Types of faults that occur in electrical system
A fault in a circuit is a failure that interfares with the normal flow of current. A short circuit fault occurs when the insulation of the system fails resulting in low impedence path either between phase or phase to ground.This cause excessively high currents to flow in the circuit requiring the operation of protective equipments to prevent damage of the equipments. The short circuit faults can be classified as
- Symmetrical faults
- Unsymmetrical faults
A three phase symmetrical fault is caused by application of three equal fault impedances Zf to the three phase as shown in fig. If Zf =0 then fault is called a solid or bolted fault. These faults can be of two types i.e Line to Line to Line to Ground(LLLG fault) and Line to Line to Line (LLL faults).since three phase equally affected, the system remains balanced this fault is called symmetrical fault or balanced fault and the fault analysis is done on per phase basis.The behavior of LLLG fault and LLL fault is identical due to the balanced nature of the fault is usually the most serve fault that can occur in the system.
Faults in which the balanced state of the network is disturbed are called unsymmetrical or unbalanced faults. The most common type of fault in unsymmetrical or unbalanced fault in a system is single line to ground fault (LG fault). Almost 60 to 75% of faults in a system are LG faults. The other types of unbalanced faults are Line to Line faults (LL faults) and Line to Line to Ground (LLG faults). About 15 to 25% faults are LLG faults and 5 to 15% are LL faults. These faults are shown in figure below.