In the case of an upward flash, the leader starts at the ground - or at a tall object - and grows upwards towards the thundercloud. It is therefore also called earth-to-cloud lightning and is easily identified by its distinctive upward branching appearance.
Upward flashes occur on very high, exposed objects such as wind turbines, buildings, radio towers or antenna systems. The distortion of the electric field at the exposed object is responsible for their formation. It leads to a high electric field strength and causes a lightning discharge.
An example: The leader of a lightning event builds up at tips of the rotor blades of a wind turbine. The leader then moves upwards towards the cloud, forming an upward flash.
Discharge mechanism of a positive upward flash, a so-called earth-to-cloud flash.
Discharge mechanism of a negative upward flash, likewise referred to as an earth-to-cloud flash.
Did you know...?
50% - 90% of all lightning events at high objects are upward flashes – according to the results of scientific research *
(*Source: Diendorfer, G.; Pichler, H.; Mair, M.: Some Parameters of Negative Upward-Initiated Lightning to the Gaisberg Tower (2000 – 2007), IEEE Trans. Electromagn. Compat., vol. 51, no. 3, pp. 443 – 452, August 2009)
An initial continuous current (ICC) flows during upward flashes. This long stroke current can have subsequent impulse currents, so-called return strokes (RS), or might be superimposed by impulse currents. Upward flashes are thus subdivided into 3 types:
ICCRS
One or more return strokes (RS) follow the ICC
RS are detected by LLS (lightning location systems)
ICCP
The ICC is not followed by RS, but impulses > 2 kA are superimposed on the ICC
Some of the ICC pulses are picked up by LLS
ICConly
The ICC is not followed by RS and there are no pulses > 2 kA
Are not detected by LLS
Studies have revealed that in practice so-called ICConly events occur very frequently. The current value of such an ICConly event is often less than 1 kA, which is well below the 200 kA required for LPL I (lightning protection level) as per DIN EN 62305-1 [3]. Nevertheless, the total electric charge of 300 C specified for lightning protection level LPL I might be greatly exceeded. One must remember that ICConly events cannot be located by the lightning detection system due to the lack of impulse current.
Modern wind turbines can be as high as 200 metres. As a result, they trigger upward flashes - often pure ICConly. Despite the low current value of initiating long-stroke currents, this type of lightning event frequently causes damage.
How is that possible?
The reason behind this is the high electric charge. It is the main cause of material melt out and erosion at the arc base. The more frequently a component, e.g., a rotor blade receptor, is hit by an upward flash, the more material is in time worn away. The damage therefore increases very gradually and often goes unnoticed. In the long run, it can lead to expensive system downtime.
With DEHNdetect. The lightning current measuring system detects all types of currents: impulse currents and dangerous long stroke currents. This clearly sets DEHNdetect apart from conventional measuring systems. These often fail to detect the low current flow of upward flashes, entirely or in part.
DEHNdetect keeps you reliably informed about every lightning event. This enables precise and early planning of repair or service work.
DEHNdetect keeps you reliably informed about every lightning event. This enables precise and early planning of repair or service work.
More information is also available in the LIGHTNING PROTECTION GUIDE, the standard reference work on all issues relating to lightning and surge protection. Chapter 2.1 deals with the topic of upward flashes.
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