Risk assessment for structure

1. Risk assessment for structure Roberto Pomponi Telecom Italia, Italy

2. Contents • Lightning as source of damages: risks and risk components • Protection need: Tolerable Risk and risk component evaluation; • Number of dangerous events; • Probability and Loss values; • Coordinated SPDs protection Reference document: IEC 62305-2 “Protection against lightning - Part 2: Risk management” (doc. 81/263/FDIS)

3. Lightning as source of damages Close to the tlc line Direct to the tlc line Direct to the structure Close to the structure RU Injury to people RV Physical damage RW Equipment failure RA Injury to people RB Physical damage RC Equipment failure RM Equipment failure RZ Equipment failure Risk components R1: Risk of loss of human life = RA+RB+RU+RV+(RC+RW+RM+RZ) R2: Risk of loss of service = RB+RC+RV+RW+RM+RZ R3: Risk of loss of cultural heritage = RB+RV R4: Risk of loss of economic value = RB+RC+RV+RW+RM+RZ Risks Question: R2 = MAX(RB+RC)+MAX(RV+RW)+RM+RZ?

4. Lightning Protection Level, LPL A set of lightning current parameters values which defines lightning as source of damage

5. Protection need • The protection is necessary when • R > RT • RT Tolerable risk • RT = 10-3 value suggested by the IEC 62305-2 standard

6. Risk components: Basic equations Rx = Nx×Px× Lx Nx Number of dangerous events Px Probability of damage Lx Consequent loss of the damage Nx = Ng× Ax • Ng Ground flash density [N/km2×anno] • Ax Collection area • Ad for direct strokes • AM for lightning close to the structure • AL for direct lightning to the service • Al for lightning near the service

7. Dangerous events

8. Dangerous events for direct flashes, Nd Nd = Ng× Ad Adcollection area of the structure

9. Dangerous events due to direct flashes to the service, NL • Al Collection area [m2] • Cd Location factor • C = 0.25 structure surrounded by higher structures or trees • C = 0.5 structure surrounded by structures or trees of the same high or smaller • C = 1 isolated structure: no other structure in the vicinity within a distance of 3H • C = 2 isolated structure on a hilltop or a knoll

10. Collection area for direct flashes to a one section service Lightning stroke To earth Earth (top view) 1:3 H 3H 3H d Buried line: L Aerial line Lc Length of the line section H High of the line Ha e Hb High of the structures connected at the ends of the line section Buried line Simplified D equation respect to K.47

11. Dangerous events due to flashes near a structure, NM 250 m Ad AM

12. Dangerous events due to flashes near a one section service, Nl Ai Collection area [m2] Ce Environmental factor Ce = 1 for rural area Ce = 0.5 suburban area (building with H < 10 m) Ce = 0,1 urban area (building with 10 < H < 20 m) Ce = 0 urban area (building with H > 20 m) Ct for power line when there is a transformer Buried line Aerial line

13. Probability factors

14. Probability PA of injury to living beings due to a direct flash to a structure The probability values PA of injury to living beings due to step and touch voltage as function of the protective measures:

15. Probability PB of physical damage due to a direct flash to a structure The probability values PB of physical damage due to direct flashes to the structure as function of the LPS class:

16. Probability PC of internal systems failure due to a direct flash to a structure Higher current withstand capability • The IEC 62305-2 assumes that: • An LPS or equivalent is installed • SPDs are installed at the entrance of the line into the structure • Coordinated SPD protection is adopted • SPDs are designed as function of the selected LPL

17. Probability PM of internal systems failure due to flashes near the structure • The probability values PM depend of the adopted lightning protection measure (LPM) according to a factor KMS: • NO SPDs: PM = PMS • SPDs: PM lower between PSPD and PMS KMS = KS1× KS2× KS3×KS4 KS1 LPS o other shields at LPZ0/LPZ1 boundary KS2 internal shields KS3 internal wiring: routing and shielding KS4 impulse withstand voltage (resistibility)

18. K.surge approach on KMS values Surge Protection Level (SPL): Peak values and waveform of the expected dangerous surge voltages or currents which could appear in different points of the telecommunication networks due to the lightning current as source of damage • USPL voltage corresponding the selected SPL, • UR reference voltage (lower than USPL) that defines the minimum resistibility voltage level of the equipment connected to the line or of the line conductor insulation; • NT(U) total number of strikes that will induce a voltage equal or greater than U.

19. Protection measures KS1= KS2 = 0,12×w w = mesh dimension b = transfer factor for cable trays and earth conductors and h = shielding factor of CBN as defined by Rec. K.56

20. Probability PU of injury to living beings due to touch voltage by a flashes to the service • The probability values PU depend on the service shield (RS), the impulse withstand voltage, the typical protection measures and the SPDs at the entrance of the structure: • NO SPDs: PU = PLD for unshielded service PLD = 1 • SPDs: PU lower between PSPD and PLD This is also valid for the probability values PV and PW.

21. Probability PZ of internal systems failure due to flashes near the service Red values missed in the IEC standard • The probability values PZ depend on the service shield (RS), the impulse withstand voltage, the typical protection measures and the SPDs at the entrance of the structure: • NO SPDs: PU = PLI • SPDs: PU lower between PSPD and PLI Kse: shield not bonded to the same B.B. to which equipment is connected Kss: shield bonded to the same B.B. to which equipment is connected

22. PX lower between PSPD and PLI? SPD between conductor and shield Equipment Tlc or signal conductor SPD1 SPD between conductor and earth of an unshielded line SPD1 Line shield Earth • I think that it is correct, but the following information is missed: • When an SPD, which has been installed in a transition point of an unshielded line with a selected PSPD, is installed in the same point of a shielded line, its P’SPD will be lower than the previous one P’SPD < PSPD PSPD = 0,05 K.surge: Direct flash to line Unshielded line Shielded line P’SPD = 0,002

23. Selection and installation of coordinated SPDsprotection(Annex D of IEC 62305-4 standard)

24. Selection SPDs with regard to voltage protection level Equipment is protected when: Up(f)≤ Uw Effective protection level, Up(f) For voltage-limiting type SPD For voltage-switching type SPD: Greater value between Up(f) = Up

25. Coordinated SPDs: clause 7 of IEC 62305-4 • SPD shall be located at the line entrance of the structure at least • Additional SPD may be required when • The distance between the location of the SPD and equipment to be protected is too long (greater than the “protection distance”): • Protection distance: maximum distance along the circuit from the equipment at which the SPD still protects the equipment • UP(f) is greater than the impulse withstand voltage UW of the equipment to be protected • The selection and the installation of coordinated SPDs shall comply with: – IEC 61643-12 e IEC 60364-5-53 (for power systems)); – IEC 61643-21 e 61643-22 (for tlc and signalling systems). • Some basic information for the selection and installation of coordinated SPDs are given in the Annex D

26. Selection and installation of coordinated SPDs • At the line entrance into the structure (e.g. at the main distribution box, MB): • SPD tested with impulse current Iimp (waveform 10/350 ms) • SPD tested with nominal current In when the risk of failure of SPDs due to direct flashes (S1 and S4) can be disregarded • Close to the equipment to be protected (e.g. at the secondary distribution box (SB) or at socket outlet, SA) • SPD tested with the nominal current In or combination waveform generation • The value of Iimp or In depends on the selected LPL (Annex E of IEC IEC 62305-1 standard : Surges due to lightning at different installation points)

27. Oscillation protection distance, lpo • lpo may be disregarded: • Up(f) 0.5  Uw • d  10 m In the other cases: k = 25 V/m

28. Induction protection distance, lpi h = 300×KS1× KS2× KS3flashes near the structure h = 30000×KS0× KS2× KS3flashes to the structure (worst case) KS0 shielding of the structure, LPS or other shields on the structure: LPS: Ks0 = Kc Kc = 1 (1 down conductor) Kc = 0.3+1/2n (n down conductors) mesh: Ks0 = 0.06×w0.5 KS1 LPS o other shields at LPZ0/LPZ1 boundary KS2 internal shields KS3 internal wiring: routing and shielding

29. Induced loop missed in the IEC standard The voltage induced in the Loop A2 is not considered

30. Conclusions • IEC 62305-2 standard gives an exhaustive risk assessment for structures, its contents and connected services • This standard should be used for protection need evaluation of the exchange or customer’s buildings and remote site • Critical points: • Risk evaluation for loss of service • Protection factor PM due to flashes near the structure • Necessary clarification and/or improvement: • Protection factor values PZ due flashes near the service are missed for 1 kV equipment resistibility • PSPD values for SPDs installed on shielded cables • Ks3 values and h and b factors of Recommendation K.56 • Induction loop between two equipment inside the structure is missed