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l [m] is the length along the air-termination system or down According to Supplement 1 of the German DIN EN 62305-3
conductor from the point where the separation dis- standard, the factor k m can be calculated for multi-layered
tance is to be determined to the next equipotential brickwork. This factor k m consists of the material thicknesses
bonding or earthing point. and the insulation properties of the materials (Figure 5.6.3).
The following formula is used to calculate the factor k m :
Coefficient k i
The coefficient k i (induction factor) of the relevant class of (l k m1 +l k ...+lx k )
mx
m2
2
1
LPS stands for the risk posed by the current steepness. It de- k mtotal = l
pends on the class of LPS and is specified in Table 10 of the g
IEC 62305-3 (EN 62305-3) standard (Table 5.6.1). where
k m total is the total material factor
Material factor k m l 1 , l 2 … l x stands for the material thicknesses
The material factor k m takes into consideration the insulation
properties of the surroundings. The electrical insulation proper- l g is the total material thickness
ties of air are assumed to be a factor 1. All other solid materials k m 1, 2 … k mx defines the insulation property of the relevant
used in the building industry (brickwork, wood, etc.) insulate material
only half as well as air. This must also be taken into account
for a roof-mounted air-termination rod. As shown in Figure For a wall construction as shown in Figure 5.6.3, the material
5.6.2, solid material (k m = 0.5) is situated between the base of factor k m total is calculated as follows:
the air-termination rod and the roof-mounted structure and an
air clearance (k m = 1) is situated between the top edge of the k = (0.35m 0.5+ 0.08m 1+ 0.12m 0.5)
roof-mounted structure and the air-termination rod. mtotal 0.55m
Since no other material factors than k m = 0.5 and 1 are speci-
fied in the standard, deviating values must be verified in tests k m total = 0.573
or calculations. A factor 0.7 is specified for glass-fibre rein-
forced plastic (GRP) which is used in the DEHN products for However, in case of multi-layered brickwork connection ele-
isolated air-termination systems (DEHNiso spacer, DEHNiso ments are commonly used between the different materials
Combi). This factor can be inserted in the calculation in the
same way as the other material factors.
concrete air clinker
k m = 0.5 k m = 1 k m = 0.5
Class of LPS k i
I 0.08
II 0.06 l 1 = 0.35 l 2 = 0.08 l 3 = 0.12
III and IV 0.04 l g = 0.55
Table 5.6.1 Induction factor k i Figure 5.6.3 k m in case of different materials with air clearance

k m = 1 wires between
concrete
s and clinker

concrete wire clinker
k m = 0.5 k m = 0 k m = 0.5

l 1 = 0.35 l 2 = 0.08 l 3 = 0.12
k m = 0.5 l g = 0.55
Figure 5.6.2 Material factors for an air-termination rod on a flat roof Figure 5.6.4 k m in case of different materials without air clearance

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