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ETL 1110-9-10(FR)
5 Jan 91
RT = RN + RW + RC
Where:
RN = Groundbed resistance (ohms)
RW = Header cable/wire resistance (ohms)
a)
Groundbed resistance (RN) from step 5.
RN = 2.26 ohms
b)
15:
LW RMFT
RW '
1000 ft
Where:
LW =
150 ft (Effective cable length. The loop
circuit makes calculating effective wire
resistance complex.
Since current is
discharged from anodes spaced all along the
cable, one-half the total cable length may
be
used
to
approximate
the
cable
resistance. Total cable length = 300 ft.
Effective cable length = (300 ft) = 150
ft.)
RMFT
=
0.254 ohm (This is the resistance per
1000 lin ft of No. 4 AWG cable, which
has been
selected for ease of
handling.)
RW =
150 ft x 0.254 ohm
1000 ft
RW =
0.038 ohm; use 0.04 ohm
c)
Structure-to-earth resistance.
Since the tanks and piping are essentially bare and
are not electrically isolated, structure-to-earth
resistance may be considered negligible. Therefore
RC = 0.
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