Installing Ground Electrode in Soft and Moist Soil - RME111 - RWS200 - AWS310-SITE - AWS430 - RWCC - AWS310 - FOC201

Grounding and Lightning Protection in Vaisala Outdoor Installations Technical Reference

Document code
M211786EN
Revision
B
Language
English (United States)
Product
RME111
RWS200
AWS310-SITE
AWS430
RWCC
AWS310
FOC201
Document type
User guide
Figure 1. Ground Electrode in Soft and Moist Soil

Use horizontal ground electrodes, ground rods, and access wells when the soil is soft and moist. This system is fast to build, but the impedance is quite high although grounding resistance is within specifications. That is because two to three rods distribute the current to a fairly small area, causing high step voltages.

  • Route the horizontal ground electrodes from the grounding connector at the mast base to the ground and towards the access wells. Bury the ground electrodes to a minimum of 60 cm (2 ft) deep or to moist soil.
  • Use two or three ground electrodes.
  • The minimum spacing of rods is at least two times the rod length, preferably more. If rod length is 2.5 … 3 m (8 … 10 ft), spacing must be 6 … 8 m (20 …26 ft).
  • To lower impedance, use more electrodes, extend the electrodes further away, and/or bury them deeper to where soil is moist.
  • Bury the ground rods into soil to a minimum of 3 m (10 ft) deep close to where you routed the horizontal ground electrodes.
  • Connect the horizontal ground electrodes to the ground rods above the ground using exothermic welding or bolts.
  • If you use bolted connections, install access wells around the connection points to protect the connections from moisture.
  • If you use exothermic welding in the connections, access wells are not needed.
Check the access wells and bolted connections regularly.

The electrodes do not need to be evenly spaced. You can guide the transient current to the most desirable direction, for example away from buildings or other equipment, or towards better conducting soil by arranging the wires to that direction.

The minimum spacing of ground rods is two times the rod length. Otherwise their effective soil volumes overlap, decreasing efficiency.

Use access wells to protect the connections and help in maintenance. The access well must have room for hands and tools so that it is possible to check and tighten the connections if necessary.

The material of access wells can be metal, concrete, or plastic, for example. You can buy access wells or build them yourself, using a plastic tube with a water-proof lid.

The following figure shows ground rod installation in an access well.

Figure 2. Installing Ground Rod in Access Well
1
Access well
2
Compression lug
3
Ground rod
4
Grounding cable