Lightning danger


More often we see that thatched roofs are finished with very beautiful copper on the ridges.

Then the question can be asked, how dangerous is copper on the ridge at lightning?
It is a fact, that metal attracts lightning a little bit more, then when there is no metal present.
Can copper be used as a lightning protector on the ridge?
Yes it should, only if the copper ridge is thick enough to bear a lightning conduction and if all transitions in the copper ridge are and stay whitout exception lasting and conductive.

Will the copper ridge be thick enough to stand melting on lightning?
Years of research has been done for melting of metal by lightning strike. In general we can state from the results that metal which is 3 mm thick, there are no results to expect. This is also the cross-section which prescribes to the standard NEN 1014.
However a copper ridge is less thick than 1 mm. In other words on direct lightning strike the copper ridge wil melt trough. A recent investigation showed that melted material drops after lightning strike, can start a fire. Since the sheer passes through to under the ridge, you have always to prevent that the copper ridge can melt trough lightning strike. To prevent meltingthrough of a copper ridge you kan construct a thicker copper reception cable, such as it is used on lightning conduction installations.


On which distance must the copper reception cable be placed according to the copper ridge?
The distance at thatched roofs will be defaulted at 15 cm. This to keep possible corona phenomena round the lightningwires on sufficient distance from the reed. As well a distance of 15 cm will be used to prevent a possible encroachment of copper reception cables to baling wires in the reed. Yet a copper ridge has such an amount of metal frame, that an encroachment only is to prevent, when the distance between the reception cable and the copper ridge is approximately 50 cm.
Cosmeticaly seen this is certainly no solution. Nevertheless encroachment has to be prevented. Because encroachment to the copper ridge can result too in melting phenomena.

The only way to prevent encroachment is toe connect the copper ridge on purpose with the reception cable of the lightningconductor. NEN 1014 tells that metal with an surface of more than 3 meters and metal wich is closer than 50 cm from the lightningconductorcable, has to be connected.
This can be done by using extra clamps, which are placed between the reception cable and the copper ridge. A copper ridge is made of parts. The parts have no mutual lasting electrical connection. This means an extra danger. Because in this case too it applies that there can be uncontrolled discharge failure between the parts, with meltingphenomena. For this reason it is important that for each part of the copper ridge, a durable mutual electric connection will be made, or that each part makes a connection with the conduction cable.
A possible solution for this is to use conductive clamps for the conduction cable, which at least has to be applied on each part of a copper ridge.
Or, such as we see with Lindenok ®, where the reception cable is integrated with the construction materials of the ridge. It is important too that the reception cable always (also at the angles) sticks out above the copper ridge, so that seen from above it is the first "point of touch" of a lightning discharge.


In this situation there still is however a possible problem.
Since the copper ridge has always must be connected on the reception cable, the ridge becomes at lightningstrike on a higher potential. Even when the ridge would not be connected, the spark jump (with all impacht of serve), the ridge will be brought on a higher potential too. Since as the copper ridge always lays on top of the reed, the risc creates junction to the metal bindingwires in the reed. Insulation between has no meaning (concerning slide-discharges).
Unfortunelately there is no solution for this. This problem applies for all metal parts which lie on the sheer or are integrated with the sheer (sink ridge, metal lead throughs, etc.).
The question is how actural is this danger.
The copper ridge has a very low impedance by its metal mass . The E-field, that eventually causes the disruption, will by means of the big surface of the ridge, build-up less rapidly than on a 50 mm cable.

Summarising:
Since a copper ridge is thinner than 3 mm, there exists a very large chance that at a possible lightning strike melts the copper, which may be the begin of a fire.
According to this a copper ridge has to be provided lightning conduction cable. Because the distance between the conduction cable and the copper ridge is not big enough (>50 cm), there a large chance exists that junction takes place from the receiving cable to the copper ridge.
In this respect the copper ridge has to be durable electric connected with the receiving cable. Because a copper ridge is mostly constructed from parts, which do not have a durable electric connection, the possibility exists here too from melting and jumpovers.
In this respect each part of the copper ridge has to be connected either mutually or with the receivingcable. Above requirements are also this way prescribed in the standard booklet for lightningsecurity, the NEN 1014.

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