, etc.).  A lightning strike at an unprotected facility could cause catastrophic damage that could result in fire, explosions and serious risk to bodily harm for personnel.  As such, a properly designed Lightning Protection System (LPS) is needed.

There are three (3) steps to consider when designing a Lightning Protection (LP) system for critical systems:

  1. Lightning Protection Planning –This tells the engineer where lightning protection systems need to be installed in order to protect the facility.
  2. Lightning Protection System Construction Documents –These documents include A/E drawings and blue prints, work plans, grounding systems, and installations specifications necessary to get applicable permits and complete the physical installation of the LP system.
  3. Lightning Strike Analysis – This analysis defines the anticipated lightning strike profile in electrical terms.  Details such as the Ground Potential Rise, the Time Domain, and the Frequency Spectrum of the strike are determined.  This data is used to reduce downtime in the event of a lightning strike by allowing custom designed surge suppression systems and critical data for the electrical coordination study.

The purpose of item #1, Lightning Protection Planning, is to determine where lightning protection systems should be installed at the facility, where lightning masts are needed, catenaries, and other protection systems in order to meet the required standards.  The standard that we recommend most is the Rolling Sphere Method (RSM) based on IEEE998.  Other methods, such as the Protection Angle Method (PAM), the Improved Electrogeometric Method (IEM), the Standard Collection Volume Method (CVM-S), and the Mesh Method (MM) are often considered during the design phase.  However, the best method of lightning protection is the IEEE998 RSM.  The level of protection is based on a risk percentage.  We often see either a Level 1 (99%) or a Level 3 (91%) selected.  This selection automatically determines the Angles, sphere radius, peak current, and basic impulse level.  Please see IEEE998 for more information.

The second step (#2) of the process is the development of actual work plans, and construction drawings, including A&E blue prints.  These documents will be used by the various contractors to obtain permits, purchase materials, and construct the actual system.  Specific details regarding what lightning masts to purchase, specific placement (x,y) of the masts, footing depths, maximum cantenary wire distances, etc. will be detailed out at during this process.  As such, such details such as this are not determined during step #1.

The third step (#3) of the process takes us back to the computer where more analysis is conducted.  This step can often be made concurrently with step #2.  The goal of this process is to analyze specific details regarding the electrical profile of a lightning strike at the specific facility.  When lightning strikes an object, the object itself becomes an antenna for the strike, thereby predictably changing the frequencies at which the strike will resonant.  Knowing the predicted frequency profile of the lightning strike at the facility can allow the Electrical Engineers conducting the facilities OCPD coordination study to compensate the timing of breakers so to minimize downtime.  The data can also be used to tune band-pass filters to the specific frequencies of the strike, thereby providing better surge protection.

Modeling a good lightning protection system requires computer program in order to ensure that all the variables have been covered.  We here at E&S Grounding Solutions use and recommend SES-Shield 3D which is part of the CDEGS software package www.sestech.com. We would of course be more than happy to help you with your Lightning Protection Plan, but if not us, please get someone with the proper software to help you with this critical task.  Feel free to call us at 310-318-7151 California time or email us at [email protected] for more information.

Best regards,

The Engineering Team at E&S Grounding Solutions

 

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2 Responses

  1. Petrosav

    I would like to know the same. Unfortunately, “The Engineering Team at E&S Grounding Solutions” didn’t give an answer to the question.

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