MIL-HDBK-1004/10
(2)
Resistance of a single anode to earth:
P
R
=
K
v
L
where
P
= earth resistivity, ohm-cm
K
= shape function (refer to para. 6.2.1.4a)
L
= backfilled anode length
where
P
=
2,000 ohm-cm
L
=
7.0 feet
K
=
0.0167 (refer to para. 6.2.1.4a)
L/D
=
84 inches/10 inches backfill size
R
=
2,000 x 0.167/7.0 ohms
Rv
=
4.77 ohms
v
(3) Number of anodes required. It was stated in the design
data that the anode bed resistance is not to exceed 2 ohms. Anode size used
is 2-inch diameter x 60 inches long with carbonaceous backfill having overall
dimensions of 10-inch diameter x 84 inches long and spaced 20 feet apart:
R
=
(1/n)R
+
P p/S
n
v
s
where
R
=
anode bed resistance
n
n
=
number of anodes
R
=
single anode resistance
v
P
=
earth resistivity with pin spacing equal to S
s
p
=
paralleling factor (refer to para. 6.2.1.4b)
S
=
spacing between adjacent anodes
where
R
=
2 ohms
n
R
=
4.77 ohms
v
P
=
2,000 ohm-cm
s
S
=
20 feet
NOTE:
p is a function of n as referred in para. 6.2.1.4b) and n
is the number of anodes which are determined by trial and
error.
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