Risk assessment for structure

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Risk assessment for structure

• Roberto Pomponi
• Telecom Italia, Italy

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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)
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Lightning as source of damages
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
RARBRURV(RCRWRMRZ) R2 Risk of loss of
service RBRCRVRWRMRZ R3 Risk of loss of
cultural heritage RBRV R4 Risk of loss of
economic value RBRCRVRWRMRZ
Risks
Question R2 MAX(RBRC)MAX(RVRW)RMRZ?
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Lightning Protection Level, LPL
A set of lightning current parameters values
which defines lightning as source of damage
Current parameters Current parameters Symbol Unit LPL LPL LPL LPL
Current parameters Current parameters Symbol Unit I (99) II (98) III (95) IV (90)
First short stroke Peak current I kA 200 150 100 100
First short stroke Short stroke charge Qshort C 100 75 50 50
First short stroke Specific energy W/R kJ/W 104 5.625 2.500 2.500
First short stroke Time parameters T1 / T2 µs/µs 10 / 350 10 / 350 10 / 350 10 / 350
Subsequent strokes Peak current I kA 50 37.5 25 25
Subsequent strokes Average steepness di/dt kA/µs 200 150 100 100
Subsequent strokes Time parameters T1 / T2 µs/µs 0,25 / 100 0,25 / 100 0,25 / 100 0,25 / 100
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Protection need

• The protection is necessary when
• R gt RT
• RT Tolerable risk
• RT 10-3 value suggested by the IEC
  62305-2 standard

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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/km2anno
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
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Dangerous events
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Dangerous events for direct flashes, Nd
Nd Ng Ad
Ad collection area of the structure
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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
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Collection area for direct flashes to a one
section service
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
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Dangerous events due to flashes near a structure,
NM
250 m
Ad
AM
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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 lt 10 m) Ce 0,1 urban
area (building with 10 lt H lt 20 m) Ce 0
urban area (building with H gt 20 m) Ct for power
line when there is a transformer
Buried line
Aerial line
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Probability factors
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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
Protective measures PA
No protective measure 1
Electrical insulation of exposed down-conductor (e.g. at least 3 mm cross-linked polyethylene) 10-1
Effective soil equipotenzialization 10-2
Warning notices 10-1
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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
Characteristics of the structure Class of LPS PPSD
Structure not protected with LPS No LPS 1
Structure protected with a LPS IV 0,2
Structure protected with a LPS III 0,1
Structure protected with a LPS II 0,05
Structure protected with a LPS I 0,02
PLS I air termination. Continuous metal down conductors PLS I air termination. Continuous metal down conductors 0,01
Continuous metal LPS system Continuous metal LPS system 0,001
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Probability PC of internal systems failure due to
a direct flash to a structure

• 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

LPL PPSD
Nessun SPD 1
III IV 0.03
II 0.02
I 0,01
lt I 0.005 0,001
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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 PMS
0,4 1
0,15 0,9
0,07 0,5
0,035 0,1
0,021 0,01
0,016 0,005
0,014 0,001
0,013 0,0001
KMS KS1 KS2 KS3KS4
KS1 LPS o other shields at LPZ0/LPZ1 boundary
KS2 internal shields
KS3 internal wiring routing and shielding
KS4 impulse withstand voltage (resistibility)
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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.

SPL Vio(K.surge) kV KMS(K.surge)
0,05 2,23 0,7
0,02 3,5 0,43
0,01 4,9 0,3
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Protection measures
KS1 KS2 0,12w
w mesh dimension
Unshielded line
No routing precaution in order to avoid loops (large buildings A 50 m2) (a d 1 m) 1
Routing precautions in order to avoid large loops (A 10 m2) (d 1 m a 0,15 m same conduit) 0,2
Routing precaution in order to avoid loops (A 0,5 m2) (d 1 m a 0,015 m same cable) 0,02
Shielded line shield resistance R W/km Ks3 0,01?Ks
5 lt R 20 0,001
1 lt R 5 0,0002
R 1 (R 0,5) 0,0001
Shield connected to B.B. at both ends and equipment connected to the same B .B. Shield connected to B.B. at both ends and equipment connected to the same B .B.
b transfer factor for cable trays and earth
conductors and h shielding factor of CBN as
defined by Rec. K.56
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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

Probability PLD Probability PLD Probability PLD Probability PLD
Uw kV 5ltRS?20 (O/km) 1ltRS? 5 (O/km) RS?1 (O/km)
1,5 2,5 4 6 1 0,95 0,9 0,8 0,8 0,6 0,3 0,1 0,4 0,2 0,04 0,02
This is also valid for the probability values PV
and PW.
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Probability PZ of internal systems failure due to
flashes near the service

• 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

Uw (kV) No shield Kse Shielding factor related to the earth KSS Shielding factor related to the shield KSS Shielding factor related to the shield KSS Shielding factor related to the shield
Uw (kV) No shield Kse Shielding factor related to the earth 5ltRS?20 (O/km) 1lt RS? 5 (O/km) RS? 1 (O/km)
1 1,5 2,5 4 6 2 1 0,4 0,2 0,1 1 0,5 0,2 0,1 0,05 0,3 0,15 0,06 0,03 0,02 0,08 0,04 0,02 0,008 0,004 0,04 0,02 0,008 0,004 0,002
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
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PX lower between PSPD and PLI?

• 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 PSPD will be lower than the
  previous one

PSPD lt PSPD
PSPD 0,05
K.surge Direct flash to line
Unshielded line
Shielded line
PSPD 0,002
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Selection and installation of coordinated
SPDsprotection(Annex D of IEC 62305-4 standard)
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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
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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

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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)

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Oscillation protection distance, lpo

• lpo may be disregarded
• Up(f) ? 0.5 ? Uw
• d ? 10 m

In the other cases
k 25 V/m
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Induction protection distance, lpi
h 300KS1 KS2 KS3 flashes near the structure
h 30000KS0 KS2 KS3 flashes 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 0.31/2n (n down
conductors) mesh Ks0 0.06w0.5
KS1 LPS o other shields at LPZ0/LPZ1
boundary KS2 internal shields KS3 internal
wiring routing and shielding
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Induced loop missed in the IEC standard
The voltage induced in the Loop A2 is not
considered
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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 customers
  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