Thales 421, Operation And Maintenance, Page: 60 / 256

LLZ 421
Installation Operation and Maintenance
ILS 420
2−32
Ed. 01.10 SOAC
2.5.3 Ground Rods
Ground rods are either pipes or U, L or T−shaped profiles driven vertically into the ground and 1 to
2 m in length. It is necessary to connect several ground rods together in order to reduce the grounding
resistance. The minimum spacing is twice the length of each ground rod. The material, cross sectional
areas and grounding resistance of the connecting leads to the shelter should be as for strip ground
conductors (see Section 2.5.2). When pipe−type grounding rods are fastened with clamps, screws
at least M10 in size should be used.
Material: Steel (hot galvanized) Pipe 30 x 3 mm
Bracket L 65 x 65 x 7 mm
U−profile U 65 DIN 1026
T−profile T 60 DIN 1024
Probable grounding resistance of a rod: Length 1 m = approx. 70
Length 2 m = approx. 40
2.5.4 Ground Plates
Ground plates are solid perforated sheet metal plates with dimensions of approx. 0.5 x 1 m or
1 x 1 m, which are embedded vertically in the ground. The top edge of the plates should always be
at least 1 m below the surface. Several ground plates must be connected in parallel in order to reduce
the grounding resistance. The minimum spacing should be 3 m. The grounding resistance of a plate
1 m
@
is approx. 30 . The data for the connecting conductors is specified in the tables in Section 2.5.2.
Material:
Sheet steel (hot galvanized) min. thickness 3 mm
Sheet copper min. thickness 2 mm
2.5.5 Estimating the Ground Resistance
The resistance of each rod, each plate and each supply line used should be determined on the basis
of Section 2.5.2 to 2.5.4. They should be considered as parallel resistances. The resulting ground
resistance should then be calculated as follows:
1/R
res
=1/R
1
+1/R
2
+1/R
3
+...+1/R
n.
Taking into ac-
count the nature of the terrain (for factors see Section 2.5.1) satisfactory grounding conditions can
be assumed if the ground resistance calculated from the base data is < 5
. If the value calculated
is 5 or more, we recommend measuring the ground resistance.
2.5.6 Measuring the Ground Resistance
See Fig. 2−28.
Three measurements must be performed over a large area in order to determine the ground resist-
ance precisely. The distance between 2 auxiliary grounds required must be large in relation to the
complete ground network of the navigation installation. Two steel rods approx. 1 m long should be
used as auxiliary grounds (reinforcing bars); these should be driven into the ground with only a few
centimeters remaining at the designated points. It is essential that the rods are of equal length and
are driven in to the same depth. An example of a GP strip conductor ground is shown in the following
figure. The drawing is not to scale.