Nace CP 1, Course Manual

Page: 470 / 535

RP0200-2000
NACE International 13
3. Establishing the Circuit (Downstream [D/S] End)
(a) The circuit shall be established by connecting
the battery negative to T2 (pipe lead).
(b) T1 (downstream vent) shall be connected to
the positive side of the battery as shown in Figure
B5.
(c) T3 and T4 shall remain the same for
measurement of potential difference as shown in
Figure B3.
(d) The percentage of distance “a” from T3 shall
be calculated as in Equation (B7).
Distance (in percent) from T3 = 100%
Current
Factor n Calibratio x (T3) V
x
∆
(B7)
V OLT
METER
CAR R IER PIPE
CASING
GRO U ND LEVE L
T1 T3 T4
-
-
BA TTERY
+
+
A M-
METER
T2
PIPE L EA D
a
D/S End
d
U/S End
VA RIA BLE RES IS TO R
b
V ENT PIPE
V ENT
PIPE
FIGURE B5: Four-Wire IR Drop Test (Establishing the Circuit) (D/S End)
CAUTION:
•
Proper placement of the insulated probe bar and test leads is required to obtain contact for measurement of the IR drop
on T3 and T4, or erroneous readings may result.
•
If more than one metal-to-metal contact exists, this test may not accurately identify the location of the shorts.
•
All safety precautions shall be implemented when lead acid batteries are used.
Example 1
A pipe casing is 76.2 cm (30.0 in.) in diameter, 12.2 m
(40.0 ft) long, and has a wall thickness of 7.92-mm
(0.312-in.).
Casing Data: Length a = 12.2 m (40.0 ft)
Length b = 15 m (49 ft)
Diameter = 76.2 cm (30.0 in.)
Wall Thickness = 7.92 mm (0.312 in.)
Step 1: Find the calibration factor.
A potential difference of 0.465 mV is measured
between T3 and T4 before the application of current
at T1 and T2. A potential difference of 1.50 mV is
measured after the application of current (9.00 A).
Potential Difference Before Current is Applied: 0.465 mV
Potential Difference After Current is Applied: -1.500 mV
Change in Potential (
∆
V):
-1.035 mV