Page 77 - 35_DS702_E_2014_Lightning_Protection_Guide

distance between the roof
conductor holders of approx. 1 m

~70
~ 300
~90 ~300

flexible connection

Figure 5.1.3.5 Plastic flat roof sheetings – Roof conductor holder of type KF / KF2

conductor holders cannot be permanently bonded to roof 5.1.4 Air-termination systems on metal roofs
sheetings since they usually do not permit the application of Modern industrial and commercial buildings often have metal
adhesives. roofs and façades. The metal sheets and plates on the roofs are
A simple and safe way of fixing air-termination conductors is usually 0.7 to 1.2 mm thick.
to use roof conductor holders of type KF in combination with
strips (strips must be cut to the desired length) made of the Figure 5.1.4.1 shows an example of the construction of a
roof sheeting material. The strip is clamped into the plastic metal roof. When such a roof is hit by a direct lightning strike,
holder and both sides are welded onto the sealing. Holder melting or vaporisation at the point of strike can leave a hole
and strip should be positioned directly next to a roof sheeting in the roof. The size of the hole depends on the energy of the
joint at a distance of approximately 1 m. The membrane strip lightning strike and the material properties of the roof (e.g.
is welded to the roof sheeting according to the roof sheeting thickness). The biggest problem is the subsequent damage,
manufacturer's instructions. This prevents air-termination con- e.g. ingress of moisture, at this point. Days or weeks can pass
ductors from being displaced on flat roofs. before this damage is noticed. The roof insulation gets damp
and / or the ceiling gets wet and is thus no longer rainproof.
If the slope of the roof is greater than 5 °, every roof conduc-
tor holder must be fixed, if it is smaller than 5 °, only every One example of damage which was assessed using the
second conductor holder must be fixed. If the slope of the roof Siemens Lightning Information Service (Blitz-Informations
is greater than 10 °, the roof conductor holder may not be suit- Dienst von Siemens (BLIDS)) illustrates this problem (Figure
able any more depending on the installation situation. 5.1.4.2). A current of approximately 20,000 A struck the sheet
If the plastic roof sheetings are mechanically fixed, the roof metal and left a hole there (Figure 5.1.4.2: Detail A). Since
conductor holders must be arranged in the immediate vicinity the sheet metal was not earthed by a down conductor, flash-
of the mechanical fixing. over to natural metal parts in the wall occurred in the area
When carrying out this work, it must be observed that the around the fascia (Figure 5.1.4.2: Detail B) which also left
roofer is liable for welding and bonding work on the sealing. a hole.
Therefore, the work may only be carried out in agreement with To prevent such kind of damage, a suitable external lightning
the roofer responsible for the particular roof or must be carried protection system with lightning current carrying wires and
out by himself (Figure 5.1.3.5). clamps must be installed even on a “thin” metal roof. The

76 LIGHTNING PROTECTION GUIDE www.dehn-international.com