# Sample calculation of lightning protection for an astrophysics observatory

17.10.16

At astrophysical observatories, observations of near and deep space are under way, which makes it possible to conduct scientific experiments, predict the approach of cosmic bodies to our planet and much more. Telescopes are our peephole to the Universe – items of hi-tech equipment comprising not only mirrors, but also high-precision electronics. The functional capability of such equipment must always be at the highest level, and thus such objects must have a lightning protection system to prevent a direct hit of lightning into the telescope.

At our technical center, we received a request to calculate a lightning protection system for one of the observatories and performed it with the customer requirements taken into account as follows.

• To implement a lightning protection system using standalone lightning diverters. To ensure a protection zone for the whole facility.
• The object comprises six telescopes, a warehouse, a parking lot and support poles with TV cameras, 3.5 m high. Maximum telescope height – 7 meters.

Soil type – loam up to 1-2 meters (100 Ohm*m), afterwards – rocky soil.

Telescopes are referred to as "ordinary" facilities, in compliance with SO 153-34.21.122-2003 and are incategory 3 as per RD 34.21.122-87. Required system reliability – 0.9

The action plan to ensure compliance with the necessary requirements for a lightning protection system is as follows:

1. Installation of 2 standalone mast interception rods of 30 meters in height (see Fig.1);
2. Installation of an earthing device for each interception rod comprises two horizontal electrodes of 3 meters in length at distances at least 5 meters apart (copper-plated band 30x4 mm). The distance between the electrodes is at least 5 meters, burial depth 0.5 – 0.7 meters, as per RD, par.2.26 (Fig.1).

Results of calculations for the protection zone in compliance with SO:

Mast interception rod No.1,2 (ZZ-201-030):

h = 30 m;

Height of the cone according to the IS chart. 3.4:

hh0=0.85·h=25.5 m;

The radius of the cone according to the IS chart. 3.4:

r0=1.2·h=36 m;

For a protection zone with the required reliability, the radius of horizontal section rx at the height hx = 7 m is determined as:

For a protection zone of the required reliability, the radius of horizontal section rx at the height hx = 3.5 m is determined as:

Results of calculations carried out with software developed by the Krzhizhanovsky Power Engineering Institute:

• Density of lightning discharges into the ground – 6 strikes / sq. km per year.
• Full number of strikes onto the system – 0.2 (once per 30 years).
• Total number of lightning breakthroughs – 0.0031
• System reliability – 0.984.
• Probability of breakthroughs into all the system objects - 0.016 (once per 318 years).

Figure 1 shows the layout of equipment and protection zones.

Павильон - Pavilion
Зона защиты на высоте 3,5 м. - Protection zone at 3.5m height.
Зона защиты на высоте 7 м. - Protection zone at 7m height.
Молниеприемник–мачта высотой 30 м (ZZ-201-030) - Lightning rod-mast 30m height (ZZ-201-030)
полоса стальная омедненная сечением 30х4 мм (GL-11075) - steel stripe copper-plated with section 30х4 mm (GL-11075)

Figure 1. Layout of the lightning protection equipment and earthing. Protection zone.

List of equipment and the required materials:

 № Fig. Product item Name Quantity, pcs. 1. ZZ-201-030 ZANDZ Vertical interception rod, 30 m (galvanized steel; with insets for the foundation) 2 2. GL-11075-10 GALMAR Copper-plated tape (30 * 4 mm / S 120 mm²; coil of 10 meters) 1 3. GL-11075-20 GALMAR copper-plated tape (30 * 4 mm / S 120 mm²; coil of 20 meters) 1 4. ZZ-005-064 ZANDZ Clamp for connecting conductors (up to 40 mm) 3