An LPS consists of both external and internal lightning protection systems.
The functions of the external LPS are
to intercept a lightning flash to the structure (with an air-termination system),
to conduct the lightning current safely to earth (with a down-conductor system),
to disperse it into the earth (with an earth-termination system).
The function of the internal LPS is to prevent dangerous sparking within the structure, using equipotential bonding or a separation distance, s, (and hence electrical isolation) between the LPS components and other electrically conducting elements internal to the structure.
Four classes of LPS (I, II, III and IV) are defined as a set of construction rules, based on the corresponding LPL. Each set includes level-dependent (e.g. rolling sphere radius, mesh width etc.) and level-independent (e.g. cross-sections, materials etc.) construction rules.
Where surface resistivity of the soil outside and of the floor inside the structure is kept low, life hazard due to touch and step voltages is reduced:
outside the structure, by insulation of the exposed conductive parts, by equipotentialization of the soil by means of a meshed earthing system, by warning notices and by physical restrictions;
inside the structure, by equipotential bonding of lines at entrance point into the structure.
The LPS shall comply with the requirements of IEC 62305-3.
8.4.2 Protection to reduce the failure of internal systems
The protection against LEMP to reduce the risk of failure of internal systems shall limit
surges due to lightning flashes to the structure resulting from resistive and inductive coupling,
surges due to lightning flashes near the structure resulting from inductive coupling,
surges transmitted by lines connected to the structure due to flashes to or near the lines, - magnetic field directly coupling with apparatus.
NOTE Failure of apparatus due to electromagnetic fields directly radiated into the equipment is negligible provided that apparatus complies with radio-frequency (RF) radiated emission and immunity tests defined by relevant EMC product standards (see IEC 62305-2 and IEC 62305-4).
The system to be protected shall be located inside an LPZ 1 or higher. This is achieved by means of electrical and electronic system protection measures (SPM) consisting of magnetic shields attenuating the inducing magnetic field and/or suitable routing of wiring to reduce the induction loop. Bonding shall be provided at the boundaries of an LPZ for metal parts and systems crossing the boundaries. This bonding may be accomplished by means of bonding conductors or, when necessary, by surge protective devices (SPDs).
The protection measures for any LPZ shall comply with IEC 62305-4.
Effective protection against overvoltages, causing failures of internal systems, may also be achieved by means of isolating interfaces and/or a coordinated SPD system, limiting overvoltages below the rated impulse withstand voltage of the system to be protected.
Isolating interfaces and SPDs shall be selected and installed according to the requirements of IEC 62305-4.Annex A
(informative)
Parameters of lightning current
A.1 Lightning flashes to earth
Two basic types of flashes exist:
downward flashes initiated by a downward leader from cloud to earth;
upward flashes initiated by an upward leader from an earthed structure to cloud.
Mostly downward flashes occur in flat territory, and to lower structures, whereas for exposed and/or higher structures upward flashes become dominant. With effective height, the probability of a direct strike to the structure increases (see IEC 62305-2:2010, Annex A) and the physical conditions change.
A lightning current consists of one or more different strokes:
impulses with duration less than 2 ms (Figure A.1)
long strokes with duration longer than 2 ms (Figure A.2).
Key
O1 virtual origin
I peak current
front time
T2 time to half value
Figure A.1 - Definitions of impulse current parameters (typically T2< 2 ms)
Key
7”l0NG duration time
QLONg lon9 stroke charge
Figure A.2 - Definitions of long duration stroke parameters
(typically 2 ms <TLOng <1 s)
Further differentiation of strokes comes from their polarity (positive or negative) and from their position during the flash (first, subsequent, and superimposed). The possible components are shown in Figure A.3 for downward flashes and in Figure A.4 for upward flashes.
IEC 2618/10
Figure A.3 - Possible components of downward flashes
(typical in flat territory and to lower structures)
Positive or negative
Negative t
Single long stroke
P
IEC 2619/10
ositive or negative Figure A.4 - Possible components of upward flashes
(typical to exposed and/or higher structures)
The additional component in upward flashes is the first long stroke with or without up to some ten superimposed impulses. But all impulse current parameters of upward flashes are less than those of downward flashes. A higher long stroke charge of upward flashes is not yet confirmed. Therefore the lightning current parameters of upward flashes are considered to be covered by the maximum values given for downward flashes. A more precise evaluation of lightning current parameters and their height dependency with regard to downward and upward flashes is under consideration.
А.2 Lightning current parameters
The lightning current parameters in this part of IEC 62305 are based on the results of the International Council on Large Electrical Systems (CIGRE) data given in Table A.1. Their statistical distribution can be assumed to have a logarithmic normal distribution. The corresponding mean value /z and the dispersion сГ|Одаге given in Table A.2 and the distribution function is shown in Figure A.5. On this basis, the probability of occurrence of any value of each parameter can be determined.
A polarity ratio of 10 % positive and 90 % negative flashes is assumed. The polarity ratio is a function of the territory. If no local information is available, the ratio given herein should be used.
The value of the probability of occurrence of lightning current peak values exceeding the previously considered is reported in Table A.3.
Table А.1 - Tabulated values of lightning current parameters taken from CIGRE
(Electra No. 41 or No. 69) Pl- l4l
|
Parameter |
Fixed values for LPL I |
Values |
Type of stroke |
Line in Figure A.5 |
||
|
95 % |
50 % |
5 % |
||||
|
/ (kA) |
|
4а |
20а |
90 |
First negative shortb |
1A+1B |
|
50 |
4,9 |
11,8 |
28,6 |
Subsequent negative shortb |
2 |
|
|
200 |
4,6 |
35 |
250 |
First positive short (single) |
3 |
|
|
QFLASH (C) |
|
1,3 |
7,5 |
40 |
Negative flash |
4 |
|
300 |
20 |
80 |
350 |
Positive flash |
5 |
|
|
®SHORT(C) |
|
1,1 |
4,5 |
20 |
First negative short |
6 |
|
0,22 |
0,95 |
4 |
Subsequent negative short |
7 |
||
|
100 |
2 |
16 |
150 |
First positive short (single) |
8 |
|
|
W/R (kJ/Q) |
|
6 |
55 |
550 |
First negative short |
9 |
|
0,55 |
6 |
52 |
Subsequent negative short |
10 |
||
|
10 000 |
25 |
650 |
15 000 |
First positive short |
11 |
|
|
d/7dt v m эх (kA/ps) |
|
9,1 |
24,3 |
65 |
First negative short6 |
12 |
|
9,9 |
39,9 |
161,5 |
Subsequent negative short6 |
13 |
||
|
20 |
0,2 |
2,4 |
32 |
First positive short |
14 |
|
|
d'/df30%/90 % (kA/ps) |
200 |
4,1 |
20,1 |
98,5 |
Subsequent negative short6 |
15 |
|
®LONG |
200 |
|
|
|
Long |
|
|
Л-ONG |
0,5 |
|
|
|
Long |
|
|
Front duration (Ц8) |
|
1,8 |
5,5 |
18 |
First negative short |
|
|
0,22 |
1,1 |
4,5 |
Subsequent negative short |
|||
|
3,5 |
22 |
200 |
First positive short (single) |
|||
|
Stroke duration (ns) |
|
30 |
75 |
200 |
First negative short |
|
|
6,5 |
32 |
140 |
Subsequent negative short |
|||
|
25 |
230 |
2 000 |
First positive short (single) |
|||
|
Time interval (ms) |
|
7 |
33 |
150 |
Multiple negative strokes |
|
|
Total flash duration (ms) |
|
0,15 |
13 |
1 100 |
Negative flash (all) |
|
|
31 |
180 |
900 |
Negative flash (without single) |
|||
|
14 |
85 |
500 |
Positive flash |
|||
a The values of I = 4 kA and / = 20 kA correspond to a probability of 98 % and 80 %, respectively.
b Parameters and relevant values reported on Electra No. 69.Table А.2 - Logarithmic normal distribution of lightning current parameters -
Mean n and dispersion a(ogcalculated from 95 % and 5 % values
from CIGRE (Electra No. 41 or No. 69) І3Ь W
|
Parameter |
Mean A |
Dispersion8 CTlog |
Stroke type |
Line in Figure A.5 |
|
/ (kA) |
(61,1) |
0,576 |
First negative short (80 %)b |
1A |
|
33,3 |
0,263 |
First negative short (80 %)6 |
1B |
|
|
11,8 |
0,233 |
Subsequent negative shortb |
2 |
|
|
33,9 |
0,527 |
First positive short (single) |
3 |
|
|
QFLASH (C) |
7,21 |
0,452 |
Negative flash |
4 |
|
83,7 |
0,378 |
Positive flash |
5 |
|
|
$SHORT(C) |
4,69 |
0,383 |
First negative short |
6 |
|
0,938 |
0,383 |
Subsequent negative short |
7 |
|
|
17,3 |
0,570 |
First positive short (single) |
8 |
|
|
W/R (kJ/Q) |
57,4 |
0,596 |
First negative short |
9 |
|
5,35 |
0,600 |
Subsequent negative short |
10 |
|
|
612 |
0,844 |
First positive short |
11 |
|
|
dfdf™ (kA/ps) |
24,3 |
0,260 |
First negative shortb |
12 |
|
40,0 |
0,369 |
Subsequent negative short6 |
13 |
|
|
2,53 |
0,670 |
First positive short |
14 |
|
|
d'^dt30%/90 % (kA/ps) |
20,1 |
0,420 |
Subsequent negative short6 |
15 |
|
$LONG (C) |
200 |
|
Long |
|
|
Л-ONG |
0,5 |
|
Long |
|
|
Front duration (US) |
5,69 |
0,304 |
First negative short |
|
|
0,995 |
0,398 |
Subsequent negative short |
|
|
|
26,5 |
0,534 |
First positive short (single) |
|
|
|
Stroke duration (US) |
77,5 |
0,250 |
First negative short |
|
|
30,2 |
0,405 |
Subsequent negative short |
|
|
|
224 |
0,578 |
First positive short (single) |
|
|
|
Time interval (ms) |
32,4 |
0,405 |
Multiple negative strokes |
|
|
Total flash duration (ms) |
12,8 |
1,175 |
Negative flash (all) |
|
|
167 |
0,445 |
Negative flash (without single) |
|
|
|
83,7 |
0,472 |
Positive flash |
|
aстіод = '°9(xie %) _ lo9<x50 %) where X is the value of parameter.
b Parameters and relevant values reported on Electra No. 69.
Table А.З - Values of probability P as function of the lightning current /
|
/ (kA) |
P |
|
0 |
1 |
|
3 |
0,99 |
|
5 |
0,95 |
|
10 |
0,9 |
|
20 |
0,8 |
|
30 |
0,6 |
|
35 |
0,5 |
|
40 |
0,4 |
|
50 |
0,3 |
|
60 |
0,2 |
|
80 |
0,1 |
|
100 |
0,05 |
|
150 |
0,02 |
|
200 |
0,01 |
|
300 |
0,005 |
|
400 |
0,002 |
|
600 |
0,001 |
NOTE
For numbering of curves see Tables A.1 and A.2.
Figure A.5 ~ Cumulative frequency distribution of lightning current parameters (lines through 95 % and 5 %>ftlue)
ro W о UI
©
ГГІ о КЗ о о mAll values fixed for LPL given in this standard relate to both downward and upward flashes.
NOTE The value of lightning parameters is usually obtained from measurement taken on tall structures. Statistical distribution of roughly estimated lightning current peak values that does not consider the effect of tall structures is also available from lightning location systems.
