1 / 17

Lightning Chapter 7

Lightning Chapter 7. Need for Lightning Protection. Major cause of farm fires. Major cause of electrical equipment failures. Cause of more that 80% of accident livestock deaths. Need for Lightning Protection-cont.

happy
Télécharger la présentation

Lightning Chapter 7

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. LightningChapter 7

  2. Need for Lightning Protection • Major cause of farm fires. • Major cause of electrical equipment failures. • Cause of more that 80% of accident livestock deaths.

  3. Need for Lightning Protection-cont. • NLSI (National Lightning Safety Institute) estimates total damages in the US each year exceeds 5 billion dollars. • Cause of human fatalities.

  4. Need-cont. The risk is not the same for everyone.

  5. Lightning Characteristics A. Certain weather conditions can cause an abundance of negative charges to gather on the bottom of clouds, while positive charges accumulate on buildings, trees, or any objects that project above the ground B. When the potential between the cloud and ground becomes great enough a streamer of negative charges moves erratically toward the earth. C. At the same time, a short leader of positive charges may move a short distance up into the air. D. When the two charges meet, the downward moving streamer completes the grounding path as the positive charges instantaneously move back up the path to the clouds. E. The resulting flash is lightning. A lightning "stroke" happens very quickly and contains a great deal of electrical energy.

  6. Characteristics-cont. • Typical lightning strike can be up to 100,000,000 volts and 300,000 amps. • Can heat the surrounding air to 50,000 oF. • Main bolt may side flash—jump from one ground object to another.

  7. Characteristics-cont. Lone trees and isolated buildings, which are closer to the clouds than their surroundings, tend to concentrate positive charges.

  8. Lightning & Buildings Lightning can enter a building in one of four ways: It can strike a metal object on the roof. It can strike a building directly (called a direct strike). It can strike a tree or silo near the building and jump to the building. This occurs when the building provides an easier path to ground. It can strike a power line or a wire fence and follow the line or fence to the building.

  9. Lightning Protection System • Five principal parts • Air terminals • Main conductors • Secondary conductors • Lightning arresters • Ground rods • The required size for all of the parts is determined by NFPA 780. • NFPA 780 divides structures into 12 categories. • Most agricultural buildings are considered ordinary structures. • Ordinary structures are wood framed buildings that are used for ordinary purposes and not more that 75 feet tall. • Ordinary structures should be protected with Class I materials.

  10. NFPA 780—Class I materials

  11. Air terminals • Pointed rods or tube mounted at the highest points and strategic locations of the building. • Two purposes • Sharp pointed rods tend to dissipate the electrical charge into the air which keeps the potential between the rod and the cloud low. • Intercept the lightning strike and direct it to the ground. What is the purpose of the glass ball on the lightening rod? Would this style of air terminal be more effective? Explain.

  12. Air Terminals—cont. • Size • Usually 10 to 24 inches high • 3/8 or 5/8 inch diameter • Location • Spaced no more than 20 feet apart on pitch > 3/12. • Within 2 feet of gable end of roof. • 20 feet along the perimeter for roofs with pitch of 3/12 or less.

  13. Conductors • Main • Connect the air terminals to the ground. • Must have a minimum of two. • Must be sized correctly. • Secondary • Used to attach bodies of conductance and bodies of inductance. Bodies of Conductance are metal objects that are attached to or are part of the building and are exposed to lightning. Bodies of inductance are metal objects near lightning conductors that may build up opposite charge to the conductor. • Common grounding • The NEC requires that all lightning grounding be bonded with the electrical and equipment grounding system.

  14. Lightening arresters • Prevent dangerous surges form entering the building wiring when lightening strikes the power lines. • Usually installed at electrical entrance, telephone entrances and on radio and television antennae.

  15. Grounding • Proper grounds are critical to ensure the lightning is dissipated without damage. • Type of grounding and number of grounds depends on the soil type and the size of the building.

  16. Example • A lightning protection system must provide a low resistant circuit to the earth. • All of the air terminals should be connected in a common grid. • Multiple grounding rods must be used. Is the barn and silo in the illustration correctly protected?

  17. Questions ?

More Related