International Telecommunication Union ITU-T SG 5 Technical Session Lightning protection 12 December...

International Telecommunication Union

ITU-T SG 5 Technical Session “Lightning protection”12 December 2005, ITU Headquarters, Geneva

Risk assessment for Risk assessment for structurestructure

Roberto Pomponi Telecom Italia, Italy

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ITU-T

ITU-T SG 5 Technical Session “Lightning Protection“12 December 2005, ITU Headquarters Geneva

Contents

o Lightning as source of damages: risks and risk components

o 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)

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Lightning as source of damages

Direct to the

structure

Close to the structure

Direct to the tlc line

Close to the tlc line

RA Injury to people

RB Physical damage

RC Equipment failure

RU Injury to people

RV Physical damage

RW Equipment failure

RM

Equipment failure

RZ

Equipment failure

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

Ris

k co

mpon

en

ts

Risks

Question: R2 = MAX(RB+RC)+MAX(RV+RW)+RM+RZ?

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Lightning Protection Level, LPL

A set of lightning current parameters values which defines lightning as source of damage

Current parameters Symbol

Unit

LPL

I(99

%)

II(98%

)

III(95

%)

IV(90%

)

First short stroke

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

Time parameters

T1 / T2 µs/µs 10 / 350

Subsequent

strokes

Peak current I kA 50 37.5 25

Average steepness

di/dt kA/µs 200 150 100

Time parameters

T1 / T2 µs/µs 0,25 / 100

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Protection need

The protection is necessary when

R > 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 × LxNx Number of dangerous events

Px Probability of damageLx Consequent loss of the damage

Nx = Ng × Ax

Ng Ground flash density [N/km2×anno] Ax Collection area

Ad for direct strokesAM for lightning close to the structureAL for direct lightning to the serviceAl 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

6dlgL 10CANN

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

)]H(H3[LH6A bacl

)]H(H3[LD2A bacl

Lc Length of the line sectionH High of the line Ha e Hb High of the structures connected at the ends of

the line section

H3H3H

1:3

Lightning strokeTo earthEarth

(top view)

d

Buried line: L

Aerial line

Buried line

Simplified D equation respect to K.47

ρ0,5D

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Dangerous events due to flashes near a structure, NM

Ad

AM

250 m

6ddMgM 10)CAA(NN

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Dangerous events due to flashes near a one section service, Nl

6teigl 10CCANN

Ai Collection area [m2]Ce Environmental factor

Ce = 1 for rural areaCe = 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

ci L1000A

Aerial line

ci L25A

Buried 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

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

The probability values PA of injury to living beings due to step and touch voltage as function of the protective measures:

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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 1Structure protected with a LPS IV 0,2

III 0,1

II 0,05

I 0,02PLS I air termination. Continuous metal down conductors

0,01

Continuous metal LPS system 0,001

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Probability PC of internal systems failure due to a direct flash to a structure

LPL PPSD

Nessun SPD 1III – IV 0.03

II 0.02I 0,01

< I 0.005 – 0,001

f(LPL)PP SPDC

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

Higher current withstand capability

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Probability PM of internal systems failure due to flashes near the structure

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

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×KS4KS1 LPS o other shields at

LPZ0/LPZ1 boundaryKS2 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

RT

SPLT

UN

UNSPL

• 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

R o I

gT dxxddiipNN ...cos...42

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Protection measures

w4S U

1.5K

?βKs3

Unshielded lineNo 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 [ /km]

K’s3 = 0,01Ks

5 < R ≤ 20 0,001 1 < R ≤ 5 0,0002 R ≤ 1 (R = 0,5) 0,0001Shield connected to B.B. at both ends and equipment connected to the same B .B.

KS1= KS2 = 0,12×w w = mesh dimension

d

daln

0,69

1or

50

AKS3

= transfer factor for cable trays and earth conductors and = shielding factor of CBN as defined by Rec. K.56

?Ks1

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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 PLDProbability PLD

Uw

kV

5<RS20

(Ω/km)

1<RS 5

(Ω/km)

RS1

(Ω/km)

1,52,546

10,950,90,8

0,80,60,30,1

0,40,2

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

shieldKse:

Shielding factor

related to the earth

KSS: Shielding factor related to the shield

5<RS20

(Ω/km)

1< RS 5

(Ω/km)

RS 1(Ω/km)

11,52,546

21

0,40,20,1

10,50,20,1

0,05

0,30,150,060,030,02

0,080,040,02

0,0080,004

0,040,02

0,0080,0040,002

Red values missed in the IEC standard

Kse: shield not bonded to the same B.B. to which equipment is connectedKss: 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 P’SPD will be lower than the previous one

SPD between conductor and shield

Line shield

SPD1 SPD1

P’SPD < PSPD

K.surge:Direct flash to line

kA5,437225,0

)135220(125,1

R0,25

)RR(mnII

s

csfp

kA25,1201

10025,0

mn

I520,If

p PSPD = 0,05Unshielded lineShielded line

SPD between conductor and earth of an unshielded line

P’SPD = 0,002

Tlc or signal conductor

Earth

Equipment

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Selection and installation of coordinated SPDs

protection

(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

For voltage-limiting type SPD

EBB

L1

SPD

L1’

U1

U1’

Up Up(f)

'11pp(f) UUUU

For voltage-switching type SPD:Greater value between

Up(f) = Up

'11p(f) UUU

Effective protection level, Up(f)

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Coordinated SPDs: clause 7 of IEC 62305-4

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

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Selection and installation of coordinated SPDs

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 s)

• 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 Inn 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

k

UUl p(f)w

po

B.B.

L1

SPD Eq. U

lpo

L

C

L1’

U1

U1’

Up Up(f)

In Iimp

lpo may be

disregarded:

• Up(f) 0.5 Uw

• d 10 m

k = 25 V/m

In the other cases:

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Induction protection distance, lpi

h

UUl p(f)w

pi

h = 300×KS1× KS2× KS3 flashes near the structureh = 30000×KS0× 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 = 0.3+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

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Induced loop missed in the IEC standard

S2

MET

S A

A1

LPS

S1

Customer’s building

A2

I

The voltage induced in the Loop A2 is not considered

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Conclusions

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 and factors of Recommendation K.56• Induction loop between two equipment inside the

structure is missed

International Telecommunication Union ITU-T SG 5 Technical Session Lightning protection 12 December...

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