UFC 3-520-01
June 10, 2002
9-16
9-7
CIRCUIT BREAKERS.
9-7.1
Paragraph 5-2 provides circuit breaker requirements, including requirements
for protection and coordination. Low voltage power breakers, insulated case breakers,
and MCCBs contain integral current sensing and trip units. Smaller breakers usually
have electromechanical trip devices and larger breakers usually have solid-state trip
units. Solid-state trip units offer greater flexibility in coordination and can be set more
precisely to obtain the desired level of protection. Figure 9-3 shows the range of
available time-current characteristics provided by a typical solid-state trip unit. Some or
all of the possible solid-state trip unit settings should be used to establish the required
level of coordination. Solid-state trip units are usually necessary to assure coordination
in the instantaneous trip region.
Figure 9-3. Simplified Solid-State Trip Unit
.01
.1
1
10
100
1000
10000
Current (Multiples of Circuit Breaker Rating)
1
10
100
1000
.02
.05
.2
.5
2
5
50
20
200
500
2000
5000
2
5
20
50
200
500
Long Time Delays
Long Time Pickup
Time (seconds)
Instantaneous
Short Time Delays
Short Time Ramp
Short Time Pickup
Summary :
Simplified Solid-State Trip Unit .01 .1 1 10 100 1000 10000 Current (Multiples of Circuit Breaker Rating) 1 10 100 1000 .02 .05 .2 .5 2 5 50 20 200 500 2000 5000 2 5 20 50 200 500 Long Time Delays Long Time Pickup Time (seconds) Instantaneous Short Time Delays Short Time Ramp Short Time Pickup
Tags :
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