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Wednesday, March 6, 2013

Lightning Protection Systems Design - Part I

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One of the most esoteric topics among electrical engineers is the Lightning Protection Systems, more specific lightning rods, as I have already mentioned lightings are a very complex natural phenomenon therefore is it difficult to establish and unified criteria, for this reason is that there exists a lot of opinions and strange myths that brings as result wrong lightning protection designs.
Before going any further I must confess that I am no expert in the subject, I have design a few Lightning rods using “the traditional criteria” that I´ve found in earlier documents, however I found necessary to develop a research about basic concepts. This is the results of it.
The lightning
Air is not a perfect isolating media, given that its dielectric resistance is around 30kV/cm, when a potential difference is reach between tow electrical conductor points a spark will occur inevitably (Family size, the one we call Lightning)
Depending of the polarization, the lightings are classified on negatives (electrons or negative charge ions) or positives (positive charged ions), according to its origin (Fig. 1) there are inside lightning (inside the cloud), intercloud (from cloud to cloud), clout – earth lightning (80% percent of the lightning produced and therefore the most important to us) and at last earth to cloud lightning.
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Fig1. Types of lightning
Despite the short duration that they have (microseconds), lightnings have a huge destructive potential given that they carry current around 30 kA typically, up to 300 kA have been register, therefore the necessity of protecting installations and ourselves.
Lightning Formation
The lightning (this point forward it will be considered as and cloud to earth and negative) is produced by the union of the ion leaders (Fig. 2) the ascendant - Up Streamer and the descendent – Stepped Leader, they precisely are the ones that make a ionize row which is used by the lightning to go through (Fig. 3). The lightning produces when the ion leaders touch each other as seen in Fig. 4.
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Fig. 2 Ascendant and Descendent Leaders
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Fig. 3 – Ionize Row for by the ascendant and descendent Leaders
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Fig 4. Lightning Formed
When a Lightning takes place it drains the negative charge of the cloud, it can occur a several times in a row, that why sometimes it looks like blinking in the sky.

Lightning Engineering Data
Parameter Magnitude
First discharge amplitude Avg. value: 30 kA, only 10% supers 75 kA
Subsequent discharge amplitude Avg. value: 8 kA, value of the 10% - 30 kA.
Increase rate of the first discharge
current
Avg. value: 7kA/us, value of the 10%
13 kA/us
Increase rate of the subsequent discharge current Avg. value: 25 kA/us, value of the 10%
80 kA/us
Discharges by lightning 25% of the lightnings have 4 discharges, the biggest have least than 18
First discharge load Avg. value: 6C, valor del 10% of 30C.
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Protection against Atmospheric Discharges
Given that a lightning is a natural phenomenon and as one it is unpredictable, it is impossible to avoid its incidence on the structures or people 100% of the times, what a protection system does is attract the lightning that otherwise will strike in an undesired area.
The most costumed way to do so is by using lightning rods, the simplest system consist on a captor element of cooper or one with and equivalent resistance, connected solid to earth trough a isolated download wire.
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The all idea is that the lightning rod has a location so it represents functions as an descendent leaders attraction element, setting it above any other structure according the location criteria.
Lightning Rod Location Criteria
Basically there are two methods to realize lightning systems designs, define the location and the amount of the lightning rods; there are the cone protection and the sphere method.
Cone Method
This method consists in setting a cone around the lightning rod and then assuming that the structures and area inside de cone will be protected.
NFPA 780 indicates two types of angles, for structures bellow 7,6m would be considered a 63 degrees angle (2 to 1 relation), for structures above 15 m would be considered a 45 degrees angle.
The next figure shows a zone considering a 45 degrees angle.
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A 3D view, shows the cone formation
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Applying transparency to the cone let us see the house and the person of reference.
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This method is considered obsolete and lacking of scientific basement, several books consulted agree with this consideration and recommend using and electro-geometrical methods.
Rolling Sphere Method
This method derives from the call electro-geometric model (EGM); it predicts that considering an imaginary sphere of determinate radium, the lightning will have a bigger probability to strike the structures or objects that are inside of the sphere area or touch its surface, staying protected everything that it’s in the outside.
The fist reference for this method comes from the work done by Ralph H. Lee in 1997.
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When the sphere is roll around the lightning rod it forms and inverted funnel as shows de next figure.
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Seen in 3D.
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Applying transparency it let us see the house and the reference person, inside the funnel formed by the protection zone.
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The protection zone have a limited range, therefore a portion of the equipment or structure might been left unprotected.
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The house on the right side is no longer been protected as it should, to overcome this problem it have to be installed another pole with a lightning rod on the top of it.
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Now as we see in the figure above the house on the right side is protected by the new lightning rod, both zones overlap and forms a bigger protection zone.
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Seen in 3D we have our new protection zone.
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But there is an additional subject to consider, when the sphere roll between and around the two lightning rods, it creates a bigger zone than the addition of them as is shows in the figure below.
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Resulting in something more likely to a circus tent.
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Design considerations
According to the book: Lightning protection for engineers at the time of the calculation of the resulting protection zone it must be considered the following:
Cone Method:
1. It is a method based in a theoretical approximation and as there is been said, the lightning in most cases wont behave as it is predicted to.
2. The concept applied for this method does NOT implied safety for people since touch voltages may affect them.
3. The angle of protection is in relation to the structures high.
Rolling Sphere Method:
1. It is a method based in a theoretical approximation and as there is been said, the lightning in most cases wont behave as it is predicted to.
2. The concept applied for this method does NOT implied safety for people since touch voltages may affect them.
3. The radium of the sphere depends of the standard that have been considered:
· (US) NFPA 780, R=46m
· (US) Dept Energy and Deot Defense, R=33m
· IEC 62305: Level I, R= 20 m / Level II, R=30m / Level III, R=45m / Level IV, R=60m
· BS 66551 (Brithis) R=20m
See the post in Spanish
 
Standards that apply to the subject:
· NFPA 780 - 2011
· IEEE Std. 998-1996. IEEE Guide for Direct Lightning
Stoke Shielding of Substations
· US Air ForceAFI 32-1065Grounding Systems (2003)
· US Air Force 91-201US Air Force Explosives Safety (2001)
· US Army385-64, Chapter 12Ammunition & Explosive Safety Standards – Lightning
· US NavyNAVSEA OP5Chapter 6, Lightning Warning and Protection (1999)
· US MilitaryMIL-STD188-124Grounding, Bonding and Shielding for Common Long Haul/Tactical Communications Systems (1992)
· US MilitaryMIL-HDBK 419A Grounding Bonding and Shielding for Electronic Equipment and Facilities (Volume I, Basic Theory, 1987)(Vol. I)
· US MilitaryMIL-HDBK 419A Grounding Bonding and Shielding for Electronic Equipment and Facilities (Volume II, Applications, 1987)(Vol. II)
· DOEM440.1-1 Department of Energy Electrical Storms and Lightning ProtectionDOEDOE/EH-0530Department of Energy Lightning Safety (1996)
· DDESBDDESB 6055.9Department of Defense Chapter 7, Lightning Protection (1997)FAAFAA-STD-019deLightning Protection, Grounding, Bonding and Shielding Requirements for Facilities (2002)
· FAAFAA 6950.19APractices & Procedures for Lightning Protection, Grounding, Bonding and Shielding Implementation (1996)
· NASA E0012EStandard for Facility Grounding and Lightning Protection (2001)
· National Weather Service30-4105Lightning Protection, Grounding, Bonding, Shielding and Surge Protection Requirements (2004)
· APIAPI-2003American Petroleum Institute Protection Against Ignitions Arising out of Static, Lightning, and Stray Currents (2008)
· British StandardBS6651Protection of Structures against Lightning (1992)
· Indian StandardIS2309Protection of Buildings and Allied Structures Against Lightning - Code of Practice (1989)(in English)
· Polish StandardPN86Lightning Protection of Structures(in Polish)7th
· AS/1768Australian Code, Lightning Protection (2004)
· Chinese Code GB50057-94Design Code for Lightning Protection of Structures(in English)
· Russian CodeRD 34.21.122-87Design Code for Buildings and Structures
 
In the part 2 it will be show how to draw the protection zones using AUTOCAD

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