5 Jan 91
= 5 anodes
To allow a factor of safety, use 6 anodes
Calculate the required length of the backfill column.
The maximum allowable spacing between anodes depends
(coke breeze). For tubular anodes of -in. diameter or
greater, maximum spacing between anodes should not
exceed four times the anode tube length.
example, we will try the maximum allowable spacing of
13 ft (39.4 in. x 4/12 = 13.1 ft).
The minimum length
6 anodes at 39.4 in. (3.3 ft) =
Spacing between anode: 5 at 13 ft =
Space above and below anode string*
(use 105 ft)
* Generally, the coke breeze column extends from 10 ft below the
bottom anode to 10 ft above the top anode.
backfill column-to-earth resistance
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.
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
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.
Determine total circuit resistance (RT) from equation