Long-Time Overcurrent Protection
Presentation
Long-time overcurrent protection helps to protect cables, busbars, and busbar trunking against overloads, based on the true RMS current. It is implemented independently for each phase and for the neutral.
This protection function is an overcurrent time-dependent protection with thermal memory. It operates as a thermal image, using the heating and cooling model of a conductor. After tripping, the protection continues to integrate the cooling of the conductor.
This protection function can be used also for transformer or generator protection thanks to the wide range of settings offered.
Availability
Long-time overcurrent protection is available on:
-
MicroLogic 2.0, 5.0, and 6.0 trip units
-
3-pole and 4-pole circuit breakers
Long-time overcurrent protection is powered by the current flowing through the internal current transformers of the circuit breaker and it does not require additional external power supply.
Operating Principle
-
Long-time overcurrent protection is based on the true RMS current of phases and neutral.
- Long-time overcurrent protection is implemented independently for each phase and for neutral when present.
Setting the Protection
The long-time overcurrent protection settings are:
-
Ir: long-time overcurrent protection pickup
-
tr: long-time overcurrent protection time delay
They can be set by using the Ir and tr multi-position dials on the front face of the MicroLogic trip unit.
Setting the Ir Pickup
The Ir pickup setting values depend on the long-time rating plug inserted in the MicroLogic trip unit. For more information on the long-time rating plug, see detailed topic.
Ir pickup = setting value x In rated current.
As standard, trip units are equipped with the standard rating plug (0.4–1 x In).
|
Rating plug |
Setting values |
||||||||
|---|---|---|---|---|---|---|---|---|---|
|
Standard |
0.4 |
0.5 |
0.6 |
0.7 |
0.8 |
0.9 |
0.95 |
0.98 |
1 |
|
Low-setting option |
0.4 |
0.45 |
0.50 |
0.55 |
0.60 |
0.65 |
0.70 |
0.75 |
0.8 |
|
High-setting option |
0.80 |
0.82 |
0.85 |
0.88 |
0.90 |
0.92 |
0.95 |
0.98 |
1 |
|
Off-plug |
No long-time overcurrent protection (Ir = In for Isd setting) | ||||||||
When the current is higher than Isd or Ii, only short-time overcurrent protection and instantaneous protection are operational.
Setting the tr Time Delay
The time delay settings indicated on the rating plugs correspond to the tripping times for an overload of 6 x Ir in cold-state conditions.
The table below gives tripping times according to tr time delay.
|
tr setting |
Accuracy |
0.5 s |
1 s |
2 s |
4 s |
8 s |
12 s |
16 s |
20 s |
24 s |
|---|---|---|---|---|---|---|---|---|---|---|
|
Resulting tripping time at 1.5 x Ir |
0 to -30% |
12.5 s |
25 s |
50 s |
100 s |
200 s |
300 s |
400 s |
500 s |
600 s |
|
Resulting tripping time at 6 x Ir |
0 to -20% |
0.7 s1 |
1 s |
2 s |
4 s |
8 s |
12 s |
16 s |
20 s |
24 s |
|
Resulting tripping time at 7.2 x Ir |
0 to -20% |
0.7 s2 |
0.69 s |
1.38 s |
2.7 s |
5.5 s |
8.3 s |
11 s |
13.8 s |
16.6 s |
|
1: Accuracy 0 to -40% 2: Accuracy 0 to -60% |
||||||||||