1. Russian Conference on Lightning Protection
St. Petersburg, May, 17-19, 2016
“Separation Distance for the Electrical Insulation of the External Lightning Protection System”
F. Heidler
University of the Federal Armed Forces, Munich

2. Lightning Protection System LPS acc. to IEC 62305-3
External Lightning Protection System (External LPS)
Air termination system
to intercept a lightning flash
Down conductor system
to conduct the lightning current safely to earth
Earth termination system
to disperse the lightning current into the earth
Internal Lightning Protection System (Internal LPS)
Lightning equipotential bonding
to prevent dangerous sparking within the structure
Separation distance

3. Side flashes and separation distance
1 MV u
250 ns t L
PEN
Bonding bar
Side flashes
LPS
Installation

4. Damages

5. Separation distance ‚s‘ acc. to IEC 62305-3
The separation distance s is the minimum distance between the external LPS and installations inside the structure to avoid side flash
General equation for the separation distance:
ki : Induction coefficient
kc: Configuration coefficient
km: Material coefficient
l: Length

6. Material coefficient km and induction coefficient ki
km considers the dielectric strength of air in relation to construction materials, e.g. bricks
ki considers the voltage induction due to current steepness of subsequent return strokes
Material km
Air 1
Concrete, bricks
0,5
Lightning protection level (LPL)
imax
imax/T1 ki I
50 kA
200 kA/µs
0.08 II
37.5 kA
150 kA/µs
0.06
III and IV
25 kA
100 kA/µs
0.04

7. Configuration coefficient kc and length l acc. to IEC 62305-3
Length l: Total length along the air termination and the down conductors from the point, where the separation distance is to be considered to the nearest equipotential bonding point
Coefficient kc: kc takes into account the percental current share through the individual air-termination conductors/down conductors. s l
50 %
100 %
Down
conductor
Installation
loop
kc = 0,5
PAS
Bonding
bar

8. Configuration coefficient kc
Evaluation of the configuration coefficient kc is the main problem
Therefore, in the standard IEC two different approaches are given with respect to practical application.
Simplified approach for the configuration coefficient kc
Detailed approach for the configuration coefficient kc

9. Simplified approach for the configuration coefficient kc
Use of pre-defined values for kc
(a) Franklin rod (1-dimensional): kc = 1
(b) 2-dimensional: kc = 0.66
(c) 3-dimensional: kc = 0.44
Correction for 3-dimensional meshed air termination and down conductor system with different values of the spacing and of the length of the down-conductors
h: Height of the structure
c: Spacing between down conductors
n: Number of down conductors

10. Detailed approach for the configuration coefficient kc
𝒔= 𝒌 𝒊 𝒌 𝒎 ∙( 𝒌 𝒄𝟏 𝒍 𝟏 + 𝒌 𝒄𝟐 𝒍 𝟐 +…+ 𝒌 𝒄𝒏 𝒍 𝒏 )
At every next junction (joint) of the air-termination mesh the current is reduced to 50 %, but the value of kcn must not be less than 1/n (n: Number of the down conductors).
The values of kc have to be considered from the point of strike to the edge of the roof using the shortest path.
The arrows indicate the paths from the different injection points (A,B,C) of the current to the edge of the roof. The lengths of the arrows correspond to the length l1, l2 … ln

11. Calculation of the separation distance s
Detailed evaluation of the separation distance is more or less only possible via calculations with computer codes:
First step: Calculation of the induced voltage at the proximity with a computer code
(used computer code CONCEPT)
Second step: Evaluation of the separation distance from the calculated voltage by the use of the “Constant- area-criterion”

12. Examples of induced voltages
LPS type “a”: Air termination: mesh,
Point of strike: corner,
Induced Voltage: Corner loop
(b) LPS type “b”: Air termination: metal roof,
Point of strike: center,
Induced Voltage: Center wire

13. Spark-over behavior of an air gap
Static onset voltage Uo
Voltage-time characteristic for impulse voltages with different steepness
Illustration of the constant-area-criterion

14. Meshed wires and Metal Attic
LPS Configuration
Motivation for the computer simulations:
Comparison of the formula of IEC
IEC standard series contains no formulae to determine the separation distance for natural components (metal roofs, metal facades, metal attics, etc.)
LPL II/LPS II is applied
LPS
Type
Air Termination
Down Conductors a
Meshed Wires
Wires b
Metal Roof c
Metal Walls d
Meshed wires and Metal Attic

15. Overview of the configurations

16. Current injection and induction loops
corner
side
center
corner
loop
center
wire

17. Induced voltages for meshed wire air termination (LPS Type “a”)
Structure
size
Induction to
Peak voltage [kV]
Point of strike
Corner
Side
Center
20 m x 20 m
10 m high
corner loop
1770
628
506
center wire
471
426
1030
20 m high
2150
974
1060
711
678
1440
40 m high
2530
1670
1660
1380
1190
1470
60 m high
2870
2010
2160
1820
1580
1930
60 m x 60 m
1780
155
243
257
247
1120

18. Separation distance for meshed wire LPS (LPS Type “a”)
Base area
Height
Configuration
Separation distance
CONCEPT
Calculation
Simplified approach
Detailed approach
20 m x 20 m
10 m
Corner loop, corner strike 29
21.8
20.0
Center wire, center strike 23
22.5
20 m 44
38.5
30.0 34
40 m 65
69.2
45.0
60 m 78
98.1 60 52
60 m x 60 m 28
19.2 32
28.8

19. Conclusion for meshed wire LPS (LPS Type “a”)
1. Corner Strike
The separation distance is underestimated with the simplified approach of IEC by about 15…30 % for heights up to 20 m.
The separation distance is even more underestimated with the detailed approach of IEC
2. Center Strike
The separation distance is overestimated with the simplified approach of IEC by up to about 100 % for heights greater than 10 m.
The detailed approach of IEC give good results.

20. Studied structures of LPS type ‚b‘ (metal roof)
corner
strike
side
center
loop
wire

21. Separation distance for metal roof (LPS Type “b”)
Structure
size
location
Separation distance s [cm]
Point of strike
Corner
Side
Center
20 m x 20 m
10 m high
corner loop
8,4
8,1
7,8
center wire
7,6
20 m high 17
40 m high 34
60 m high 49 48
60 m x 60 m
6,1
2,8
2,2
2,4
2,6

22. Conclusions for metal roofs
Using a metal roof as a natural component in combination with down conductor wires significantly reduces the separation distances (by a factor of 2 and more).
The separation distances are fairly independent of the point of strike and the location of the induction loop.
The separation distance is predominantly a function of the structure’s height.
The separation distance can be calculated by the following formula:
LPS I II
III / IV k
0,012
0,009
0,006

23. Studied structures of LPS type ‚c‘ (metal walls)
Center strike
Corner strike
Side strike
Corner
loop

24. Peak voltages for metal walls (LPS type “c”)
Structure
size
Location
Peak voltage [kV]
Point of strike
Corner
Side
Center
20 m x 20 m
10 m high
corner loop
441
167
395
center wire 99 60
1010
20 m high
440
110
500
160 75
1430
60 m x 60 m
430 85
190
100
1120

25. Conclusion for metal walls (LPS type “c”)
1. Strike to the corner or rim of the roof
Induced voltages were less than 500 kV (for LPL II)
Separation distance of s  15 cm is sufficient
2. Strike to the center of the roof and center wire induction
Peak voltages are not much different to an LPS type „a“ structure
The reduction of ‘s’ compared to a type “a” LPS is ≤ 20 %

26. Meshed wire LPS with an attic (LPS Type “d”)
center strike
side strike
corner strike
LPS type “d”
center
wire
corner
loop
Base Area 20 m x 20 m: Height 20 m
Width of attic: 0,5 m and 1 m

27. Meshed wire LPS with an attic (LPS Type “d”) (Structure size 20 m x 20 m x 20 m)
Separation distance in cm
Center strike
Side strike
Corner strike
For the cases of side strike and center strike no significant reduction of the separation distance is found, irrespective of the sheet width
Only for the worst-case combination of “corner strike/corner loop” the separation distance is reduced by about 30 % for the sheets width of 0.5 m and by about 40 % for the sheets width of 1 m.

28. Conclusion 1. Meshed wire LPS 2. Metal roof 3. Metal wall
Corner strike: Simplified approach and detailed approach of IEC underestimate the separation distance, but simplified approach gives better results
Center strike: Detailed approach gives satisfying results for the separation distance, but the simplified approach may overestimate the separation distance strongly
2. Metal roof
The separation distance is reduced by the factor of about
The separation distance is proportional to the height over the equipotential plane.
3. Metal wall
If the point of strike is at the top of the metal wall, the separation distances are relatively small. About 15 cm are sufficient to keep the required separation distances for class II LPS
In case of center strike the use of a metal facade gives only marginal reduction of the separation distances
4. Metal attic
The use of a circumferential metal attic is of minor influence, whereat the separation distance is reduced by some tens of percents, at best.