Explain how lightening occurs
Electrical separation occurs in the clouds. The clouds become negatively charged and the ground becomes positively charged. Eventually the cloud sends down a stepped downwards leader and the ground sends up a streamer from high points such as the metalic top of buildings or trees. When the leader and the streamer touch then discharch occurs which releases 3-200KA at 100million to 1billion volts. Duration of the event is from 1 microsecond to 1 second.
What are the sources of lightening damage and what type of damage can they cause.
S1: Direct hit to the structure can cause fire explosion, structural damage, loss of life or injury,
S2: Flashes near the structure can cause damage to the services from electro magnetic pulse.
S3: Flashes to services attached to the structure can cause fire, mechanical failure and services damage.
S4:Flashes near the services connected to the structure can cause all of the above problems.
What are the three types of lightning damage
D1: Injury or death
D2: Physical damage such as from fire, explosion or chemical release
D3: Failure or damage to internal systems
What are the 4 levels of lightning protection
•LPL I can see a range of peak current from 3kA to 200kA with a probability that: 99% of strikes will be < 200kA, 99% of strikes will be >3kA
•LPL II can see a range of peak current from 5kA to 150kA with a probability that: 98% of strikes will be < 150kA 97% of strikes will be > 5kA
•LPL III can see a range of peak current from 10kA to 100 kA with a probability that: 97% of strikes will be < 100kA 91 % of strikes will be > 10kA
•LPL IV can see a range of peak current from 16kA to 100kA with a probability that: 97% of strikes will be < 100kA 84% of strikes will be > 16kA
What are the 4 components of a lightning protection system?
Explain an air termination system
Air termination system
BS EN 62305-3 advocates the following,
Air Rods (or Finials)
Catenary (or Suspended). Could be Enhanced by Spanned wires and cables
Meshed Conductor Network
The combination of All
To capture the lightning discharge current and dissipate it to earth via the down conductor and earth termination system.
What are the types of air termination system
1)Air Rods (or Finials or Lightning Rod) whether they are free standing masts or linked with conductors to form a mesh on the roof (Figure a)
2)Catenary (or suspended) conductors, whether they are supported By Free Standing Masts or linked with conductors to form a mesh on the roof. (Figure b)
3)Meshed conductor network that may lie in direct contact with the roof or be suspended above it (in the event that it is of paramount importance that the roof is not exposed to a direct lightning discharge). See Figure C.
What is the typical distance between down conductors for the different classes of lightning protection system?
Levels 1&2: 10M
Level 3: 15M
Level 4: 20M
Describe the rolling sphere method of calculating lightning protection coverage
Use an imaginary sphere of radius S over the surface of a Building (or Substations).
•The sphere rolls up and over and is supported by Air rods (or finials), lightning masts, Meshed conductor, shield wires, substation fences, and other grounded metallic objects that can provide lightning shielding.
•This method is particularly useful where the building roof/top is not flat and a lot of protruding parts.
A Part of Building Construction, equipment is said to be protected from a direct stroke if it remains below the curved surface of the sphere.
What is the radius of the rolling sphere for each level of lightning protection
Level 1: 20M
Level 2: 30M
Level 3: 45M
Level 4: 60M
What is the protective angle method
A protective angle is a method of calculating area protected using an imaginary cone. The angle a function of the height of the air terminal and the level of protection. The higher the level of protection the steeper the angle and the lower the volume of the cone.