Nace CP 1, Course Manual

Page: 528 / 535

TM0101-2001
NACE International 17
10.3.5.8 Install and place in operation necessary
interrupter equipment in the DC current sources
influencing the tank at the test site.
The “off”
interval should be kept as short as possible but still
long enough to read a polarized tank-to-electrolyte
potential after any “spike” (see Figure 2b) has
collapsed.
10.3.5.9 Measure and record the tank-to-
electrolyte “on” and instant-off potentials and their
polarities with respect to the reference electrode.
The difference between the instant-off potential
and the original potential is the amount of
polarization formation.
10.3.5.9.1 If spiking may be present, delay
measurement
of the tank-to-electrolyte
potential to eliminate the spike voltages from
the measured value. Spiking usually occurs
within 0.5 second of the interruption of the
cathodic
protection currents. Appropriate
instrumentation such as an oscilloscope or
high-speed recording device may be used to
verify the presence and duration of the
spiking.
10.3.5.10 Record a sufficient number of
measurements to determine the level of cathodic
protection over the entire structure.
10.3.6 Evaluation of Data
10.3.6.1 Cathodic protection shall be judged
adequate if 100 mV or more of polarization
formation is measured with respect to a standard
reference electrode.
10.3.7 Monitoring
10.3.7.1 When at least 100 mV or more of
polarization formation has been measured, the
tank “on” potential may be used for monitoring
unless
significant environmental, structural,
coating integrity, or cathodic protection system
parameters have changed.
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Section 11: Test Methods for Continuity Testing of Steel Tank Systems
11.1 The following test methods may be used to determine
whether a tank is electrically continuous with piping,
electrical equipment, conduit, and other appurtenances or
structures in the immediate area. This list of tests is not all-
inclusive. Other methods or equipment may be used to test
for electrical continuity or discontinuity.
11.2 Underground storage tank (UST) systems may have
been designed to be electrically isolated from other metallic
structures such as piping, conduit, grounded electrical
equipment, and hold-down devices.
A lack of electrical
isolation from these structures can result in lowered levels
of cathodic protection or a reduction in the life of the
cathodic protection system.
11.2.1 Some UST systems are designed to have
electrical continuity between the tank and the related
piping, electrical equipment, and other appurtenances
or accessories.
Discontinuity between the tank and
another structure or appurtenance can result in a lack
of protection, and in some cases, damage to the tank
structure and/or appurtenances that are isolated.
11.3 Fixed Cell/Moving Ground Technique
11.3.1 This test method uses basic cathodic protection
test equipment to test for an indication of possible
electrical continuity through the use of structure-to-
electrolyte potential comparison measurements.
11.3.2 The following procedure shall be followed when
testing for continuity using the fixed cell/moving ground
technique:
11.3.2.1 Make electrical contact between the
reference electrode and the electrolyte at a
location remote from the system to be tested.
11.3.2.1.1 The location should not be within
the potential gradient of an anode or any
other structure. Placement of the reference
electrode in a location that is shielded by
another tank or structure may result in
erroneous data concerning the continuity of
the shielded tank(s) or structures. Alternate
reference
electrode placements may be
necessary to determine the continuity of all of
the structures at a test site.
11.3.2.1.2 Once placed, the reference
electrode shall not be moved for the duration
of this test procedure.
11.3.2.2 Connect the voltmeter to the tank and
reference electrode as described in Paragraph
5.6.
11.3.2.3 Measure and record the tank-to-
electrolyte potential with respect to the reference
electrode.
11.3.2.4 Disconnect the test lead from the tank
and continue to test other structures by connecting
that lead to the structure in question.