Nace CP 1, Course Manual

Page: 483 / 535

TM0497-2002
NACE International 3
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Section 3: Safety Considerations
3.1 Appropriate safety precautions, including the following,
shall be observed when making electrical measurements.
3.1.1 Be knowledgeable and qualified in electrical
safety
precautions before installing, adjusting,
repairing, removing, or testing impressed current
cathodic protection equipment.
3.1.2 Use properly insulated test lead clips and
terminals to avoid contact with unanticipated high
voltage (HV). Attach test clips one at a time using a
single-hand technique for each connection.
3.1.3 Use caution when long test leads are extended
near overhead high-voltage alternating current (HVAC)
power lines, which can induce hazardous voltages onto
the test leads.
High-voltage direct current (HVDC)
power lines do not induce voltages under normal
operation,
but transient conditions may cause
hazardous voltages.
3.1.3.1 Refer to NACE Standard RP01772 for
additional information about electrical safety.
3.1.4 Use caution when making tests at electrical
isolation devices. Before proceeding with further tests,
use appropriate voltage detection instruments or
voltmeters with insulated test leads to determine
whether hazardous voltages may exist.
3.1.5 Avoid testing when thunderstorms are in the
area. Remote lightning strikes can create hazardous
voltage surges that travel along the pipe under test.
3.1.6 Use caution when stringing test leads across
streets, roads, and other locations subject to vehicular
and pedestrian traffic. When conditions warrant, use
appropriate barricades, flagging, and/or flag persons.
3.1.7 Before entering, inspect excavations and
confined spaces to determine that they are safe.
Inspections may include shoring requirements for
excavations and testing for hazardous atmospheres in
confined spaces.
3.1.8 Observe appropriate electrical codes and
applicable safety regulations.
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Section 4: Instrumentation and Measurement Guidelines
4.1 Cathodic protection electrical measurements require
proper selection and use of instruments. Pipe-to-electrolyte
potential, voltage drop, potential difference, and similar
measurements require instruments that have appropriate
voltage ranges. The user should know the capabilities and
limitations of the equipment, follow the manufacturer’s
instruction manual, and be skilled in the use of electrical
instruments. Failure to select and use instruments correctly
causes errors in cathodic protection measurements.
4.1.1 Analog instruments are usually specified in
terms of input resistance or internal resistance. This is
usually expressed as ohms per volt of full meter scale
deflection.
4.1.2 Digital instruments are usually specified in terms
of input impedance expressed as megaohms.
4.2 Factors that may influence instrument selection for field
testing include:
(a) Input impedance (digital instruments);
(b) Input
resistance or internal resistance (analog
instruments);
(c) Sensitivity;
(d) Conversion speed of analog-to-digital converters used
in digital or data logging instruments;
(e) Accuracy;
(f) Instrument resolution;
(g) Ruggedness;
(h) Alternating current (AC) and radio frequency (RF)
signal rejection; and
(i)
Temperature and/or climate limitations.
4.2.1 Some instruments are capable of measuring and
processing voltage readings many times per second.
Evaluation of the input wave-form processing may be
required if an instrument does not give consistent
results.
4.2.2 Measurement of pipe-to-electrolyte potentials on
pipelines affected by dynamic stray currents may
require the use of recording or analog instruments to
improve
measurement accuracy. Dynamic stray
currents include those from electric railway systems,
HVDC transmission systems, mining equipment, and
telluric currents.
4.3 Instrument Effects on Voltage Measurements
4.3.1 To measure pipe-to-electrolyte potentials
accurately, a digital voltmeter must have a high input
impedance (high internal resistance, for an analog
instrument) compared with the total resistance of the
measurement circuit.