MIL-HDBK-1004/10
where
W
=
180 pounds
S
=
1.0 lbs/A yr
E
=
0.50 efficiency
I
=
2.36 A
Y
=
(180 x 0.50)/(1.0 x 2.36) = 38.1 years
It should be noted that the expected ground bed life greatly
exceeds the design requirement of 15 years. This is brought about by the
additional anode material required to establish a 2-ohm ground bed. The lower
ground bed resistance saves energy (power, P = I R).
g)
Resistance of dc circuit:
(1)
Ground bed-to-soil resistance, 2.0 ohms maximum.
(2) Resistance of ground bed feeder conductor (length 500
feet, type HMWPE, size No 2 AWG):
Conductor resistance (refer to Table 10):
0.159 ohm/1,000 feet
R = 500 feet x 0.159 ohms/1,000 feet
R = 0.080 ohms
(3)
Total resistance of
circuit:
R
= 2.0 + 0.080 = 2.08 ohms:
T
h)
Rectifier rating:
(1)
Minimum current requirement, 2.36 A.
(2)
Circuit resistance, 2.08 ohms.
(3)
Voltage rating:
E = IR
where
I
=
2.36 A
R
=
2.08 A
E
=
2.36 x 2.08 V
E
=
4.9 or 5.0 V
To allow for rectifier aging, film formation, and seasonal changes
in the soil resistivity, it is considered good practice to use a multiplying
factor of 1.5 to establish the rectifier voltage rating.
E = 5.0 x 1.5 = 8.0 V
183
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