Introduction:
IS/IEC 62305 part 2 explains about the damages due to lightning strikes, sources of such damages, Losses due to lightning and the corresponding risks. In this article, we shall explore about the assessment of individual risk components and the probability of damages due to lightning flashes as specified by IS/IEC 62305 part 2.
Probability of Damage (PX ):
The second important parameter in calculating the risk due to lightning strikes is the probability (PX ) of damage. For every risk component, there exists a probability of damage. The different probability of damages corresponding to the risk components are as follows.
Probability PA - a flash to a structure will cause injury to living beings by electric shock.
Probability PB - a flash to a structure will cause physical damage.
Probability PC - a flash to a structure will cause failure of internal systems.
Probability PM - a flash near a structure will cause failure of internal systems.
Probability PU - a flash to a line will cause injury to living beings by electric shock.
Probability PV - a flash to a line will cause physical damage
Probability PW - a flash to a line will cause failure of internal systems
Probability PZ - a lightning flash near an incoming line will cause failure of internal systems
Calculation:
IS/IEC 62305 part 2 provides the procedure for calculating the different probabilities of damages corresponding to the risk components. The formulae for calculating the probabilities as specified in Annexure B of IS/IEC 62305 part 2 are as follows.
PA = PTA * PB
PC = PSPD * CLD
PM = PSPD * PMS
PU = PTU * PEB * PLD * CLD
PV =PEB * PLD * CLD
PW = PSPD * PLD * CLD
PZ = PSPD * PLI * CLI
Among these probabilities, PA and PU are related to injury to living beings due to electric shocks. The injuries due to electric shocks are mainly because of the step and touch potentials. The terms PTA and PTU depends on the protection measures taken against the step and touch potentials.
The probabilities PB and PV are related to the physical damages due to lightning strikes on structure and power lines respectively. The probability PB mainly depends on the class of LPS.
The probabilities PC, PM, PU, PW and PZ are related to the failure of internal systems. The internal systems shall be protected from the lightning impulses by using Surge Protection Devices. From the above formulae we can observe that these probabilities depend on the factor SPD.
All the parameters and factors involved in calculating the probability are provided in Annexure B of IS/IEC 62305 part 2.
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