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ETL 1110-9-10(FR)
5 Jan 91
RT = RA + RW + RC (Because the cathodic protection
system uses a single deep anode groundbed RA = RN)
Where:
RA =
Backfill
column-to-earth
resistance
(ohms).
RW
=
Wire resistance (ohms).
RC
=
Structure-to-earth resistance (ohms).
a)
Backfill
column-to-earth
resistance
(RA)
from
step 5.
RA = 0.65 ohm
b)
LW RMFT
RW '
1000 ft
A deep anode groundbed is defined as one where the top
of the backfill column is at least 50 ft below the
surface of the earth.
Actual depth will vary,
depending on subsurface geology and the distance over
which the current is expected to spread.
In this
example, the depth to the top of the coke breeze
backfill column was determined to be 100 ft. The
anodes can be supplied through a single or dual feed.
Dual feed is preferred to reduce both the resistance
of the circuit and the chance of a failure due to a
cable break.
The assembly is made on a single length of cable,
beginning at the rectifier, running down through the
tubular anodes to the bottom anode and then back up
adjacent to the anodes to the rectifier. Resistance
calculations are made as though there were three
cables as noted below. The conductor provided with
some tubular ceramic anodes is designated EPR/HY5O for
one-in. diameter anodes and EPR/HY16 for 0.63-in.
diameter anodes. These cables have an ethylene-
propylene
rubber
inner
insulation
and
a
chlorosulphonated polyethylene outer jacket and are
suitable for deep anode use if the insulation is
protected with a chlorine-resistant sheath or shield.
63
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