Side-by-side comparison
| Parameter | Symmetric | Unsymmetric Fault |
|---|---|---|
| Fault Type | 3-phase (LLL or LLLG) — balanced | LG, LL, LLG — unbalanced |
| Frequency of Occurrence | < 5% of all faults | > 95%; SLG alone is ~65–70% |
| Symmetry of Currents | All three phase currents equal; 120° apart | Phase currents unequal; negative/zero sequence appear |
| Analysis Method | Single-phase equivalent (positive sequence only) | Symmetrical components (Z1, Z2, Z0 networks) |
| Sequence Networks Used | Positive sequence only | All three: Z1, Z2, Z0 connected per fault type |
| Fault Severity (current magnitude) | Highest — up to 20–25 kA at 220 kV bus | SLG fault current often lower (limited by Z0) |
| Zero Sequence Current | Zero (balanced — no neutral current) | Present in LG and LLG; flows through earth/neutral |
| Relay Response | Distance relay (21), overcurrent — all three phases trip | Earth fault relay (50N/51N) for SLG; 67 for LL |
| Transformer Neutral Grounding Effect | Not relevant | Z0 path depends on neutral grounding; isolated neutral blocks earth fault current |
| Example Scenario | Three-phase bus fault at 220 kV substation | Single conductor touching tower at 33 kV feeder |
Key differences
Three-phase faults produce the highest fault current (up to 25 kA at 220 kV) but are analysed with only the positive sequence network — no Z2 or Z0 needed. Single-line-to-ground faults require all three sequence networks in series: If = 3Ea/(Z1+Z2+Z0), making zero-sequence impedance critical. A solidly grounded system has low Z0 and produces high SLG current; an isolated neutral system has Z0 → ∞ and almost zero SLG current. Negative sequence current (present in all unsymmetric faults) heats generator rotors and triggers negative-sequence relays (46), which never operate during balanced faults.
When to use Symmetric
Use symmetric three-phase fault analysis when sizing circuit breakers and selecting equipment MVA ratings at substations. Example: a 220 kV bus at RRVPNL Heerapura is rated for a 40 kA symmetric short-circuit level determined by a 3-phase fault calculation.
When to use Unsymmetric Fault
Use unsymmetric fault analysis (symmetrical components) when setting earth-fault relays and studying power quality issues like negative-sequence heating. Example: a 33 kV feeder earth-fault relay (IDMT 50N, pick-up 10% of CT primary) is set based on the single-line-to-ground fault current calculated using Z0 of the delta–star transformer.
Recommendation
For equipment rating and breaker selection, always use the three-phase symmetric fault — it gives maximum current. For relay co-ordination involving earth faults and negative-sequence protection, choose unsymmetric fault analysis with symmetrical components. Both calculations are needed on any real project; they answer different questions.
Exam tip: Examiners ask students to calculate SLG fault current using If = 3Ea/(Z1+Z2+Z0) with given sequence impedances and then state what happens to this current if the neutral is isolated — zero, because Z0 becomes infinite.
Interview tip: A placement interviewer at a power consultancy will ask you to distinguish why a delta winding on a transformer blocks zero-sequence current — explain that delta provides no return path for zero-sequence, so Z0 seen from the delta side is infinite.