UFC 3 -520-01
June 10, 2002
9-7
CIRCUIT BREAKERS .
9-7.1
Paragraph 5-2 provides circuit breaker requirements, including requirements
for protection and coordination. L ow 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
10000
Long Time Pickup
5000
2000
1000
Long Time Delays
500
200
100
50
20
10
5
Short Time Pickup
2
Short Time Ramp
1
.5
Short Time Delays
.2
.1
.05
Instantaneous
.02
.01
1
2
5
10
20
50
100
200
500 1000
Current (Multiples of Circuit Breaker Rating)
9-16
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