Nace CP 1, Course Manual

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RP0200-2000
10 NACE International
Analysis
−
Resistance Calculation using Ohm’s Law
The pipe-to-casing potential difference (
∆
V) is calculated
by subtracting the result of Step 4 from the result of Step
1. Because Ohm’s Law states that the direct current
flowing in an electric circuit is directly proportional to the
voltage applied to the circuit,
∆
V shall be divided by the
current (I). A casing-to-pipe (metal-to-metal) contact may
exist if the resultant value is less than 0.01 ohm.
Example 1
A potential survey indicates that the difference between the casing potential and the pipeline potential is less than 100 mV.
Using the above procedure to perform an internal resistance test, a potential of 0.090 V is measured on terminals T1 and T2
prior to the application of current. After a current of 1.70 A is applied on terminals T3 and T4, a pipe-to-casing potential of
0.106 V is measured.
Pipe-to-Casing Potential Before Current is Applied: 0.090 V
Pipe-to-Casing Potential with the Current Applied: -0.106 V
Change in Potential (
∆
V):
-0.016 V
By dividing
∆
V by current (I), as in Equation (B1), the value of the resistance (R) is obtained:
0.016 V
÷
1.7 A = 0.0094
Ω
(B1)
The resistance is 0.0094
Ω
. Because the pipe-to-casing resistance is less than 0.01
Ω
, the presence of a short is indicated.
Note: Because the pipe to casing resistance is so low considering the circuit resistance, the presence of an electrical short is
indicated.
Example 2
A pipe-to-casing potential of 0.100 V is measured between terminals T1 and T2. After 1.70 A of current is applied between
terminals T3 and T4, a pipe-to-casing potential of 0.302 V is measured between terminals T1 and T2.
Pipe-to-Casing Potential Before Current is Applied: 0.100 V
Pipe-to-Casing Potential After Current is Applied: -0.302 V
Change in Potential (
∆
V):
-0.202 V
By dividing
∆
V by current (I), as in Equation (B2), the value of the resistance (R) is obtained:
0.202 V
÷
1.7 A = 0.12
Ω
(B2)
The resistance is 0.12
Ω
. Because the pipe-to-casing resistance is greater than 0.01
Ω
, pipe-to-casing (metal-to-metal)
contact is not indicated.
Note: Because the pipe to casing resistance is still quite low considering the circuit resistance, the presence of an electrical
short is not indicated; however, additional testing may be necessary to confirm the casing condition.
3. Four-Wire IR Drop Test for Cased Crossings
Purpose
This method may indicate the presence and location of a
metallic short to the casing.
Procedure
1.
Measuring the Linear Resistance of the Casing
(a) The potential difference shall be measured
between terminals T3 and T4 while a measured
battery current is simultaneously passed between
terminals T1 and T2 (see Figure B3). This can be
done with suitable test points (vents or test leads), or
the use of probe bars.
(b) The battery current in amperes is divided by
the change in potential difference from T3 to T4 (
∆
V)
in mV to express the calibration factor in A/mV, as
shown in Equation (B3).