Short circuit fault

In a healthy three-phase system, the red, yellow, and blue conductors are separated by insulation and by air gaps. A short-circuit occurs when insulation fails and a very low-impedance path suddenly appears between conductors or between a conductor and earth. Because the path impedance collapses, the current rises abruptly to a value limited only by the source, transformer, line, and grounding impedances seen from the fault point. The voltage at the fault location drops close to zero, and the system experiences intense thermal and mechanical stress.” Important physical facts: The first cycles of fault current contain an AC component plus a DC offset whose magnitude depends on the system X/R ratio and the fault inception angle. Forces on conductors and windings scale with current squared; heating in arcs and copper/iron is very rapid. Depending on which conductors are involved, the system may remain balanced (symmetrical fault) or become unbalanced (unsymmetrical fault). 1) Classification (what you’ll see on screen) Symmetrical faults: all three phases are affected equally → balanced currents in R, Y, B. Examples: Three-phase short (LLL) and Three-phase to earth (LLL-G). Unsymmetrical faults: only one or two phases are involved → unbalanced currents and voltages. Examples: Single line-to-earth (L-G), line-to-line (L-L), double line-to-earth (L-L-G). We’ll now walk through the two cases shown in your diagrams, mapping everything to the colors. 2) Three-Phase to Earth Fault (LLL-G) — use your top figure On screen: Red R, Orange Y, Blue B; a Green vertical link touches all three and continues down to the Green earth symbol. Narration: “All three colored conductors—red R, orange Y, and blue B—are shorted together and tied to earth through the green path. This is a symmetrical fault: the three phase currents are equal in magnitude and 120° apart, so the system remains electrically balanced even though the voltages at the fault collapse. Because the return includes earth, the surrounding ground rises in potential (ground-potential rise), creating step- and touch-voltage hazards.” Deep theory points: For a bolted single-point LLL-G, the current distribution is essentially the same as a pure LLL fault; analysis uses the positive-sequence network because the three phase currents are balanced. The initial current is governed by the subtransient reactances of nearby generators and the positive-sequence impedance of transformers and lines. It decays to transient and steady values over a few cycles. Consequences: maximum interrupting duty for breakers and busbars; severe electromechanical forces on windings; arc energy and metal vaporization at the fault; GPR considerations for earthing systems and telecom coupling. Protection narrative: “Because current is at or near the maximum possible, high-speed phase and ground elements in relays must trip within tens of milliseconds; robust substation grounding limits step/touch voltages.” 3) Phase-to-Phase Fault (L-L) — use your bottom figure On screen: Green short link connecting red R and orange Y only; blue B stays untouched. Narration: “Here the fault path—drawn in green—bridges just two conductors, red R and orange Y. The blue B line remains healthy. This is an unsymmetrical fault because not all three phases are equally involved.” Deep theory points: With no earth connection, there is no zero-sequence current. The system carries positive-sequence and negative-sequence currents of equal magnitude but opposite phase in the two sequences. The negative-sequence component is crucial: it produces a double-frequency magnetic field in the rotors of generators and motors, causing rapid rotor heating even if the total current is lower than in an LLL fault. Healthy phase B (blue) can experience over-voltage relative to earth because of the unbalance and capacitances, stressing insulation on unfaulted equipment. Protection narrative: “L-L faults are cleared by phase overcurrent and distance elements; many plants also apply negative-sequence overcurrent to protect rotating machines from thermal damage due to unbalance.” 4) Where do these faults come from? Insulation breakdown from aging, moisture, contamination, or over-voltage. Mechanical events: conductor clashing in wind, broken cross-arms, animal/bird contact, or tool drops. External phenomena: lightning flashover, pollution, or salt fog creating surface conduction. Equipment failures: bushing rupture, cable sheath damage, failed breakers/isolators left partly closed. 5) System-level effects (common to all short circuits) Voltage depression around the fault bus; upstream generators experience electromagnetic torque shocks.