Nace CP 1, Course Manual

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Field Measurements 5:20
CP 1 – Cathodic Protection Tester Course Manual
©
NACE International, 2000
02/01/05
4-wire Line Current Test
A 4-wire current test station can be used to determine the line current even
if the dimensions of the pipeline are unknown or there are anomalies within
the test span. Figure 5.10 shows the set-up.
Pipe Span for Measuring Current
Wires
must be
color
coded
0.17
mV
+
_
Pipeline
Power
Source
+
_
Current
Interrupter
+
_
AMPS
VOLTS
Figure 5.10 4-wire Line Current Test
The test procedure is as follows:
1.
Calibrate the span by passing a known amount of battery current
between the outside leads and measuring the change in voltage drop
across the span (
∆
E) using the inside leads. Divide the current flow in
amperes (I) by the change in voltage drop in millivolts to express the
calibration factor (K) in “amperes per millivolt.” The calibration factor
is calculated as follows:
K = I /
∆
E = I / E with current applied – E with no current applied
Normally, if the pipeline operating temperature is stable, this can be
done only once as the calibration factor may be recorded for subsequent
tests at the same location. On pipelines where the temperature of the
pipe changes considerably (with accompanying changes in resistance),
more frequent calibration may be necessary.
2.
Measure the voltage drop in millivolts across the measuring span
(without the battery current) using the inside potential measurement
leads. This voltage drop is due to normal pipeline current.