# Design Calculations of Lightning Protection Systems – Part Twenty

In Article " Design Calculations of Lightning Protection Systems – Part Two ", I indicated the lightning protection design process involves a number of design steps as in below Fig.1.

 Fig.1: The Lightning Protection Design Process

 Step#1: Characteristics of the Structure to Be Protected Explained in Article " Design Calculations of Lightning Protection Systems – Part Two "

 Step#2: Risk Assessment Study

Also, In above Article, I indicated that the risk assessment study can be done by (4) different methods as follows:

 Methods Of Calculations For Risk Assessment Study Articles First: Manual Method (Equations And Tables Method) as per IEC 62305-2 Design Calculations of Lightning Protection Systems – Part Two First: Manual Method (Equations And Tables Method) as per NFPA 780 Second: Software Method For Performing The Risk Assessment Study Third: Excel Sheets Method For Performing The Risk Assessment Study Fourth: Online Calculators Method Used for Need for Lightning Protection calculations

 Step#3: Selection Of External LPS Type and Material Explained in Article " Design Calculations of Lightning Protection Systems – Part Fifteen "

 Step#4: Sizing of Air Termination System Components

In Article " Design Calculations of Lightning Protection Systems – Part Sixteen ", I explained the following points:
• Types and forms of Strike Termination Subsystem,
• Sizing of Air Terminals Based on IEC 62305-3 and Based on BS EN 62305-3,
• Sizing of Natural Air Terminals,
• Positioning / Placement of Air Termination System Components.
• The Class of LPS/LPL influences on the (3) Positioning Methods.

And I explained the Methods for Positioning of Air Terminals in the following Articles:

 Method for Positioning of Air Terminals Article The Rolling Sphere Method (RSM) The Protective Angle Method (PAM) The Mesh Method

Also, I explained the Recommendations for the Best Positioning of Air Terminals in Article " Design Calculations of Lightning Protection Systems – Part Nineteen ".

Today, I will explain Other Steps of the Lightning Protection Design Process.

 Step#5: Design of Down-Conductor System

 This Step was explained before in the following Articles: Article " Conventional Lightning Protection System Components – Part Two ", which includes the following points: 1- Function of Conductor Subsystems, 2- Effects of Lightning Strikes on Conductor Subsystems, 3- Conductor Subsystem Material Requirements: 3.1Comparison between Copper and Aluminum as a Lightning Conductor Material 3.2 Use of Dissimilar Metals in the Same Lightning Conductor Subsystem 3.3 Lightning Conductor Geometry Article " Conventional Lightning Protection System Components – Part Three ", which includes the following points: 1- The Correct Choice Of Lightning Protection Components (LPC) 2- Types of the Conductors in Conductor Subsystems: 2.A Down-Conductors for a Non-Isolated Lightning Protection System 2.A.1 Temperature Rise Criteria: Case#1: If the wall is made of flame-resistant material or material with a normal level of flammability Case#2: If the wall is made of highly flammable material Case#2-A: The temperature rise of the down-conductor systems is not hazardous Case#2-B:  The temperature rise of the down-conductor systems presents a hazard 2.A.2 Requirements of Down Conductor Installation in Non-Isolated Lighting Protection System 2.B Down conductors of an isolated external lightning protection system 3- Installation Requirements For Down Conductors: 3.1 General Consideration For Down Conductors Installation 3.2 Down Conductor Routing: 3.2.A Structures with overhangs 3.2.B Routing Down Conductors within Walls, Or the Wall Cavity 3.2.C Large Flat Structures 3.2.D Courtyards 3.3 Fixing Of Down Conductors 3.4 Measuring Points (Test Joints) Article " Conventional Lightning Protection System Components – Part Four ", which includes the following points: 1- Natural Components Used as Down Conductors 1.1 Metal Installations 1.2 Facade Elements, Mounting Channels and the Metal Substructures of Facades 1.3 Metal downpipes 1.4 Rebar in Reinforced Concrete 1.5 Rebar in Precast Concrete 1.6 Rebar in Prestressed Concrete

 Determination Of The Number Of Down Conductors

First: According to IEC 62305-3

• The number of down conductors depends on the perimeter of the external edges of the roof (perimeter of the projection on the ground surface).( but at least two down-conductors should be used on a structure).
• The down conductors must be arranged to ensure that, starting at the exposed corners of the structure, they are distributed as uniformly as possible to the perimeter. (Note: A down-conductor should be installed at each exposed corner of the structure, where this is possible).
• However an exposed corner does not need a down conductor if the distance between this exposed corner to the nearest down-conductors complies with the following conditions:

1. The distance to both adjacent down-conductors is half the distance according to Table#1 or smaller; or
2. The distance to one adjacent down-conductor is one-quarter of the distance according to Table#1 or smaller.

• The IEC 62305-3 (EN 62305-3) standard gives typical distances between down conductors and ring conductors for each class of lightning protection system in Table#1 in below.

 Class of LPS Typical Distance between down conductors I 10 m II 10 m III 15 m IV 20 m

Table#1: Distance between down conductors according to
IEC 62305-3 (EN 62305-3)

• Depending on the structural features (e.g. gates, precast components), the distances between the various down conductors can be different. In each case, there must be at least the total number of down conductor required for the respective class of lightning protection system. The exact number of down conductors can only be determined by calculating the separation distance s.

Note:

If down-conductors cannot be spaced symmetrically, a variation of ± 20% of the distance requirements of Table#1 is permitted, provided the mean spacing of down-conductors conforms to the values shown.

Reducing the separation distance between down conductors to meet the requirements of Table#1 by one of the following methods

• Method#1: If the separation distance between down-conductors and the internal installations, calculated on the basis of the down-conductor spacing according to Table#1, is too large, the number of down-conductors should be increased to meet the required separation distance.
Note:
• In large, flat structures (typically industrial structures, exhibition halls, etc.) with dimensions over four times the spacing of the down-conductors, extra internal down-conductors should be provided, wherever possible, approximately every 40 m to minimize the separation distance when the lightning current is flowing long distances over a flat roof.

• Method#2: By interconnecting the down conductors at ground level (base conductor) and using ring conductors for higher structures (see Fig.2), it is possible to balance the distribution of the lightning current which, in turn, reduces the separation distance s.

 Fig.2: Total view on a newly installed external lightning protection system

• Method#3: It is also possible to use a new type of high voltage-resistant isolated conductors (HVI) which can be installed or even integrated in the facade (see Fig.2).

Second: According to NFPA 780-2011

1- General Rule:

• At least two down conductors shall be provided on any kind of structure, including steeples.
• Structures exceeding 76 m (250 ft) in perimeter shall have a down conductor for every 30 m (100 ft) of perimeter or fraction thereof.
• Lower roofs or projections that are located within a zone of protection shall not be required to be included in the perimeter measurement.

2- Special Rules:

2.1 For flat or gently sloping roofs:

• The total number of down conductors on structures having flat or gently sloping roofs shall be such that the average distance between all down conductors does not exceed 30 m (100 ft).
• For a flat or gently sloping roof structure, only the perimeter of the roof areas requiring protection shall be measured.

2.2 For irregularly shaped structures:
• Irregularly shaped structures shall have additional down conductors as necessary to provide a two-way path from each strike termination device.

Example#1:

For a pitched roof structure shown in Fig.3 with spacing between shown points as follows:

 Points Space 1–2 40 m (130 ft) 2-3 26 m (85 ft) 3-4 26 m (85 ft) 4-5 26 m (85 ft) 5-1 26 m (85 ft)

Find the Minimum required number of down conductors?

 Fig.3

Solution:

Total perimeter =  40 m (130 ft) + 26 m (85 ft) + 26 m (85 ft) + 26 m (85 ft) + 26 m (85 ft) = 144 m (470 ft)
Since, the average distance between down conductors does not exceed 30 m (100 ft).

Required down conductors = 144 m / 30 m = 4.8 = 5 down conductors

Third: According to Rules of Thumb

• The number of down conductors depends upon the building area or upon its circumference in such away that the minimum numbers of down conductors is (2).
• A building with area not exceeding 100 m2 or its perimeter up to 30m or less, the number of down conductor = one.
• For buildings with height <30 m, the minimum number resulted from one of the following two methods:

1. One down conductors for first 100m2 in addition of one conductor for each 300m2 or a part of it.
2. One down conductor for each 30m of building perimeter.

• For buildings with height ≥30 m, the minimum number of down conductor is given in the following Table#2:

 Area of building in m2 Number of down conductors 100 - 400 2 400 - 700 3 700 - 1000 4 1000 - 1300 5 1300 – 1600 6 1600 - 1900 7
Table#2

Example#2:

If the total perimeter of intermediate school = 625 mt, Find the Minimum required number of down conductors by the rules of thumb method if the intermediate school height < 30 m?

Solution:

Since intermediate school height < 30 m , then use One down conductor for each 30m of building perimeter.

the No. of down conductor = 625 / 30 = 20.83333 = 21 down conductors

 Practical Recommendations For Determining Number Of Down Conductors 1- Number of down conductors for isolated LPS: If the air termination system consists of rods on separate masts (or one mast), at least one down conductor is needed for each mast. In the case of masts made of metal or interconnected reinforced steel, no additional down conductor is necessary. If the air termination system consists of stretched wires (or one wire), at least one down conductor is needed at each wire end. If the air termination system consists of meshed conductors, at least one down conductor is needed for each support. 2- Number of down conductors for non-isolated LPS: If the air termination system consists of one rod, at least one down conductor is needed. If the air termination system consists of individual rods, at least one down conductor is needed for each rod. If the air termination system consists of stretched wires, at least one down conductor is needed at each wire end. If the air termination system consists of meshed conductors, at least two down conductor are needed, distributed around the perimeter of the building to be protected.

 Step#6: Design of Earth Termination System

 This Step was explained before in Article " Conventional Lightning Protection System Components – Part Five ", which includes the following points: 1- Grounding Electrode Subsystem (Earth Termination System) 2- Functions of Grounding Electrode Subsystem: 3- Grounding System – General Overview 4- Resistance value for Grounding Electrode Subsystem 5- Grounding Electrode Subsystem Types 5.A Type A arrangement 5.A.1 Type A Arrangement Criteria 5.A.2 Calculating the minimum total length of electrode at each down-conductor for Type A arrangement 5.A.3 Type A Arrangement For Sites With Extreme Weather Conditions 5.B Type B arrangement, 5.B.1 Type B Arrangement Criteria 5.B.2 Calculating the minimum length of the ring earth electrode for Type B arrangement 5.C Comparison of Type A and Type B arrangements 5.D Type C arrangement: Foundation earth electrodes. 5.D.1 Type C Arrangement Criteria 6- Earth Termination System Testing

 Step#7: Design of Internal LPS System

This Step was explained before in the following Articles:

Article " Conventional Lightning Protection System Components – Part Six ", which includes the following points:

1- The Internal Lightning Protection System
2- Function of Internal Lightning Protection System
3- How Internal Lightning Protection System prevent hazardous sparking inside the building or structure?
4- Components Of The Internal Lightning Protection System

5- The Internal Lightning Protection System subsystems
5.A Equipotential Bonding Subsystem,
5.A.1 General Notes For Equipotential Bonding
5.A.2 Parts Of Equipotential Bonding Subsystem
5.A.2.a Main equipotential bonding,
5.A.2.b Supplementary equipotential bonding.
5.A.2.c The Difference Between Main And Supplementary Equipotential Bonding
5.A.2.d Considerations For Different Cases Of Main Equipotential Bonding In Any Installation
5.A.2.d.1 Lightning Equipotential Bonding For External LPS  And External Conductive Parts
5.A.2.d.1.2 Lightning Equipotential Bonding Of External Services
5.A.2.d.1.3 Lightning Equipotential Bonding For Internal Systems
5.A.2.d.1.4 Lightning Equipotential Bonding Of Roof Top Fixtures

Article " Conventional Lightning Protection System Components – Part Seven ", which includes the following points:

1- Components Of Equipotential Bonding Subsystem
1.1 Equipotential bonding conductors
1.1.A Sizing Of Equipotential Bonding Conductors
1.2 Equipotential bonding bars,
1.3 Connection Components .

2- Separation (Isolation) Distance Requirements
2.1 Why a separation distance between the external LPS and the structural metal parts is needed?
2.2 Calculation of Separation distance (S)

3- Test And Inspection Of The Equipotential Bonding Subsystem

4- Surge Protection Subsystem.
4.1 General Overview of Surge Protection Subsystem
4.2 What Is A Surge Protective Device?
4.3 Importance of Surge Protective Devices SPDs
4.4 Working Principle for Surge Protective Devices SPDs
4.5 Selection and Installation Of Surge Protective Devices SPDs

 We will not discuss the issues of selection and installation of surge protective devices and the LEMP Protection Measures System (LPMS) in this course, and they will be discussed later in a separate course.

 Step#8: LPS Design Drawings and Specifications

 First: LPS Design Drawings Complete design drawings shall be submitted to the engineer for approval and  showing at least the following: Location of all grounding, Location of all roof conductors, Location of all through-roof / through-wall assemblies, Location of all down conductors, Location of all air terminals, Location of all bonding bars, Location of all welding points to Rebars, Location of all Roof Top Equipments, Details for installation of different materials and equipments. You can download Example of Lightning Protection System Design Drawings by click on the link. Second: LPS Design Specifications A general format of a common LPS Design Specifications will include the following parts as a minimum: PART 1 – GENERAL 1.1. Objective:  To provide safety for the building and occupants by eliminating  damage to the entire structure caused by lightning, surges and other related  occurrences. 1.2. Standards: list the specifications and standards used in design. 1.3. System Design: The design of this system is to be in strict accordance  with this section of the standards and specifications. If any departure from  the contract drawings or submittal drawings covered below are deemed  necessary by the Contractor, details of such departures and reasons therefore  shall be submitted as soon as practical to the architect and engineer for approval. 1.4. Submittals: Complete design drawings, product catalogs and calculations  shall be submitted to the engineer for approval. 1.5. Quality Assurance: The LPS shall conform to the requirements and standards  for LPS(s) in accordance with section of the standards and specifications. PART 2 – PRODUCTS 2.1 Standard: The system furnished under this specification shall be the standard  product of a manufacturer regularly engaged in the production of lightning  protection equipment and shall be the manufacturer’s latest approved design.  2.2 Equipment: Provide and install a complete LPS in compliance with  the specifications and standards of the most current editions listed in section  of the standards and specifications. 2.3 Materials: All lightning protection materials and components shall comply in  weight, size and composition with the specifications and standards of the most  current editions listed in section of the standards and specifications. PART 3 – EXECUTION 3.1 Installation: An experienced installation company shall directly supervise  the installation to accomplish its completion. All equipment shall be installed  in a neat, workmanlike manner.  The system shall consist of a complete network of conductor cables at the roof  and include air terminals, connectors, splicers, appropriate bonding, down lead  cables and proper ground terminals. 3.2 Coordination: The lightning protection contractor will work with other  trade contractors to ensure a correct, neat and inconspicuous as practical  installation. It shall be the responsibility of the lightning protection contractor  to assure a proper common bond to the appropriate grounded utilities; such as  the electric service ground, incoming water and gas pipe, etc. 3.3 Inspection and Certification: Upon completion of the installation, the contractor shall furnish the Master Label Certification issued by a certified inspector for this system. If the existing structure does have a LPS, the contractor shall advise the Owner of any additional work required on  the existing system to achieve compliance with regulations requirements. You can download an Example of A General Format for Lightning Protection  System Specification by click on the link.

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