LIGHTNING PROTECTION

This chapter presents the ITU recommendation for Lightning protection.
The purpose of lightning protection is to protect a vulnerable part of the Spectrum Monitoring Sensor withminimum damage. The lightning rod has been the front line of defence against the direct lightning strike. For the induced lightning, the total lightning protection plan for grounding, interconnection or bonding, shielding and surge suppression shall be implemented.

Lightning effect
Lightning is the result of separation of charges in thunderclouds. The maximum dumped current (power) of flash can amount to almost 400 KVA. However, from the statistics, the majority of dumped current is less than 100 KVA, and about 50% of them are between 20 – 25 KVA, the range when the end of leader launches lightning stroke toward ground (stroke range) is between 30 m and 70 m for 20 KVA and between 70 m to 150 m for 50 KVA .

There are two categories of lightning effects known as:

1. Direct lightning strike that causes enormous damage due to huge amount of lightning strike current. Normally conductor elements, e.g. building and antennas are exposed of this kind of effect.
2. Induced lightning effect, caused by intense electromagnetic field, results transient voltage and current surge in equipment, and can affect up to several kilometres away actual strike.



As shown in Figure 1, the extent of protected area for concerned structure largely relies on the magnitude of stroke current and the height of the lightning rod.

When the height of the lightning rod is bigger than the stroke range ( h>rs) as shown in Figure 1 (b), the lightning rod is ineffective to attract the lightning, hence, the lightning may strike the upper portion of the structure.





Because the stroke range relies on the stroke current, IEC 1024-1 (Basic principal of structure protection)defined the protection level of structure as listed below.


The lightning protection system with lightning rod is not the complete system for protection of lightning strike. It is important to determine to what extent the Spectrum Monitoring Sensor shall be protected.

Antenna protections
Besides of earthing and outdoor protection measures the following also should be considered:
· The antenna system / array should be brought under ‘45° protection area’, and / or ‘ rolling sphere protection radius’ of lightning - rod.
· Lightning-rod should be mounted vertically above the antenna system / array, or if not possible, it may be mounted in the vicinity of system so as to make a proper protection area over the system.