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ETL 1110-9-10(FR)
5 Jan 91
b. Computations.
1)
Find the surface area to be protected.
Area of each tank bottom = B r2:
A = B x 37.52 = 4418 sq ft
Area of the four bottoms:
A = 4 x 4418 = 17,672 sq ft
2)
Determine the current requirement (I) from equation
1-1:
I = (A) (I')(l.0-CE)
Where:
A
=
17,672 sq ft (Total surface area from
previous calculation)
I'
=
2 mA/sq ft (Current density from item
4 of paragraph 2-4a)
CE
=
0.0 (bare) (Coating efficiency from
item 8 of paragraph 2-4a)
I
=
17,672 sq ft x 2 mA/sq ft x (1.0 -
0.0)
I
=
35 amp
3)
Select an anode and calculate the number of anodes (N)
required to meet the design life.
The deep anode
groundbed will consist of a series of rods connected to
a continuous header cable. For physical strength, the
1/8-in. or 1/4-in. diameter rods are usually chosen. To
ensure good transmission of current in to the backfill
column, long anodes, typically 6 ft or 8 ft are used.
For this example, 1/4-in. diameter by 72-in. long rods
have been chosen. Using equation 1-2:
N=
I
IA
Where:
I = 35
amp
(Current
requirement
from
previous calculation)
IA = 6.6 amp/anode (Current rating per heavy
duty coated anode from table 3-3)
N=
35
= 5.3; use 6 anodes
6.6
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