Circuit Breakers: Thermal Magnetic (TM-D) Protection
Introduction
Circuit breakers equipped with thermal-magnetic trip units are used for protection of cables on distribution systems supplied by transformers.
Thermal-magnetic trip units provide:
-
Thermal protection against overload with an adjustable pickup Ir, and a non-adjustable time-delay.
-
Magnetic protection against short circuits, with a fixed pickup Ii.
The following figure shows the tripping curve.
Ir Thermal protection pickup
Ii Instantaneous pickup
Thermal Protection (Ir)
The thermal protection pickup Ir is set by using an adjustment dial.
Turning the thermal protection adjustment dial (A) modifies the tripping curve (B), as shown.
The values of Ir are adjustable from 0.7 to 1 x In (circuit breaker rating) as shown in the following table:
|
Thermal protection |
Circuit breaker rating In (A) |
||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|
|
16 |
25 |
32 |
40 |
50 |
63 |
80 |
100 |
125 |
160 |
||
|
Pickup trip between 1.05 and 1.20 x Ir |
Ir (A) |
Adjustable in amps from 0.7 to 1 x In |
|||||||||
|
Time delay (s) |
tr (s) |
Non-adjustable |
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Magnetic Protection (Ii)
The instantaneous pickup value Ii cannot be adjusted. Its value for each circuit breaker rating is shown in the following table:
|
Magnetic protection |
Circuit breaker rating In (A) |
||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|
|
16 |
25 |
32 |
40 |
50 |
63 |
80 |
100 |
125 |
160 |
||
|
Pickup ±20% |
Ii (A) |
500 |
600 |
600 |
600 |
600 |
800 |
1000 |
1250 |
1250 |
1250 |
The time delay for magnetic protection cannot be adjusted:
-
Non-tripping time: 10 ms
-
Maximum breaking time: 200 ms for I > 1.5 Ii