When three phase fault occur
In a three phase power system, the type of faults that can occur are classified by the combination of conductors or buses that are faulted together.In addition, faults may be classified as either bolted faults or faults that occur through some impedance such as an arc.
Four types of Three Phase Faults
1. Three Phase bolted faults
2. Bolted line to line faults
3. Line to line to ground faults
4. Line to ground faults
1. Three Phase Bolted Faults
A three phase bolted fault describes the condition where the three conductors are physically held together with zero impedance between them, just as if they were bolted together. For balanced symmetrical system, the fault current magnitude is balanced equally within the three phase.
While this type of fault does not occur frequently, its results are used for protective device selection, because this fault type generally yields the maximum short circuit current value.
2. Bolted Line to line Faults
Bolted line to line faults are more common than three phse faults and have fault current that are approximately 87% of the three phase bolted fault current.
3. Line to line to Ground Faults
Line to line to ground faults are typically line to ground faults that have escalated to include a second phase conductor. This is an unbalanced fault. The magnitudes of double line to ground fault currents are usually greater than those of line to line faults, but are less than those three phase faults.
4. Line to Ground Faults
Line to ground faults are most common type of faults and are usually the lease disturbing to the system. The current in the faulted phase can range from near zero to a value slightly greater than the bolted three phase fault current.
