Thermal-Magnetic Protection for Circuit Breakers
Thermal-magnetic protection provides the following features for general-purpose applications:
-
Thermal protection against overload, with fixed threshold In.
-
Instantaneous protection against short circuits, with fixed threshold Ii.
The following figure shows the trip curve.
Thermal Protection
The thermal protection pick-up value cannot be adjusted. Its value for each frame rating is shown below.
Frame rating In (A) |
|||||||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
15 |
20 |
25 |
30 |
35 |
40 |
45 |
50 |
60 |
70 |
80 |
90 |
100 |
110 |
125 |
|
In pick-up (A) |
15 |
20 |
25 |
30 |
35 |
40 |
45 |
50 |
60 |
70 |
80 |
90 |
100 |
110 |
125 |
AC Magnetic Trip Levels
Ii |
Rated Current (A) |
||||||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
15 |
20 |
25 |
30 |
35 |
40 |
45 |
50 |
60 |
70 |
80 |
90 |
100 |
110 |
125 |
|
Hold (A) |
400 |
400 |
400 |
400 |
400 |
400 |
400 |
480 |
640 |
640 |
800 |
1000 |
1000 |
1000 |
1000 |
Trip (A) |
600 |
600 |
600 |
600 |
600 |
600 |
600 |
720 |
960 |
960 |
1200 |
1500 |
1500 |
1500 |
1500 |
DC Magnetic Trip Levels
The Time Current Curves (trip curves) provide the complete time-current characteristics of the circuit breaker when applied on an AC system. When applying thermal-magnetic circuit breakers on DC systems, they retain the same thermal tripping characteristics, but the magnetic trip levels vary. See table for the appropriate DC magnetic hold and trip levels.
Ii |
Rated Current (A) |
||||||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
15 |
20 |
25 |
30 |
35 |
40 |
45 |
50 |
60 |
70 |
80 |
90 |
100 |
110 |
125 |
|
Hold (A) |
460 |
460 |
510 |
510 |
510 |
510 |
510 |
510 |
790 |
900 |
900 |
1200 |
1200 |
1200 |
1200 |
Trip (A) |
680 |
680 |
800 |
800 |
800 |
800 |
800 |
800 |
1240 |
1420 |
1420 |
1850 |
1850 |
1850 |
1850 |