National Instruments PS-17 User Manual page 28

Chapter 2 Installation and Configuration
Resistance 0.5 mm
180 mΩ 2.3 m
120 mΩ 3.3 m
150 mΩ 4.2 m
NI PS-17 Power Supply User Manual
Test 2: Short circuit with S1 on the load end (additional impedance is
included in the form of longer load wire length). The input voltage was
230 V and the load current was 0 A.
AC
Power
Supply
AC
The following circuit breaker tripped during the test:
A- or Z-Characteristic
B-Characteristic
C-Characteristic
Table 2-3 provides a comparison of resistances in terms of wire gauge and
length.
Table 2-3. Resistances for Wire Gauges and Lengths
2 2
0.7 mm 1.0 mm
3.2 m 4.6 m
4.7 m 6.7 m
5.9 m 8.4 m
For example:
Which wire gauge must be used to trip a C-Characteristic circuit breaker
with a rating of 8 A? The load wire length is 19 m.
Answer: A 8 A C-Characteristic circuit breaker requires a loop impedance
of less than 150 mΩ (test results). Table 2-3 shows that up to 20.9 m of wire
with a cross section of 2.5 mm
smaller than 2.5 mm
Circuit
Breaker
I
R
+
–
DC
Figure 2-10. Breaker Trip Test 2
≤20 A and R<82 mΩ
≤13 A and R<120 mΩ
≤8 A and R<150 mΩ
2 2
1.5 mm
6.9 m
10.0 m
12.5 m
2
is below 150 mΩ. You should not use a wire
2
.
2-14
+
S1 Load
–
2
2.5 mm 4.0 mm
11.4 m 18.3 m
16.7 m 26.7 m
20.9 m 33.4 m
2
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