MIL-HDBK-1004/10
Simplified expressions for the determination of the anode-to-
electrolyte resistance for a single vertical anode is given in para. 6.2.1.4.
This formula is valid for sacrificial and impressed current anodes. The basic
equations given in para. 6.2.1.3 are valid for sacrificial anode and impressed
current systems. In some cases, it is desirable to use groups of two or three
sacrificial anodes in order to provide the required current or anode life
using stock size anodes. In this case, the paralleling factors given in para.
6.2.1.4 can be used to calculate the equivalent resistance of the anodes in
parallel. In some cases where this method is used, an adjustable resistor or
nichrome wire resistor is installed in the anode-to-structure cable to limit
the current to the required value. In this case, the determination of the
anode-to-electrolyte resistance is used to calculate the value of the resistor
required.
Determination of Structure-to-Electrolyte Resistance. The
7.4.2.2
structure-to-electrolyte resistance is commonly disregarded in the design of
sacrificial anode cathodic protection systems since it is usually small with
respect to the anode-to-electrolyte resistance.
Connecting Cable Resistance. The connecting cable resistance is
7.4.2.3
determined by the size and length of cables used. The selection of appropriate
wire sizes is described in para. 6.6.1. No. 12 AWG solid copper wires are
commonly supplied on sacrificial anodes and No. 10 AWG wires are commonly used
as connecting cables. These wires have a resistance of 1.02 and 1.62 ohms per
1,000 feet, respectively. Since connecting cables are short and currents are
low in most sacrificial anode cathodic protection systems, connecting cable
resistance can usually be neglected.
Resistance of Connections and Splices. The need to maintain low
7.4.2.4
resistance throughout the life of the sacrificial anode cathodic protection
system is more important than the initial resistance of connections. Although
deterioration of connections in sacrificial anode cathodic protection systems
is protected, the connections are still subject to corrosion resulting in
increased resistance. As in the case of impressed current cathodic protection
systems, the number of connections should be kept to an absolute minimum, and
they should be very carefully assembled, insulated, inspected, and installed.
The number and location of each connection should be installed per the system
design and not at the discretion of the installer.
Total Circuit Resistance. The total circuit resistance (usually
7.4.2.5
only the anode-to-electrolyte resistance is a major factor) is then determined
by adding all of the resistances of the circuit elements.
Anode-to-Structure Potential. The potential difference between the
7.4.2.6
anode and the protected structure is then determined. In most cases, the open
circuit anode potential and a structure potential (for steel) of -850 mV
versus copper/copper sulfate is used. Other structure potential criteria can
be used as necessary. Use of an anode potential lower than the open circuit
potential may be required when anode outputs are high as in very low
resistivity environments.
Anode Output Current. The anode output current is then determined
7.4.2.7
from the circuit resistance and the structure-to-anode potential using Ohm's
Law.
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