International Telecommunication Union
Risk assessment for
structure
Roberto Pomponi
Telecom Italia, Italy
ITU-T SG 5 Technical Session “Lightning protection”
12 December 2005, ITU Headquarters, Geneva
Contents
ITU-T o Lightning as source of damages: risks
o
and risk components
Protection need: Tolerable Risk and
risk component evaluation;
• Number of dangerous events;
• Probability and Loss values;
o
Coordinated SPDs protection
Reference document: IEC 62305-2
“Protection against lightning - Part 2:
Risk management” (doc. 81/263/FDIS)
dates
ITU-T SG 5 Technical Session “Lightning Protection“
12 December 2005, ITU Headquarters Geneva
2
Lightning as source of damages
Direct to the
structure
Close to the
structure
Risk
components
ITU-T
RA Injury to
people
RB Physical
damage
RC Equipment
failure
RM Equipment
failure
Risks
R1: Risk
R2: Risk
R3: Risk
R4: Risk
Direct to the
tlc line
RU Injury to
people
RV Physical
damage
RW Equipment
failure
Close to the
tlc line
RZ Equipment
failure
of loss of human life = RA+RB+RU+RV+(RC+RW+RM+RZ)
of loss of service = RB+RC+RV+RW+RM+RZ
of loss of cultural heritage = RB+RV
of loss of economic value = RB+RC+RV+RW+RM+RZ
Question: R2 = MAX(RB+RC)+MAX(RV+RW)+RM+RZ?
dates
ITU-T SG 5 Technical Session “Lightning Protection“
12 December 2005, ITU Headquarters Geneva
3
Lightning Protection Level, LPL
ITU-T
A set of lightning current parameters values which
defines lightning as source of damage
LPL
Current parameters
First short
stroke
Symbol
Unit
I
(99%)
II
(98%)
Peak current
I
kA
200
150
100
Short stroke
charge
Qshort
C
100
75
50
Specific energy
W/R
kJ/W
104
5.625
2.500
T1 / T2
µs/µs
Peak current
I
kA
50
37.5
25
Average
steepness
di/dt
kA/µs
200
150
100
T1 / T2
µs/µs
Time parameters
Subsequent
strokes
Time parameters
dates
III
(95%)
IV
(90%)
10 / 350
ITU-T SG 5 Technical Session “Lightning Protection“
12 December 2005, ITU Headquarters Geneva
0,25 / 100
4
Protection need
ITU-T
The protection is necessary when
R > RT
• RT
Tolerable risk
• RT = 10-3 value suggested by the IEC
62305-2 standard
dates
ITU-T SG 5 Technical Session “Lightning Protection“
12 December 2005, ITU Headquarters Geneva
5
Risk components: Basic equations
Rx = Nx × Px × Lx
ITU-T
Nx
Px
Lx
Number of dangerous events
Probability of damage
Consequent loss of the damage
Nx = Ng × Ax
Ng
Ax
Ground flash density [N/km2×anno]
Collection area
Ad
AM
AL
Al
dates
for direct strokes
for lightning close to the structure
for direct lightning to the service
for lightning near the service
ITU-T SG 5 Technical Session “Lightning Protection“
12 December 2005, ITU Headquarters Geneva
6
ITU-T
Dangerous events
dates
ITU-T SG 5 Technical Session “Lightning Protection“
12 December 2005, ITU Headquarters Geneva
7
Dangerous events for direct flashes,
Nd
ITU-T
N d = Ng × A d
Ad collection area
of the structure
dates
ITU-T SG 5 Technical Session “Lightning Protection“
12 December 2005, ITU Headquarters Geneva
8
Dangerous events due to direct
flashes to the service, NL
ITU-T
N L  Ng  Al  Cd 106
Al
Cd
Collection area [m2]
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
dates
ITU-T SG 5 Technical Session “Lightning Protection“
12 December 2005, ITU Headquarters Geneva
9
Collection area for direct flashes to a
one section service
ITU-T
Aerial line
A l  6  H  [L c  3  (H a  H b )]
Lc
H
H a e Hb
3H
Length of the line section
High of the line
High of the structures connected at the ends of
the line section
Lightning stroke
To earth
Earth
(top view)
d
Buried line
A l  2  D  [L c  3  (H a  H b )]
D  0,5  ρ
Buried line: L
dates
1:3
3H H
ITU-T SG 5 Technical Session “Lightning Protection“
12 December 2005, ITU Headquarters Geneva
Simplified D
equation
respect to K.47
10
Dangerous events due to flashes near
a structure, NM
ITU-T
NM  Ng  (AM  Ad  Cd ) 106
250 m
Ad
AM
dates
ITU-T SG 5 Technical Session “Lightning Protection“
12 December 2005, ITU Headquarters Geneva
11
Dangerous events due to flashes near
a one section service, Nl
ITU-T
Nl  Ng  Ai  Ce  Ct 106
Ai
Ce
Collection area [m2]
Environmental factor
Ce = 1 for rural area
Ce = 0.5 suburban area (building with H < 10 m)
Ct
dates
Ce = 0,1 urban area (building with 10 < H < 20 m)
Ce = 0 urban area (building with H > 20 m)
for power line when there is a transformer
Aerial line
Buried line
Ai  1000  Lc
Ai  25    Lc
ITU-T SG 5 Technical Session “Lightning Protection“
12 December 2005, ITU Headquarters Geneva
12
ITU-T
Probability factors
dates
ITU-T SG 5 Technical Session “Lightning Protection“
12 December 2005, ITU Headquarters Geneva
13
Probability PA of injury to living beings due
to a direct flash to a structure
ITU-T
The probability values PA of injury to living beings due
to step and touch voltage as function of the protective
measures:
Protective measures
No protective measure
Electrical insulation of exposed
down-conductor (e.g. at least 3 mm
cross-linked polyethylene)
Effective soil equipotenzialization
Warning notices
dates
ITU-T SG 5 Technical Session “Lightning Protection“
12 December 2005, ITU Headquarters Geneva
PA
1
10-1
10-2
10-1
14
Probability PB of physical damage due to a
direct flash to a structure
ITU-T
The probability values PB of physical damage due to
direct flashes to the structure as function of the LPS
class:
Characteristics of the structure
Structure not protected with LPS
Structure protected with a LPS
Class of
LPS
No LPS
IV
III
II
I
PLS I air termination. Continuous metal down
conductors
Continuous metal LPS system
dates
ITU-T SG 5 Technical Session “Lightning Protection“
12 December 2005, ITU Headquarters Geneva
PPSD
1
0,2
0,1
0,05
0,02
0,01
0,001
15
Probability PC of internal systems failure due
to a direct flash to a structure
ITU-T
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
PC  PSPD  f(LPL)
Higher current
withstand capability
dates
LPL
Nessun SPD
III – IV
II
I
<I
PPSD
1
0.03
0.02
0,01
0.005 – 0,001
ITU-T SG 5 Technical Session “Lightning Protection“
12 December 2005, ITU Headquarters Geneva
16
Probability PM of internal systems failure due
to flashes near the structure
ITU-T
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
dates
KS1
LPS o other shields at
LPZ0/LPZ1 boundary
KS2
internal shields
KS3
internal wiring: routing
and shielding
KS4
impulse withstand voltage
(resistibility)
KMS
≥ 0,4
0,15
0,07
0,035
0,021
0,016
0,014
≤ 0,013
ITU-T SG 5 Technical Session “Lightning Protection“
12 December 2005, ITU Headquarters Geneva
PMS
1
0,9
0,5
0,1
0,01
0,005
0,001
0,0001
17
K.surge approach on KMS values
ITU-T
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
N T U SPL 
SPL 
N T U R 
NT
dates
• 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

 2
 4.N g    pi .di. cos  .d .x.dx the line conductor insulation;
R o I
• 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
ITU-T SG 5 Technical Session “Lightning Protection“
12 December 2005, ITU Headquarters Geneva
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Protection measures
K S4 
KS1= KS2 = 0,12×w w = mesh dimension
Unshielded line
ITU-T
K S3 
No routing precaution in order to avoid loops
(large buildings A = 50 m2) (a = d = 1 m)
Routing precautions in order to avoid large loops
(A = 10 m2) (d = 1 m; a = 0,15 m; same conduit)
Routing precaution in order to avoid loops (A =
0,5 m2) (d = 1 m; a = 0,015 m; same cable)
Shielded line:
shield resistance R [ W/km]
1.5
Uw
A
1
ad
or
 ln
50 0,69
d
1
0,2
0,02
K’s3 = 0,01Ks
5 < R ≤ 20
0,001
1<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.
K s1  h ?
K s3  β ?
dates
b = transfer factor for cable trays and earth conductors
and h = shielding factor of CBN as defined by Rec. K.56
ITU-T SG 5 Technical Session “Lightning Protection“
12 December 2005, ITU Headquarters Geneva
19
Probability PU of injury to living beings due
to touch voltage by a flashes to the service
ITU-T
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
Uw
kV
5<RS20
(Ω/km)
1<RS 5
(Ω/km)
RS1
(Ω/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.
dates
ITU-T SG 5 Technical Session “Lightning Protection“
12 December 2005, ITU Headquarters Geneva
20
Probability PZ of internal systems failure due
to flashes near the service
ITU-T
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)
Red
values
missed
in the
IEC
standard
1
1,5
2,5
4
6
No
shield
2
1
0,4
0,2
0,1
Kse:
Shielding
factor related
to the earth
1
0,5
0,2
0,1
0,05
KSS: Shielding factor related to
the shield
5<RS20 1< RS 5
(Ω/km) (Ω/km)
0,3
0,15
0,06
0,03
0,02
0,08
0,04
0,02
0,008
0,004
RS 1
(Ω/km)
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
dates
ITU-T SG 5 Technical Session “Lightning Protection“
12 December 2005, ITU Headquarters Geneva
21
PX lower between PSPD and PLI?
ITU-T
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
SPD between conductor and shield
P’SPD < PSPD
Tlc or signal conductor
SPD between conductor and
earth of an unshielded line
SPD1
SPD1
Equipment
Line shield
Earth
K.surge: Unshielded line If 
0,25  I p
n m

0,25 100
 1,25 kA
1 20
PSPD = 0,05
Direct
Shielded I  I f  n  (m  R s  R c )  1,25 1 (20  2  135)  437,5 kA
p
flash to line
0,25  R s
0,25  2
P’SPD = 0,002
line
ITU-T SG 5 Technical Session “Lightning Protection“
dates
12 December 2005, ITU Headquarters Geneva
22
ITU-T
Selection and installation of
coordinated SPDs
protection
(Annex D of IEC 62305-4 standard)
dates
ITU-T SG 5 Technical Session “Lightning Protection“
12 December 2005, ITU Headquarters Geneva
23
Selection SPDs with regard to voltage
protection level
ITU-T
Equipment is protected when:
Up(f) ≤ Uw
Effective protection level, Up(f)
For voltage-limiting type SPD
U p(f)  U p  U1  U1'
For voltage-switching type SPD:
L1
U1
SPD
Up
L1
Greater value between
’
U1
Up(f)
’
EBB
Up(f) = Up
dates
Up(f)  U1  U1'
ITU-T SG 5 Technical Session “Lightning Protection“
12 December 2005, ITU Headquarters Geneva
24
Coordinated SPDs: clause 7 of IEC
62305-4
ITU-T o SPD shall be located at the line entrance of the structure at
least
o 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
o 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).
o Some basic information for the selection and installation of
coordinated SPDs are given in the Annex D
dates
ITU-T SG 5 Technical Session “Lightning Protection“
12 December 2005, ITU Headquarters Geneva
25
Selection and installation of
coordinated SPDs
ITU-T
o 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
o 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
o 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)
dates
ITU-T SG 5 Technical Session “Lightning Protection“
12 December 2005, ITU Headquarters Geneva
26
Oscillation protection distance, lpo
ITU-T
lpo
In
Iimp
L1
U1
SPD
Up
L1’
L
Up(f)
U
U1’
Eq.
C
lpo may be disregarded:
• Up(f)  0.5  Uw
• d  10 m
B.B.
In the other cases:
l po 
dates
U w  U p(f)
k
k = 25 V/m
ITU-T SG 5 Technical Session “Lightning Protection“
12 December 2005, ITU Headquarters Geneva
27
Induction protection distance, lpi
l pi 
ITU-T
U w  U p(f)
h
h = 300×KS1× KS2× KS3 flashes near the structure
h = 30000×KS0× KS2× KS3 flashes to the structure (worst case)
KS0
KS1
KS2
KS3
dates
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
LPS o other shields at LPZ0/LPZ1 boundary
internal shields
internal wiring: routing and shielding
ITU-T SG 5 Technical Session “Lightning Protection“
12 December 2005, ITU Headquarters Geneva
28
Induced loop missed in the IEC
standard
S1
ITU-T
LPS
Customer’s building
S
MET
A
A1
S2
I
A2
The voltage induced in the Loop A2
is not considered
dates
ITU-T SG 5 Technical Session “Lightning Protection“
12 December 2005, ITU Headquarters Geneva
29
Conclusions
ITU-T
o 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
o Critical points:
• Risk evaluation for loss of service
• Protection factor PM due to flashes near the structure
o 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
dates
ITU-T SG 5 Technical Session “Lightning Protection“
12 December 2005, ITU Headquarters Geneva
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