MIL-HDBK-1004/10
current anode beds in appropriate locations. If the anodes are not connected
through resistors, resistors may be added. Protective output can be increased
or decreased locally by installing additional anodes or disconnecting anodes
as necessary.
Rectifier Voltage and Current Capacity. If the voltage capacity of
11.4.2
the rectifier is inadequate to provide sufficient current to achieve
protection, reduction of system resistance by the installation of additional
anodes may be appropriate. If the current capacity of the rectifier proves
inadequate, and correction of any interference or shorts does not result in
adequate protection over a period of 1 year, then additional current must be
supplied either by installing an additional rectifier in parallel with the
existing rectifier, or installing a larger capacity rectifier in place of the
existing rectifier.
Adjustment of Sacrificial Anode Systems. Overprotection in
11.5
sacrificial anode cathodic protection systems is rare as the open circuit
(maximum) potential of most anode materials is normally less than that
potential considered to be excessive. Potentials more negative than those
required for protection are, however, wasteful and will result in premature
anode consumption. Potentials in sacrificial anode cathodic protection
systems can be easily adjusted by inserting an adjustable resistor in the
anode lead and adjusting it to give the current required for adequate
protection.
Low Anode Current Levels. Low anode current output resulting in
11.5.1
inadequate protection is common in sacrificial anode cathodic protection
systems. Low anode current output can be the result of high soil resistivity
or high circuit resistance. If high soil resistivity is suspected, wet down
the soil over the anodes and remeasure the current output after a few days.
If the current increases to give adequate protection, then high soil
resistivity problems have been confirmed. If lowering the soil resistivity in
the vicinity of the anodes does not result in increased anode output, then
high circuit resistance may be the problem and all leads and connections
should be checked. If all leads and connections are all right and the output
is still too low, then more anodes are required to provide additional current.
In the case of an impressed system, the rectifier voltage may be increased.
Inadequate Protection at Designed Current Levels. If a situation
11.5.2
is encountered where anode current output is within design limits and adequate
protection is not obtained, the current required for protection may be more
than originally anticipated. This may be due to interference, unusually
corrosive soil conditions, a poorly performing coating on the structure, or a
shorted dielectric fitting. If any interference has been corrected and
inadequate protection is still encountered, either more anodes will have to be
installed, or the rectifier current output will have to be increased, or both.
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