Ground-Fault Protection on MicroLogic 6.0 P Trip Unit
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A ground fault in the protection conductors can provoke local temperature rise at the site of the fault or in the conductors.
The purpose of the ground-fault protection function is to eliminate this type of fault.
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There are two types of ground-fault protection, depending on the type of installation.
Type
Description
Residual
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The function determines the zero-phase sequence current, i.e. the vector sum of the phase and neutral currents.
It detects ground faults downstream of the circuit breaker.
Source Ground Return
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Using a special external sensor, this function directly measures the fault current returning to the transformer via the grounding cable.
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It detects ground faults both upstream and downstream of the circuit breaker
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The maximum distance between the sensor and the circuit breaker is 10 m (33 ft).
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Ground-fault and neutral protection are independent and can therefore be combined.
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Zone selective interlocking (ZSI).
The short-time and ground-fault protection functions enable time selectivity by delaying the upstream devices to provide the downstream devices the time required to clear the fault. Zone selective interlocking can be used to obtain total selectivity between circuit breakers using external wiring.
For the characteristics and external wiring of the zone selective interlocking function, refer to Zone Selective Interlocking in the technical appendix.
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For the factory settings, the setting ranges, increment steps and setting accuracies, refer to Threshold and Time-Delay Settings.
Ground-Fault Pick-Up Ig
The pickup values can be set independently and are identical for both the residual and source ground return ground-fault protection functions.
|
Ig pickup (accuracy ± 10%) |
In ≤ 400 A |
Ig = In x... |
A | B | C | D | E | F | G | H | I |
|
0.3 |
0.3 |
0.4 |
0.5 |
0.6 |
0.7 |
0.8 |
0.9 |
1 |
|||
|
400 A < In ≤ 1200 A |
Ig = In x... |
0.2 |
0.3 |
0.4 |
0.5 |
0.6 |
0.7 |
0.8 |
0.9 |
1 |
|
|
In > 1200 A |
Ig = ... |
500 A |
640 A |
720 A |
800 A |
880 A |
960 A |
1040 A |
1120 A |
1200 A |
Time Delay tg
The time delay tg can be set independently and is identical for both the residual and source ground return ground-fault protection functions.
|
tg time delay (s) |
I²t OFF |
0 |
0.1 |
0.2 |
0.3 |
0.4 |
|
I²t ON |
0.1 |
0.2 |
0.3 |
0.4 |
— |
|
|
Tripping time (ms) at In or 1200 A with I²t ON or I²t OFF |
Maximum resettable time |
20 |
80 |
140 |
230 |
350 |
|
Maximum break time |
80 |
140 |
200 |
320 |
500 |