Zara frowned. “Should be isn’t a calculation. We need to know the prospective short-circuit current at the motor terminals.”
[ I_{SC} = \frac{I_{Full\ Load}}{Z_{pu}} ] short ckt current calculation
“Your full load current on the low side (400V) is about 3,600 amps. With 5% impedance (0.05 pu), your short-circuit current is 3,600 / 0.05 = (72 kA).” Mike’s face went pale. “But our breaker is only rated for 25 kA.” Zara frowned
“Exactly,” said Leo. “That breaker isn’t a protection device anymore. It’s a bomb. When that 72 kA fault occurs, the breaker will try to open, but the arc inside will be so violent it will vaporize the contacts, explode the casing, and turn the panel into a fireball. You won’t have a ‘short circuit.’ You’ll have a ‘long explosion.’” With 5% impedance (0