In Article " Design Process for Lightning Protection Systems ", I indicated that the Design Process for 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:
- Planning phase,
- Consultation phase,
- Detailed Design phase.
Today, I will explain Design Calculations of Lightning Protection Systems.
Design Calculations of Lightning Protection Systems
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:
1.2 Types of Loss
Loss LX mean amount of loss (humans and goods) consequent on a specified type of damage due to a dangerous event, relative to the value (humans and goods) of the structure to be protected.
While a Dangerous event means lightning flash to or near the structure to be protected, or to or near a line connected to the structure to be protected that may cause damage
Each type of damage relevant to structure to be protected, alone or in combination with others, may produce different consequential loss. The type of loss that may appear depends on the characteristics of the structure itself.
The following types of loss, which may appear as consequence of damages relevant to structure, are considered:
The relationship between source of damage, type of damage and loss is reported in Fig.2.
1.3 Types of Risks Associated with Losses
Risk R: is the value of probable average annual loss (humans or goods) due to lightning, relative to the total value (humans or goods) of the structure to be protected.
For each type of loss which may appear in a structure, the relevant risk shall be evaluated corresponding to their equivalent type of loss. The risks to be evaluated in a structure may be as follows: (see Fig.3)
R1: Risk of loss of human life:
R2: Risk of loss of service to the public:
R3: Risk of loss of cultural heritage:
R4: risk of loss of economic value:
1.4 Lightning protection zones (LPZ)
Lightning protection zone LPZ are used to define the lightning electromagnetic environment. The zone boundaries of an LPZ are not necessarily physical boundaries (e.g. walls, floor and ceiling). The zones are areas characterized according to threat of direct or indirect lightning flashes and full or partial electromagnetic field. Protection measures such as LPS, shielding wires, magnetic shields and SPD determine lightning protection zones (LPZ).
With respect to the threat of lightning, the following LPZs are defined (see Figures 4 and 5):
A comparison between the exposure threats for each Lightning Zone can be listed in Fig.6 in below:
1.5 Lightning protection levels (LPL)
Lightning protection level LPL: is a number related to a set of lightning current parameters values relevant to the probability that the associated maximum and minimum design values will not be exceeded in naturally occurring lightning.
In the IEC 62305 series, (4) lightning protection levels are introduced and the design rules are based on the LPS being able to protect against maximum values (“sizing efficiency”) and minimum values (“interception efficiency”) of current.
The four lightning protection levels are:
LPL I, LPL II, LPL III and LPL IV.
LPL I offers the highest protection level (greatest level of protection), with LPL IV offering the lowest level of protection.
Fig.7 indicates for these lightning protection levels the maximum current expected and the probability that this may be exceeded. The probability of occurrence of lightning with minimum or maximum current parameters outside the range of values defined for LPL I is less than 2 %.
As the lightning downward leader approaches the ground or structure, the electric field increases to the point that the ground or structure launches an upward leader that may eventually intercept the downward leader. This is termed the “striking distance” (see fig.8). The larger the amount of charge carried by the lightning leader, the greater will be the distance at which this happens. The larger the charge of the leader, the larger the resulting lightning current. It is generally accepted that the striking distance r is given by:
r = 10 I 0.65
Where I is the peak current of the resulting stroke.
For each of the lightning protection levels, a minimum current level to be protected against has been determined (selected). Fig.9 details these current levels, together with probability percentages that lightning may be greater than these levels.
LPL I positions terminals such that 99% of all lightning flashes are intercepted (all those of 3 kA or greater). There is only a 1% probability that lightning may be smaller than the 3 kA minimum, and may not be close enough to an air-terminal to be intercepted. It should be noted that flashes of less than 3 kA are rare, and typically would not be expected to cause damage to the structure. Protection greater than LPL I (99%) would require significantly more material, is not covered by the standard and generally is not required for commercial construction.
To further explain Fig.9, a lightning protection system to provide LPL IV, designed using the rolling sphere method, would use air-terminals placed using a rolling sphere radius of 60 m.
These air-terminals would be positioned such that they would capture all lightning flashes of 16 kA or greater, thus offering protection to at least 84% of the lightning (the term “at least” is used to indicate that the percentage of lightning captured might be greater, since smaller lightning flashes could be captured if they were closer to the air-terminal).
To offer a greater lightning protection level (e.g. LPL I, II or III) a smaller rolling sphere radius would be used. This would result in a reduced spacing between air-terminals (more air-terminals), thus positioning the air-terminals to capture smaller lightning flashes, and increasing the total percentage of lightning flashes captured.
1.6 Class of LPS
Class of LPS is a number denoting the classification of an LPS according to the lightning protection level for which it is designed power line or telecommunication line connected to the structure to be protected.
Four classes of LPS (I to IV), as shown in Fig.10, are defined in this standard corresponding to lightning protection levels defined in IEC 62305-1.
Each class of LPS is characterized by the following:
A- Data dependent upon the class of LPS:
B- Factors not dependent upon the class of LPS:
The choice of what Class of LPS shall be installed is governed by the result of the risk assessment calculation. Thus it is prudent to carry out a risk assessment every time to ensure a technical and economic solution is achieved.
1.7 Protection Measures
Protection Measures are measures to be adopted for the structure to be protected in order to reduce the risk, according to the type of damage, in the event of a lightning strike to or near a structure or connected service.
For each type of loss, there is a number of protection measures which, individually or in combination, make the condition R ≤RT. Lightning protection science include (3) types of protection measures as follows:(see fig.11)
1- LPS Protection Measures:
It used to reduce physical damage, Protection is achieved by the lightning protection system (LPS) which includes the following features:
2- LPMS Protection Measures:
It used to reduce failure of electrical and electronic systems, Possible protection measures (LPMS) include:
3- Other Protection Measures:
It used to reduce injury of living beings by electric shock, other Possible protection measures include:
Details of the methodology and criteria for deciding the most suitable protection measures are given in the Risk management study which will be explained in next Articles.
In the next Article, I will continue explaining Design Calculations of Lightning Protection System. Please, keep following.