b) In areas where the soil is extremely wet or loose, such as in a
swampy area, it may not be possible to properly install or tamp the backfill
material. Packaged anodes with the backfill contained in metal cylinders
(cans) surrounding the anodes may be useful in these circumstances but
increase the cost. Anodes prepackaged with backfill, usually contained in
metal cans which are rapidly corroded away during operation, are easier to
install than separate installation of anode and backfill. The prepackaged
anodes are higher in cost and have the following additional disadvantages:
(1) High unit weight reduces ease of handling.
(2) Possibility of voids developing in backfill during
transportation and handling.
(3) The critical anode cable and connection between the anode and
cable are hidden and difficult to inspect.
The choice of packaged versus unpackaged impressed current anodes must be made
based upon economics and local site conditions. Packaged anodes are usually
used only where unstable soil conditions exist, where the hole excavated for
installation caves in, and where prepackaged anodes are stocked for augmenting
Structure-to-Electrolyte Resistance. The structure-to-electrolyte
resistance is commonly disregarded in the design of impressed current cathodic
protection systems since it is usually very small with respect to the anode-
to-electrolyte resistance. When total circuit resistance is measured (refer
to para. 18.104.22.168), the structure-to-electrolyte resistance is included in the
Connecting Cable Resistance. The connecting cable resistance is
determined by the size and length of cables used. The selection of appropriate
wire sizes is described in para. 6.6.1.
Resistance of Connections and Splices. In addition to the fact
that connections and splices are sources of resistance in impressed current
cathodic protection systems, they are a site of failure. These connections
should be kept to an absolute minimum, and they should be very carefully
assembled, insulated, inspected, and installed. The cable from the positive
lead of the power source to the anodes carries a high positive charge and will
deteriorate rapidly at any point where the insulation is breached and the
conductor contacts the electrolyte. The number and location of each
connection should be installed per the system design and not at the discretion
of the installer.
Determination of Power Supply Requirements. The power supply
requirements, namely current and voltage, are determined by Ohm's Law from the
required current for protection of the structure and the calculated or
measured total circuit resistance. The actual power supply requirement should
allow for future loads and rectifier aging. Generally, a factor of 1.5 over
calculated output is used.