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 |