“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.”
“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.’” short ckt current calculation
“See that?” Leo said, pointing at the darkened but intact panel. “That’s the sound of a proper calculation. A ‘sneeze,’ not a ‘heart attack.’” “Your full load current on the low side
In the bustling industrial park of Voltavia, three electrical engineers—Zara, Mike, and old-hand Leo—were tasked with commissioning a new manufacturing line. The line included a massive 2,000 horsepower motor, several conveyors, and a brand-new 2.5 MVA transformer. “That breaker isn’t a protection device anymore
[ I_{SC} = \frac{I_{Full\ Load}}{Z_{pu}} ]
“Just connect it to the main 11 kV bus,” said the project manager. “The breaker we have in stock is rated for 25 kA. That should be plenty.”
Zara frowned. “Should be isn’t a calculation. We need to know the prospective short-circuit current at the motor terminals.”