Nace CP 1, Course Manual

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Field Measurements 5:17
CP 1 – Cathodic Protection Tester Course Manual
©
NACE International, 2000
02/01/05
2-wire Line Current Test
Where 2-wire test points span a known length of a structure such as a
pipeline and the diameter and wall thickness or the weight per foot are
known, current flow across the span can be calculated. This is done by
measuring the voltage drop across the span, determining the resistance of
the span from a pipe table, and using Ohm’s Law as you would with a
shunt. Figure 5.9 shows the test setup. Table 5.3 provides some resistance
values for common pipe sizes.
Pipe Span in Feet
Wires must be
color coded
0.17
mV
+
_
Pipe size and
wall thickness or
weight per foot
must be known
Pipeline
N
West East
Pipe Span in Feet
Wires must be
color coded
0.17
mV
+
_
Pipe size and
wall thickness or
weight per foot
must be known
Pipeline
N
Pipe Span in Feet
Wires must be
color coded
0.17
mV
+
_
Pipe size and
wall thickness or
weight per foot
must be known
Pipeline
Pipe Span in Feet
Wires must be
color coded
0.17
mV
+
_
Pipe size and
wall thickness or
weight per foot
must be known
Pipeline
N
West East
Figure 5.9 2-wire Line Current Test
Because the voltage drop across a pipe span is relatively small, with earlier
instruments it was necessary to correct for the voltage drop in the test leads
caused by the current drawn by the meter. However, with the high-input
resistance meters available today, this correction is not necessary.
Using a high-input impedance meter, the procedure involves simply
measuring the voltage drop between the test leads. For example, if the
voltage drop across a 200-ft span of 30-in. (76.2 cm) pipe weighing 118.7
lbs./ft (176.65 kg/m) is 0.17 mV, then current flow is calculated as follows: