Question

The maximum short circuit current occurs in case of :

• A. Three phase bolted fault
• B. Double line to ground fault
• C. Line to line fault
• D. Single line to ground fault

Hint:

Fault current severity: \[ \text{3-Phase Fault} > \text{DLG Fault} > \text{LL Fault} > \text{LG Fault} \] Three-phase bolted fault gives maximum current.

Answer 1

The Correct Option is A

Concept: A short circuit fault causes extremely large current flow in a power system. Different faults produce different magnitudes of fault current. Common faults are:
• Three-phase fault
• Line-to-line fault
• Double-line-to-ground fault
• Single-line-to-ground fault Among all these: \[ \text{Three-phase bolted fault produces maximum current} \]

Step 1: Understanding a bolted fault. A bolted fault means:
• zero fault impedance,
• direct metallic short circuit,
• maximum current path. Therefore current becomes extremely large.

Step 2: Understanding three-phase fault. A balanced three-phase fault involves:
• all three phases simultaneously,
• symmetrical current flow,
• minimum effective impedance. Fault current is: \[ I_f = \frac{V}{Z} \] Since fault impedance is minimum: \[ I_f \text{ becomes maximum} \]

Step 3: Comparing with unsymmetrical faults. Unsymmetrical faults such as:
• single line-to-ground,
• line-to-line,
• double line-to-ground involve sequence network combinations that increase effective impedance. Hence their fault currents are lower.

Step 4: Selecting the correct option. Thus maximum short-circuit current occurs for: \[ \text{Three phase bolted fault} \] Hence correct answer is: \[ \boxed{(1)} \]