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ETL 1110-9-10(FR)
5 Jan 91
LW RMFT
RW '
1000 ft
Where:
LW =
240 ft (After reviewing figure 2-20,
it is estimated that 240 ft of wire
will be required to
connect
the
rectifier to the riser anode string)
RMFT =
0.640 ohm (Wire resistance per 1000
lin ft of No. 8 AWG HMWPE insulated
wire)
RW =
240 ft x 0.640 ohm
= 0.15 ohm
1000 ft
Since this anode string is very long, the metal
of
the anode can represent a significant
resistance. Since the current discharges all
along the anode, one half of its length is used
in the resistance calculation.
Manufacturer's
data
show
the
longitudinal
resistance of a single 4-ft anode segment to be
0.053 ohm.
Effective resistance of the string
is:
0.053 ohm/anode x 28 anodes
= 0.74 ohm
2
This resistance is very high compared to the
anode resistance (greater than 10 percent) as a
result, too much current will discharge near the
top of the anode and not enough current will be
discharged near the bottom of the anode.
Therefore, either a double-end feed method will
have to be used or copper-cored rods must be
used. (Note: This is usually only a problem in
fresh water applications when the anode rod
length is greater than 30 ft.) In this case, we
will elect to use copper cored 1/8-in. diameter
rods. Manufacturer's data show the longitudinal
resistance of a single 4-ft long copper-cored rod
of this diameter is 0.0034 ohm.
Effective
resistance of this string is therefore:
= 0.048 ohm
0.0034 ohm/anode x 28 anodes
2
99
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