0064im.jpg)
ETL 1110-9-10(FR)
5 Jan 91
N=
40
= 5 anodes
8
To allow a factor of safety, use 6 anodes
4)
Calculate the required length of the backfill column.
The maximum allowable spacing between anodes depends
primarily on the resistivity of the backfill column
(coke breeze). For tubular anodes of -in. diameter or
greater, maximum spacing between anodes should not
exceed four times the anode tube length.
For this
example, we will try the maximum allowable spacing of
13 ft (39.4 in. x 4/12 = 13.1 ft).
The minimum length
of
the
backfill
column
is
calculated as:
6 anodes at 39.4 in. (3.3 ft) =
19.8 ft
Spacing between anode: 5 at 13 ft =
65.0 ft
Space above and below anode string*
20.0 ft
Total length
104.8 ft
(use 105 ft)
* Generally, the coke breeze column extends from 10 ft below the
bottom anode to 10 ft above the top anode.
5)
Calculate
the
backfill column-to-earth resistance
(RA).
This can be done from equation 1-4. Because several
attempts may have to be made to obtain the required
resistance-to-earth, the process is facilitated by
using the curve in figure 2-9C.
Typical hole
diameters are 6 or 8 in.; a 6-in. diameter hole has
been selected for this groundbed.
From figure 2-9C for a 105 ft long coke breeze backfill
column, the resistance to earth per 1000 ohm-cm is
0.325 ohm.
In 2000 ohm-cm soil, the resistance is
0.325 x 2 = 0.65 ohm, which is below the design
requirement defined in item 7 of paragraph 2-5a.
6)
1-3:
62
Previous Page
