Design Calculations of Lightning Protection Systems – Part Three


In Article Design Process for Lightning Protection Systems ", I indicated the (3) phases of the Design Process for Lightning Protection Systems as follows:





Design Process For Lightning Protection Systems

The design process of lightning protection systems is commonly broken into discrete phases, allowing the lightning protection designer to present an integrated design package. These phases can be listed as follows:

  1. Planning phase,
  2. Consultation phase,
  3. Detailed Design phase.


A Quality assurance is required in each phase in above.



Also, in Article 
Design Calculations of Lightning Protection Systems – Part One ", I explained an Introduction to design calculations of lightning protection systems as follows:





1- Introduction To Design Calculations Of Lightning Protection Systems

It is very important before explaining the design calculations of lightning protection systems to highlight some important topics or expressions that will be used in these calculations. These topics can be listed as follows:

  1. Sources and Types of Damage to a Structure,
  2. Types of Loss,
  3. Types of Risks Associated with Losses,
  4. Lightning Protection Levels (LPL),
  5. Lightning Protection Zones (LPZ),
  6. Class of LPS,
  7. Protection Measures.





And in Article " Design Calculations of Lightning Protection Systems – Part Two ", I explained the following:





Design Calculations of Lightning Protection Systems – Continued
Third: Detailed Design Phase



The Lightning protection design process involves a number of design steps as in Fig.1.


Fig.1: Lightning Protection Design Process




Step#1: Characteristics of the Structure to Be Protected






Step#2: Risk Assessment Study





Methods Of Calculations For Risk Assessment Study

The risk assessment study can be done by (4) different methods as follows:

1- Manual Method (equations and tables method),which will be explained as per:
  • IEC 62305-2,
  • NFPA780.

2-Software Method,
3- Excel Sheets Method,
4-Online Calculators Method.





First: Manual Method (Equations And Tables Method) as per IEC 62305-2






Procedure For Performing The Risk Assessment Study By Manual Method

Procedure for performing the risk assessment study includes three parts as follows:

  • Part#1: evaluating Need for lightning protection,
  • Part#2: Determination of Required Protection Level,
  • Part#3: evaluating the cost-effectiveness of protection measures.







Part#1: Evaluating Need For Lightning Protection

To evaluate the need for lightning protection, the following steps need to be carried out a follows:

Step#2-1: Identify the structure to be protected.

Step#2-2: Identify the types of loss relevant to the structure to be protected Rn, where:

R1 risk of loss of human life,
R2 risk of loss of services to the public,
R3 risk of loss of cultural heritage.

Step#2-3: For each loss to be considered, identify the tolerable level of risk RT (tolerable means still acceptable).


Step#2-4: For each type of loss to be considered , identify and calculate the risk components Rx that make up risk Rn which are: RA, RB, RC, RM, RU, RV, RW, RZ.

Step#2-5: Calculate Rn = Σ Rx

Step#2-6: Comparing the calculated actual risk Rn of each loss to a tolerable level of risk (RT), then we have (2) cases:

Case#1: If the calculated risk Rn is equal or less than the respective tolerable risk RT i.e. Rn ≤ RT , then Structure is adequately protected for this type of loss and no lightning protection is required for this type of loss,
Case#2: If the calculated risk Rn is higher than the tolerable risk RT i.e.  Rn > RT, then Install lightning protection measures in order to reduce Rn.

Step#2-7: go back to step#2-4 and make a series of trial and error calculations until the risk Rn is reduced below that of RT (Rn ≤ RT).

Note:

In cases where the risk cannot be reduced to a tolerable level, the site owner should be informed and the highest level of protection provided to the installation.

The following flow diagram in Fig.2 shows this procedure for evaluating Need for lightning protection.


Fig.2: Procedure for Evaluating Need of Lightning Protection




Today, I will explain, in detail and step by step, Part#1: Evaluating Need for Lightning Protection Systems.





Part#1: Evaluating Need for Lightning Protection Systems




Step#2-1: Identify The Structure to be Protected





The structure to be protected includes:

  1. The structure itself;
  2. Installations in the structure;
  3. Contents of the structure;
  4. Persons in the structure or in the zones up to 3 m from the outside of the structure;
  5. Environment affected by damage to the structure.


Note that Protection does not include connected lines outside of the structure.

For more information about Structure Characteristics, Please review Step#1 in Article Design Calculations of Lightning Protection Systems – Part Two ".






Step#2-2: identify the Types Of Loss relevant to the structure to be protected (Rn)





The following risks have been identified, corresponding to their equivalent type of loss

  • R1 Risk of loss of human life,
  • R2 Risk of loss of service to the public,
  • R3 Risk of loss of cultural heritage.


Hereafter the primary risks will be referred to collectively as Rn where the subscript n indicates 1, 2 or 3 as described above.

Please review the paragraph " Types of Risks Associated with Losses " in Article Design Calculations of Lightning Protection Systems – Part One ".





Step#2-3: for each Loss to be considered, identify the Tolerable Level Of Risk RT





The Tolerable Risk is expressed in the form of number of events per year and is given in engineering units (e.g. 10-x) as in the following Table-1 which includes the values of RT from different Standards and codes.


Types Of Loss
RT(y-1)
IEC 62305-2
BS EN 62305-2
NFPA-780
Loss Of Human Life
10-5
(risk of 1 in 100,000)
10-5
(risk of 1 in 100,000)
10-6
(risk of 1 in 100,000)
Loss Of Service To The Public
10-3
(risk of 1 in 1,000)
10-4
(risk of 1 in 10,000)
10-3
(risk of 1 in 1,000)
Loss Of Cultural Heritage
10-4
(risk of 1 in 1,000)
10-4
(risk of 1 in 10,000)
10-3
(risk of 1 in 1,000)
Table-1: Values of RT from different Standards and Codes


Notes:

  • Use the national standard appropriate to the country of installation, for example don’t use BS EN 62305-2 in United States.
  • If your country didn’t have a national standard or local regulation which better reflect the localized conditions and acceptable local tolerable risk in your country, so select a national standard for a country which experiences similar lightning risk (ground flash density/ thunderdays) and similar social/economic values.






Step#2-4: For each type of loss to be considered , identify and calculate the risk components Rx that make up Primary risk Rn





Step#2-4 includes two main parts as follows:

  • Identification of the Risk Components Rx,
  • Calculations of the Risk Components Rx.




First Part: identification of the Risk Components Rx



Fig.3: Risk Components Rx


Each primary risk Rn (R1 to 3) is composed of several risk components Rx. Each risk component Rx relates to a different relationship between source of damage (S1, S2 and S3) and type of damage (D1, D2 and D3) as shown in Fig.3, such that:


1- Risk components for a structure due to flashes to the structure:

  • RA: Component related to injury to living beings caused by electric shock due to touch and step voltages inside the structure and outside in the zones up to 3 m around downconductors. Loss of type L1 and, in the case of structures holding livestock, loss of type L4 with possible loss of animals may also arise.


Note:

  • In special structures, people may be endangered by direct strikes (e.g. top level of garage parking or stadiums). These cases may also be considered using the principles of this part of IEC 62305.


  • RB: Component related to physical damage caused by dangerous sparking inside the structure triggering fire or explosion which may also endanger the environment. All types of loss (L1, L2, L3 and L4) may arise.

  • RC: Component related to failure of internal systems caused by LEMP. Loss of type L2 and L4 could occur in all cases along with type L1 in the case of structures with risk of explosion, and hospitals or other structures where failure of internal systems immediately endangers human life.


2- Risk component for a structure due to flashes near the structure:

  • RM: Component related to failure of internal systems caused by LEMP. Loss of type L2 and L4 could occur in all cases, along with type L1 in the case of structures with risk of explosion, and hospitals or other structures where failure of internal systems immediately endangers human life.



3- Risk components for a structure due to flashes to a line connected to the structure:

  • RU: Component related to injury to living beings caused by electric shock due to touch voltage inside the structure. Loss of type L1 and, in the case of agricultural properties, losses of type L4 with possible loss of animals could also occur.

  • RV: Component related to physical damage (fire or explosion triggered by dangerous sparking between external installation and metallic parts generally at the entrance point of the line into the structure) due to lightning current transmitted through or along incoming lines. All types of loss (L1, L2, L3, L4) may occur.

  • RW: Component related to failure of internal systems caused by overvoltages induced on incoming lines and transmitted to the structure. Loss of type L2 and L4 could occur in all cases, along with type L1 in the case of structures with risk of explosion, and hospitals or other structures where failure of internal systems immediately endangers human life.


Notes:

  • The lines taken into account in this assessment are only the lines entering the structure.
  • Lightning flashes to or near pipes are not considered as a source of damage based on the bonding of pipes to an equipotential bonding bar. If an equipotential bonding bar is not provided, such a threat should also be considered.


4- Risk component for a structure due to flashes near a line connected to the structure

  • RZ: Component related to failure of internal systems caused by overvoltages induced on incoming lines and transmitted to the structure. Loss of type L2 and L4 could occur in all cases, along with type L1 in the case of structures with risk of explosion, and hospitals or other structures where failure of internal systems immediately endanger human life.


Notes:

  • The lines taken into account in this assessment are only the lines entering the structure.
  • Lightning flashes to or near pipes are not considered as a source of damage based on the bonding of pipes to an equipotential bonding bar. If an equipotential bonding bar is not provided, such a threat should also be considered.



Each Risk Components Rx is identified as in Fig.4 in below


Fig.4: Risk Components related to Source and Type of Damage


Also, the Characteristics of the structure and of possible protection measures influencing risk components for a structure are given in Table-2 as follows:


Characteristics of structure or of internal systems Protection measures
RA
RB
RC
RM
RU
RV
RW
RZ
Collection area
X
X
X
X
X
X
X
X
Surface soil resistivity
X







Floor resistivity
X



X



Physical restrictions, insulation, warning notice, soil equipotentialization
X



X



LPS
X
X
X
Xa
Xb
Xb


Bonding SPD
X
X


X
X


Isolating interfaces


Xc
Xc
X
X
X
X
Coordinated SPD system


X
X


X
X
Spatial shield


X
X




Shielding external lines




X
X
X
X
Shielding internal lines


X
X




Routing precautions


X
X




Bonding network


X





Fire precautions

X



X


Fire sensitivity

X



X


Special hazard

X



X


Impulse withstand voltage


X
X
X
X
X
X
a Only for grid-like external LPS.
b Due to equipotential bonding.
c Only if they belong to equipment.
Table-2: Factors influencing the Risk Components






Single Zone Or Multi-Zone Structure

The designer can deal with a structure to be protected as a single Lightning Protection Zone or a Multi Lightning Protection zone as follows:


1- Single Zone Structure

  • In this case only one zone ZS made up of the entire structure is defined. The risk R is the sum of risk components RX in this zone.
  • Defining the structure with a single zone may lead to expensive protection measures because each measure must extend to the entire structure.



2- Multi-Zone Structure



Fig.5: Multi-Zone Structure

  • In this case, the structure is divided into multiple zones ZS to highlight the high Risk areas in the structure (see Fig.5).
  • The risk for the structure is the sum of the risks relevant to all zones of the structure; in each zone, the risk is the sum of all relevant risk components in the zone.
  • Dividing a structure into zones allows the designer to take into account the characteristics of each part of the structure in the evaluation of risk components and to select the most suitable protection measures tailored zone by zone, reducing the overall cost of protection against lightning


Please review the paragraph " Lightning Protection Zones (LPZ) " in Article Design Calculations of Lightning Protection Systems – Part One ".


2.1 Partitioning Of A Structure In Zones ZS

  • To assess each risk component, a structure could be divided into zones ZS each having homogeneous characteristics. However, a structure may be, or may be assumed to be, a single zone.
  • Zones ZS are mainly defined by:

  1. Type of soil or of floor (risk components RA and RU);
  2. Fireproof compartments (risk components RB and RV) ;
  3. Spatial shields (risk components RC and RM).

  • Further zones may be defined according to:

  1. layout of internal systems (risk components RC and RM),
  2. protection measures existing or to be provided (all risk components),
  3. Losses LX values (all risk components).


Notes:

  • Partitioning of the structure in zones ZS should take into account the feasibility of the implementation of the most suitable protection measures.
  • If the structure is partitioned in zones ZS, each risk component shall be evaluated for each zone ZS. The total risk R of the structure is the sum of risks components relevant to the zones ZS which constitute the structure.
  • For components RA, RB, RU, RV, RW and RZ, only one value is to be fixed in each zone for each parameter involved. Where more than one value is applicable, the highest one shall be chosen.
  • Zones ZS according to this part of IEC 62305 may be LPZ in line with IEC 62305-4 (see Fig.5). However they may also be different from LPZs.



2.2 Partitioning Of A Line (Telecom Line or LV Power Line or HV Power Line) Into Sections SL

  • To assess the risk components due to a flash to or near a line, the line could be divided into sections SL. However a line may be, or may be assumed to be, a single section.
  • For all risk components, sections SL are mainly defined by:

  1. Type of line (aerial or buried),
  2. Factors affecting the collection area (CD, CE, CT),
  3. Characteristics of line (shielded or unshielded, shield resistance).



Notes:

  • If more than one value of a parameter exists in a section, the value leading to the highest value of risk is to be assumed.
  • If the line has more than one section, the values of RU, R, RW and RZ are the sum of the RU, RV, RW and RZ values relevant to each section of the line. The sections to be considered are those between the structure and the first node.






In the next Article, I will continue explaining Step#2 - Second Part: 
Calculations of the Risk Components Rx. Please, keep following.


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