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:
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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.
Thermal Protection (Ir)
The thermal protection pickup Ir is set by using a setting dial.
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) |
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|---|---|---|---|---|---|---|---|---|---|---|---|
|
16 |
25 |
32 |
40 |
50 |
63 |
80 |
100 |
125 |
160 |
||
|
Pickup trip between 1.05 and 1.30 x Ir |
Ir (A) |
Adjustable in amps from 0.7 to 1 x In |
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|
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:
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Non-tripping time: 10 ms
-
Maximum breaking time: 200 ms for I > 1.5 Ii