Design Calculations of Lightning Protection Systems – Part Six


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:


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 as per IEC 62305



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




Fig.1: The 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



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 For Lightning Protection



And In Article " Design Calculations of Lightning Protection Systems – Part Three ", I explained the following:
  • 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,
  • Step#2-3: For each loss to be considered, identify the tolerable level of risk RT,
  • Step#2-4 First Part: Identification of the Risk Components Rx.


Also, in Article " Design Calculations of Lightning Protection Systems – Part Four ", I explained Step#2-4 Second Part: Calculations of the Risk Components Rx and I indicated that:


Each of the risk components Rx is obtained using further calculations, sub-calculations and reference tables based on the general equation:

RX = NX x PX x LX

Where

NX = number of dangerous events per year,
PX = probability of damage to structure,
LX = amount of consequent loss.
X = A, B, ...

So, The task of the risk assessment therefore involves the determination of the three parameters NX, PX and LX.


In this Article, I explained how to calculate the first Parameter NX: number of dangerous events per year.

And in Article " Design Calculations of Lightning Protection Systems – Part Five ", I explained how to calculate the second Parameter: PX = probability of damage to structure.

Today, I will explain how to calculate the Third Parameter: LX = Amount of Consequent Loss.


Step#2-4 Second Part: Calculations of the Risk Components Rx




Calculations of Third Parameter: LX = Amount of Consequent Loss



Amount of Consequent Loss LX

The loss LX refers to the mean relative amount of a particular type of damage for one dangerous event caused by a lightning flash, considering both its extent and effects. The loss value LX varies with the type of loss considered (see Fig.3):

  • L1 (Loss of human life, including permanent injury): the endangered number of persons (victims);
  • L2 (Loss of public service): the number of users not served;
  • L3 (Loss of cultural heritage): the endangered economic value of structure and content;
  • L4 (Loss of economic values): the endangered economic value of animals, the structure (including its activities), content and internal systems, and, for each type of loss, with the type of damage (D1, D2 and D3) causing the loss.


Fig.3: Damage and loss relevant to a structure according to the point of Strike


For more information, please review Article " Design Calculations of Lightning Protection Systems – Part One ", paragraph “Types of Loss”.

Notes:

  • In this Article, we will not discuss the L4 Loss (Loss of economic values) and it will be discussed later when explaining Part#3: evaluating the cost-effectiveness of protection measures.
  •  The loss LX should be determined for each zone of the structure into which it is divided.
  • The typical mean values of loss LX in a structure given in this Article are merely values proposed by the IEC.
  • Different values may be assigned by each national committee or after detailed investigation.
  • Usually, The values of amount of loss LX should be evaluated and fixed by the lightning protection designer (or the owner of the structure) especially When the damage to a structure due to lightning may also involve surrounding structures or the environment (e.g. chemical or radioactive emissions), then a more detailed evaluation of LX that takes into account this additional loss should be performed.



1- Loss of Human Life (L1)

The loss value LX for each zone can be determined according to Table-1, considering that:

  • Loss of human life is affected by the characteristics of the zone. These are taken into account by increasing (hz) and decreasing (rt, rp, rf) factors;
  • The maximum value of loss in the zone shall be reduced by the ratio between the number of persons in the zone (nz) versus the total number of persons (nt) in the whole structure;
  • The time in hours per year for which the persons are present in the zone (tz), if it is lower than the total 8 760 h of a year, will also reduce the loss.



Type of damage
Typical loss
D1
LA = rt  x LT x nZ / nt  x tz / 8760
D2
LU = rt  x LT x nZ / nt x tz/ 8760
D3
LB = LV = rp x rf  x hz  x LF x nZ / nt x tz / 8760
D4
LC = LM = LW = LZ = LO x nZ / nt  x tz / 8760

Table-1: Type of loss L1: Loss values for each zone

Where:

  • LT is the typical mean relative numbers of victims injured by electric shock (D1) due to one dangerous event (see Table-2);
  • LF is the typical mean relative numbers of victims by physical damage (D2) due to one dangerous event (see Table-2);
  • LO is the typical mean relative numbers of victims by failure of internal systems (D3) due to one dangerous event (see Table-2);
  • rt is a factor reducing the loss of human life depending on the type of soil or floor (see Table-3);
  • rp is a factor reducing the loss due to physical damage depending on the provisions taken to reduce the consequences of fire (see Table-4);
  • rf is a factor reducing the loss due to physical damage depending on the risk of fire or on the risk of explosion of the structure (see Table-5);
  • hz is a factor increasing the loss due to physical damage when a special hazard is present (see Table-6);
  • nz is the number of persons in the zone;
  • nt is the total number of persons in the structure;
  • tz is the time in hours per year for which the persons are present in the zone.



Type of damage
Typical loss value
Type of structure
D1
injuries
LT
10–2
All types
D2
physical damage
LF
10–1
Risk of explosion
10–1
Hospital, hotel, school, civic building
5 X 10–2
Public entertainment, church, museum
2 X 10–2
Industrial, commercial
10–2
Others
D3
failure of internal systems
LO
10–1
Risk of explosion
10–2
Intensive care unit and operation block of hospital
10–3
Other parts of hospital
Table-2: Type of loss L1: Typical mean values of LT, LF and LO


Notes:

  • Values of Table-2 refer to a continuous attendance of people in the structure.
  • In case of a structure with risk of explosion, the values for LF and LO may need a more detailed evaluation, considering the type of structure, the risk explosion, the zone concept of hazardous areas and the measures to meet the risk.


When the damage to a structure due to lightning involves surrounding structures or the environment (e.g. chemical or radioactive emissions), additional loss (LE) should be taken into account to evaluate the total loss (LFT):

LFT = LF + LE

Where:

LE = LFE x te / 8760

  • LFE being the loss due to physical damage outside the structure;
  • te being the time of presence of people in the dangerous place outside the structure.


Note:

  • If values of LFE and te are unknown, LFE x te /8760 = 1 should be assumed.


Type of Surfaceb
Contact Resistance k Ωa
rt
Agricultural, concrete
≤ 1
10–2
Marble, ceramic
1 – 10
10–3
Gravel, moquette, carpets
10 – 100
10–4
Asphalt, linoleum, wood
≥100
10–5
a Values measured between a 400 cm2 electrode compressed with a uniform force of 500 N and a point of infinity.
b A layer of insulating material, e.g. asphalt, of 5 cm thickness (or a layer of gravel 15 cm thick) generally reduces the hazard to a tolerable level.
Table-3: Reduction factor rt as a function of the type of surface of soil or floor


Provisions
rp
No provisions
1
One of the following provisions: extinguishers; fixed manually operated extinguishing installations; manual alarm installations; hydrants; fire compartments; escape routes
0,5
One of the following provisions: fixed automatically operated extinguishing installations; automatic alarm installationsa
0.2
a Only if protected against overvoltages and other damages and if firemen can arrive in less than 10 min.
Table-4: Reduction factor rp as a function of provisions taken to reduce the consequences of fire


Notes:

  • If more than one provision has been taken, the value of rp should be taken as the lowest of the relevant values.
  • In structures with risk of explosion, rp = 1 for all cases.



Risk
Amount of risk
rf
Explosion
Zones 0, 20 and solid explosive
1
Zones 1, 21
10–1
Zones 2, 22
10–3
Fire
High
10–1
Ordinary
10–2
Low
10–3
Explosion or fire
None
0
Table-5: Reduction factor rf as a function of risk of fire or explosion of structure


Notes:

  • In case of a structure with risk of explosion, the value for rf may need a more detailed evaluation.
  • Structures with a high risk of fire may be assumed to be structures made of combustible materials or structures with roofs made of combustible materials or structures with a specific fire load larger than 800 MJ/m2.
  • Structures with an ordinary risk of fire may be assumed to be structures with a specific fire load between 800 MJ/m2 and 400 MJ/m2.
  • Structures with a low risk of fire may be assumed to be structures with a specific fire load less than 400 MJ/m2, or structures containing only a small amount of combustible material.
  • Specific fire load is the ratio of the energy of the total amount of the combustible material in a structure and the overall surface of the structure.
  • For the purposes of this part of IEC 62305, structures containing hazardous zones or containing solid explosive materials should not be assumed to be structures with a risk of explosion if any one of the following conditions is fulfilled:

  1. The time of presence of explosive substances is lower than 0,1 h/year;
  2. The volume of explosive atmosphere is negligible according to IEC 60079-10-1 and IEC 60079-10-2;
  3. The zone cannot be hit directly by a flash and dangerous sparking in the zone is avoided.

  • For hazardous zones enclosed within metallic shelters, condition c) is fulfilled when the shelter, as a natural air-termination system, acts safely without puncture or hot-spot problems, and internal systems inside the shelter, if any, are protected against overvoltages to avoid dangerous sparking.



Kind of special hazard
hz
No special hazard
1
Low level of panic (e.g. a structure limited to two floors and the number of persons not greater than 100)
2
Average level of panic (e.g. structures designed for cultural or sport events with a number of participants between 100 and 1 000 persons)
5
Difficulty of evacuation (e.g. structures with immobile persons, hospitals)
5
High level of panic (e.g. structures designed for cultural or sport events with a number of participants – greater than 1 000 persons)
10
Table-6: Factor hz increasing the relative amount of loss in presence of a special hazard




2- Unacceptable Loss of Service to The Public (L2)

The loss value LX for each zone can be determined according to Table-7, considering that:

  • Loss of public service is affected by the characteristics of the zone of the structure. These are taken into account by decreasing (rf, rp) factors;
  • The maximum value of loss due to the damage in the zone must be reduced by the ratio between the number of users served by the zone (nz) versus the total number of users (nt) served by the whole structure.



Type of damage
Typical loss
D2
LB = LV = rp x rf  x LF x nz/nt
D3
LC = LM = LW = LZ = LO x nz/nt
Table-7: Type of loss L2: Loss values for each zone

Where:

  • LF is the typical mean relative number of users not served, resulting from physical damage (D2) due to one dangerous event (see Table-8);
  • LO is the typical mean relative numbers of users not served resulting from failure of internal systems (D3) due to one dangerous event (see Table-8);
  • rp is a factor reducing the loss due to physical damage depending on the provisions taken to reduce the consequences of fire (see Table-4);
  • rf is a factor reducing the loss due to physical damage depending on the risk of fire (see Table-5);
  • nz is the number of users served by the zone;
  • nt is the total number of users served by the structure.



Type of damage
Typical loss value
Type of service
D2
physical damage
LF
10–1
Gas, water, power supply
10–2
TV, telecommunications lines
D3
failure of internal systems
LO
10–2
Gas, water, power supply
10–3
TV, telecommunications lines
Table-8: Type of loss L2: Typical mean values of LF and LO



3- Loss Of Irreplaceable Cultural Heritage (L3)

The loss value LX for each zone can be determined according to Table C.9, considering that:

  • Loss of cultural heritage is affected by the characteristics of the zone. These are taken into account by decreasing (rf, rp) factors;
  • The maximum value of loss due to the damage of the zone must be reduced by the ratio between the value of the zone (cz) versus the total value (ct) of the whole structure (building and content).



Type of damage
Typical loss value
D2
physical damage
LB = LV = rp x rf  x LF x cz / ct
Table-9: Type of loss L3: Loss values for each zone

Where:

  • LF is the typical mean relative value of all goods damaged by physical damage (D2) due to one dangerous event (see Table-10);
  • rp is a factor reducing the loss due to physical damage depending on the provisions taken to reduce the consequences of fire (see Table-4);
  • rf is a factor reducing the loss due to physical damage depending on the risk of fire (see Table-5);
  • cz is the value of cultural heritage in the zone;
  • ct is the total value of building and content of the structure (sum over all zones).



Type of damage
Typical loss value
Type of structure or zone
D2
physical damage
LF
10–1
Museums, galleries
Table-10: Type of loss L3: Typical mean value of LF



After the calculation of the three parameters NX, PX and LX and For evaluation of risk components RX related to lightning flashes to the structure and based on the general equation:

RX = NX x PX x LX

 The following relationships for each Risk Component are illustrated in the following Table-11 with their corresponding source and type of damage:


Source of
Damage






Type of
damage
Lightning strike (with regard to the structure)
Direct
Indirect
S1
Direct lightning
strike into the
structure
S2
Lightning strike
into the earth
next to the
structure
S3
Direct lightning
strike into the
entering supply
line
S4
Lightning strike
into the earth
next to the entering
supply line
D1
Electric shock to
living beings
RA = ND x PA x LA

RU = (NL + NDJ) x PU x LU

D2
Fire, explosions,
mechanical and
chemical effects
RB = ND x PB x LB

RV = (NL + NDJ) x PV x LV

D3
Interferences on
electrical and
electronic systems
RC = ND x PC x LC
RM = NM x PM x LM
RW = (NL + NDJ) x PW x LW
RZ = NI x PZ x LZ
Table-11: Risk Component Equations

Note: In many cases NDJ may be neglected.



Step#2-5: Calculate Rn = Σ Rx = R1 + R2 +R3



Each risk, R, is the sum of its risk components. When calculating a risk, the risk components may be grouped according to the source of damage and the type of damage. So, the risks R1, R2 and R3 will be calculated from the following equations:


R1: Risk of loss of human life:

R1 = RA1 + RB1 + RC11) + RM11) + RU1 + RV1 + RW11) + RZ11)

1) Only for structures with risk of explosion and for hospitals with life-saving electrical equipment or other structures when failure of internal systems immediately endangers human life.

R2: Risk of loss of service to the public:

R2 = RB2 + RC2 + RM2 + RV2 + RW2 + RZ2

R3: Risk of loss of cultural heritage:

R3 = RB3 + RV3

The risk components corresponding to Risk of each type of loss are also indicated in below Table-12:

Source of
damage
Flash to a structure S1
Flash near a structure S2
Flash to a line connected to the structure S3
Flash near a line connected to the structure S4
           Risk         component



Risk for each type of loss
RA
RB
RC
RM
RU
RV
RW
RZ
R1
*
*
*a
*a
*
*
*
*a
R2

*
*
*

*
*
*
R3

*



*


R4
*b
*
*
*
*b
*
*
*
a Only for structures with risk of explosion, and for hospitals or other structures where failure of internal systems immediately endangers human life.
b Only for properties where animals may be lost.
Table-12: Risk Components Corresponding to Risk of Each Type of Loss



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



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.



End of 
Manual Method (equations and tables method) for Risk Assessment Study as per IEC 62305-2



In the next Article, I will explain Manual Method (Equations and Tables Method) for Risk Assessment Study but as per NFPA-780. Please, keep following.

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