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Examoogle • User • info@examoogle.com • EE-2024-8821
Chapter 1 of 12 • Page 1 of 248🔒 Protected PDF • Watermarked
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Which is symmetrical type fault?
LG
LLL
LLG
LL
LLL
Quick Summary: A symmetrical fault, also known as a balanced fault, is a condition where all three phases of a power system are involved in the fault simultaneously. Because the impedance of the system is equal for all three phases, the resulting fault currents remain balanced, maintaining the symmetry of the system.
A symmetrical fault, also known as a balanced fault, is a condition where all three phases of a power system are involved in the fault simultaneously. Because the impedance of the system is equal for all three phases, the resulting fault currents remain balanced, maintaining the symmetry of the system.
If=Z1Vph — Fault current calculation for LLL fault where Z1 is the positive sequence impedance.
Isym=0 — In symmetrical faults, negative and zero sequence components are absent.
In a symmetrical (LLL) fault, the three phases are short-circuited together, potentially to the ground as well. Since the fault impedance is identical in all three phases, the sequence networks (positive, negative, and zero) are interconnected only in the positive sequence network, leading to equal magnitude currents with a phase displacement of 120°. This is the most severe type of fault due to the lack of phase-to-neutral impedance limitation.
Symmetrical faults are the most severe but least frequent fault types.
Only positive sequence network is involved in the calculation.
Used for calculating the MVA capacity of circuit breakers.
The fault current is limited only by the source impedance and line impedance.
Simplified analysis using single-phase equivalent circuit.
Provides the worst-case scenario for determining short-circuit ratings.
High magnitude currents cause significant thermal stress.
Electromagnetic forces can cause mechanical damage to busbars and windings.
Circuit breaker sizing and rating.
Selection of protective relay settings.
Symmetrical faults account for only about 2-5% of total system faults.
LG (Line-to-Ground), LL (Line-to-Line), and LLG (Double Line-to-Ground) are classified as unsymmetrical faults because they disturb the balance between the phases.
B is correct — LLL (Three-phase fault) is the only type where the system remains balanced/symmetrical across all phases.
Always remember that symmetrical faults involve all three phases equally; if a question mentions 'balanced' or 'symmetrical', think LLL fault.