Protection in Low Voltage Systems:
Voltage between 50V and 1000V AC is considered low voltage. The short circuit current varies in Low Voltage and High Voltage systems. The fault current is largely determined by the impedance of the fault. In the LV system, the impact is thermal only with a higher short time rating is about 1 to 3 seconds.
The LV Protective Devices:
Based on the ongoing rating of the current protection devices are listed below.
| S.No | Protective Devices | Continuous Ratings |
| 1 | Fuse | 400A |
| 2 | MCB | 0.63A – 125A |
| 3 | MCCB | 6A – 1600A |
| 4 | ACB | 400A – 6300A |
For LV release, clearance is necessary for coordination between characteristic curves.
Consider a small system with a utility, transformer, two motors and MCCB and ACB protectors. The system is protected by coordinating the highest rated power of the motor with the LV side of the transformer. Figure 2 and Figure 3 of the TCC curves illustrates the coordination. In the Figure 1 the parameters used for the system is Utility, 2.5 MVA 11/0.4kV Transformer, 37.5 kW and 175 kW Motors, 100A and 400A MCCB and 4000A ACB.
Figure 1: System with Protective Devices
Figure 2: Phase Protection TCC Curves
Figure 3: Ground Protection TCC Curves
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