Datasheet

QTD20.241
Q-Series
24V, 20A, DC INPUT
Mar 2012 / Rev. 1.2 DS-QTD20.241-EN
All parameters are specified at 24V, 20A, 600Vdc input, 25°C ambient and after a 5 minutes run-in time unless otherwise noted.
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26.4. OUTPUT CIRCUIT BREAKERS
Standard miniature circuit breakers (MCBs or UL1077 circuit breakers) can be used for branch protection. Ensure that
the MCB is rated for DC voltage, too. The following tests show which circuit breakers the power supply typically trips.
Circuit breakers have huge tolerances in their tripping behavior. Therefore, these typical tests can only be used as a
recommendation or for comparing two different power supplies. Furthermore, the loop impedance has a major
influence on whether a breaker trips or not. Two tests were performed, representing typical situations:
Test 1: Short circuit with S1 on the power supply end of the cable (loop impedance approx. 20mOhm)
Fig. 26-3 Branch protectors, test circuit 1
Circuit
Breaker
Power
Supply
Input
DC
+
-
I
Load
+
-
S1
Parameters:
Input voltage: 600Vdc, load current: 0A
Tripping time shorter than 5s.
The following circuit breaker tripped during the test:
A- or Z- Characteristic:: equal or smaller 10A *)
B- Characteristic: equal or smaller 10A *)
C- Characteristic: equal or smaller 10A *)
Test 2: Short circuit with S1 on the load end (additional impedance included; represents longer load wire length).
Fig. 26-4 Branch protectors, test circuit 2
R
Circuit
Breaker
Power
Supply
Input
DC
+
-
I
S1 Load
+
-
Parameters:
Input voltage: 600Vdc, load current: 0A
Tripping time shorter than 5s.
The following circuit breaker tripped during the test:
A- or Z- Characteristic:: ≤ 10A and R< 120mOhm *)
B- Characteristic: ≤ 10A and R< 120mOhm *)
C- Characteristic: ≤ 8A and R<= 150mOhm *)
What does this resistance mean in wire length?
0.5mm
2
0.7mm
2
1.0mm
2
1.5mm
2
2.5mm
2
4.0mm
2
120mOhm
3.3m 4.7m 6.7m 10.0m 16.7m 26.7m
150mOhm
4.2m 5.9m 8.4m 12.5m 20.9m 33.4m
*) A list of the circuit breakers under test is available on request.
Example:
Which wire gauge must be used at a length of 15m before a B-Characteristic circuit breaker with 10A will trip?
Answer: A 10A B-Characteristic circuit breaker requires a loop impedance of less than 120mOhm based on the test
results. The wire length table shows that a length of 16.7m wire with a cross section of 2.5mm
2
is measured at
120mOhm. A wire not smaller than 2.5mm
2
shall be used.
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