Nace CP 1, Course Manual

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Field Measurements 5:11
CP 1 – Cathodic Protection Tester Course Manual
©
NACE International, 2000
02/01/05
A current shunt is installed in series with the circuit the same as the
ammeter. Current is obtained by measuring voltage across the current shunt
and calculating current using Ohm’s Law. Figure 5.8 illustrates a shunt
measurement.
V
measured
R
shunt
I
calculated
=
+
_
+
_
VOLTS
R
A
R
B
R
C
E
I
Current Shunt with
known resistance value
is in series with the
circuit
Voltmeter is
connected in
parallel across
the current
shunt
V
measured
R
shunt
I
calculated
=
V
measured
R
shunt
I
calculated
=
+
_
+
_
VOLTS
R
A
R
B
R
C
E
I
Current Shunt with
known resistance value
is in series with the
circuit
Voltmeter is
connected in
parallel across
the current
shunt
+
_
+
_
VOLTS
R
A
R
B
R
C
E
I
Current Shunt with
known resistance value
is in series with the
circuit
Voltmeter is
connected in
parallel across
the current
shunt
+
_
+
_
VOLTS
R
A
R
B
R
C
E
I
Current Shunt with
known resistance value
is in series with the
circuit
Voltmeter is
connected in
parallel across
the current
shunt
Figure 5.8 Shunt Measurement
Current Calculation
The resistance value of a current shunt may be given in ohms or
amps/millivolts. If the resistance unit is ohms, use Ohm’s Law, but do not
forget to perform any conversion that may be necessary to maintain the
units of the law (amps and volts or milliamps and millivolts).
For example:
Given:
Shunt – 0.01 Ω
Voltage across shunt = 50 mV
Calculate Current:
1. Convert units of voltage: 50 mV = .05 V
2. Calculate current using Ohm’s Law I = .05 V / .01 Ω = 5 A