Nace CP 1, Course Manual

Page: 487 / 535

TM0497-2002
NACE International 7
5.11.2.3 Impressed current devices on foreign
piping; and
5.11.2.4 Numerous cross bonds to parallel
pipelines.
5.11.3 Natural and Manmade Stray Currents
5.11.3.1 Telluric currents; and
5.11.3.2 Manmade DC stray currents, such as
those from mass transit and mining operations.
5.12 When voltage drops have been evaluated at a test
location and the pipe-to-electrolyte potential found to be
satisfactory, the “on” pipe-to-electrolyte potential value may
be used for monitoring until significant environmental,
structural,
or cathodic protection system parameters
change.
5.12.1 Significant environmental, structural, or
cathodic protection system parameter changes may
include:
(a) Replacement or addition of piping;
(b) Addition, relocation, or deterioration of cathodic
protection systems;
(c) Failure of electrical isolating devices;
(d) Effectiveness of coatings; and
(e) Influence of foreign structures
.
5.13 After a cathodic protection system is operating, time
may be required for the pipe to polarize. This should be
considered when measuring the potential at a test site on a
newly protected pipe or after reenergizing a cathodic
protection device.
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Section 6: Causes of Measurement Errors
6.1 Factors that contribute to faulty potential
measurements include:
6.1.1 Pipe and instrument test leads
(a) Broken or frayed wire strands (may not be visible
inside the insulation);
(b)
Damaged or defective test lead insulation that
allows the conductor to contact wet vegetation, the
electrolyte, or other objects;
(c)
Loose, broken, or faulty pipe or instrument
connections; and
(d)
Dirty or corroded connection points.
6.1.2 Reference electrode condition and placement
(a) Contaminated reference electrode solution or
rod, and solutions of insufficient quantity or saturation
(only laboratory-grade chemicals and distilled water, if
water is required, should be used in a reference
electrode);
(b)
Reference electrode plug not sufficiently porous
to provide a conductive contact to the electrolyte;
(c)
Porous plug contaminated by asphalt, oil, or
other foreign materials;
(d)
High-resistance contact between reference
electrode and dry or frozen soil, rock, gravel,
vegetation, or paving material;
(e)
Reference electrode placed in the potential
gradient of an anode;
(f)
Reference electrode positioned in the potential
gradient of a metallic structure other than the one with
the potential being measured;
(g)
Electrolyte between pipe and disbonded coating
causing error due to electrode placement in electrolyte
on opposite side of coating;
(h) Defective permanently installed reference
electrode;
(i)
Temperature correction not applied when
needed; and
(j)
Photo-sensitive measurement error (in CSE with
a clear-view window) due to light striking the electrode
electrolyte solution (photovoltaic effect).
6.1.3 Unknown isolating devices, such as unbonded
tubing or pipe compression fittings, causing the pipe to
be
electrically discontinuous between the test
connection and the reference electrode location.
6.1.4 Parallel path inadvertently established by test
personnel contacting instrument terminals or metallic
parts of the test lead circuit, such as test lead clips and
reference electrodes, while a potential measurement is
being made.
6.1.5 Defective or inappropriate instrument, incorrect
voltage range selection, instrument not calibrated or
zeroed, or a damp instrument sitting on wet earth.
6.1.6 Instrument having an analog-to-digital converter
operating at such a fast speed that the voltage spikes
produced by current interruption are indicated instead
of the actual “on” and “off” values.
6.1.7 Polarity of the measured value incorrectly
observed.
6.1.8 Cathodic protection current-carrying conductor
used as a test lead for a pipe potential measurement.