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ETL 1110-9-10(FR)
5 Jan 91
Therefore, one half the cable length is used in this
calculation. The calculation is also conservative since not
all the current flows through the complete intertank bond
circuit.
Total ft of negative circuit wire = 295 ft of No. 4 AWG cable
Negative resistance if calculated from equation
1-15:
LW RMFT
RNEG '
1000 ft
Where:
LW =
295 ft (Negative cable length)
RMFT =
0.254 ohm (Negative cable resistance
per 1000 un ft of No. 4 AWG cable
[table A-6])
RNEG =
295 x 0.254
1000
RNEG =
0.074
Total wire resistance therefore is:
RW
=
RT/B + RPOS + RNEG = 0.030 + 0.012 + 0.074
RW
=
0.116 ohm
Cable
insulation
is
also
important.
High
molecular
weight
polyethylene
insulation,
commonly used for cathodic protection work, tends to
blister, become brittle, and then crack in deep
groundbed use where chlorine gas generation can occur.
This has been most prevalent in open holes containing
brackish water, but may occur in coke breeze
backfilled holes also. Consequently, to minimize the
chances of cable failure, one of the two following
types of insulation, which show good resistance to
these oxidizing environments should be used:
Polyvinylidene fluoride (Kynar)1
____________________
1
Registered trademark of Penwalt Corp.
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