Air Terminal - RWS200 - RWCC - AWS310-SITE - AWS310 - FOC201 - AWS430 - RME111

Grounding and Lightning Protection in Vaisala Outdoor Installations Technical Reference

Document code
M211786EN
Revision
B
Language
English (United States)
Product
RWS200
RWCC
AWS310-SITE
AWS310
FOC201
AWS430
RME111
Document type
User guide

The preferred air terminal materials are:

  • Copper (Cu)
  • Aluminum (Al)
  • Stainless steel

Use only conductive coating materials, if any. Conducting mast structures are sometimes used as air terminals.

To ensure intended performance of the air terminal, do not paint the air terminal or use isolating coating, such as anodized aluminum.
  • Typically Vaisala sites require only one air terminal. To protect larger areas, you can use two separate air terminals or two air terminals that are connected with a wire, but this is seldom applicable to a meteorological site.
  • Prefer an air terminal that has a blunt tip because it attracts lightning strikes better than a pointed tip.
  • Extend the air terminal tip so high that it keeps the protected equipment in the protected zone. A traditional protected zone estimate for a 10 meter (33 ft) wind mast is a 1 to 1 line (45º cone).
Figure 1. Protected Zone

Most standards model the protected area with an imaginary rolling sphere with a radius of 50 m (150 ft). However, it has been observed several times that lightning is more interested in the actual impedance to ground than physical dimensions of objects. Thus no 100% guarantee is given to any design rule for any lay-out. Just raise the air terminal tip as high as you safely can!

  • To avoid air turbulence, minimize air terminal thickness versus horizontal distance ratio in a wind sensor mast. If you have a wind vane and an anemometer, place the air terminal further away from the wind vane because it is more easily affected by air terminal turbulence than the anemometer.
  • Ensure structural safety under foreseeable wind and ice load.

Equipment with "special design" are sometimes used as air terminals, including early streamer emitters, repellers, and dissipators. If the customer requires a specific brand or type, and the design does not interfere with local legislation or safety and operational requirements, Vaisala follows the requirement, but does neither recommend nor specify such designs. In protected-zone or terminal-positioning calculations these "special designs" are treated as ordinary blunt-tip passive air terminals.