ETL 1110-9-10(FR)

5 Jan 91

4)

Calculate the required length of the backfill column.

Spacing between anodes depends primarily on the

resistivity of both the backfill column (coke breeze).

If low resistivity calcined fluid petroleum coke is

used, than the spacing between rod anode (1/8-in. and

1/4-in. diameter) should not be greater than twice the

individual rod length.

Based on previous design

experience, a spacing between anode rods of 11 ft is

selected.

The minimum length of the backfill column is then

calculated as follows:

6 anodes at 6 ft per anode =

36 ft

Spacing between anodes: 5 at 11 ft =

55 ft

Space above and below anode string* =

20 ft

Total length

111 ft

* Generally, the coke breeze column extends from

10 ft below the bottom anode to 10 ft above the top

anode as shown in figures 2-7 and 2-8.

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

groundbed resistance, the process is facilitated by

using the curves in figures 2-9A through 2-9C. Typical

hole diameters are 6 or 8 in.; a 6-in. diameter hole

has been selected for this groundbed. Figure 2-9C will

be used for this design.

Figure 2-9C shows that, for a 111-ft long coke breeze

backfill column, the resistance to earth per 1000 ohm-

cm is 0.31 ohm.

In 1500 ohm-cm soil, the resistance

is 0.31 x 1500/1000 = 0.47 ohm (groundbed resistance in

1000 ohm-cm soil, times actual soil resistivity in ohm-

cm per 1000).

This is well below the design

requirement of 0.75 ohm.

6)

Determine total circuit resistance (RT) from equation

1-3 :

RT =

RA + RW + RC (Because the cathodic

protection system utilizes a single

deep anode groundbed RA = RN)

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