jar546
CBO
While public field reports rarely specify “interrupting rating below available fault current” as the root cause, controlled manufacturer testing has shown exactly what happens when it occurs.
Eaton subjected a molded case breaker with a 14 kA interrupting rating to a 50 kA short circuit event at 480 volts. The result was not a simple trip; the breaker ruptured violently, ejecting molten metal and damaging surrounding components. In another test, fuses rated 10 kA under the same fault level suffered catastrophic failures.
NEC 110.9 requires that equipment intended to interrupt current must have an interrupting rating at least equal to the available fault current at its line terminals. NEC 110.10 further requires the overcurrent protective devices, conductors, and other equipment to be coordinated so that a fault is cleared without damage to the electrical system.
These are not hypothetical scenarios. They are physical demonstrations of what the Code exists to prevent. When the available fault current exceeds a device’s interrupting rating, the device itself becomes the hazard.
Key takeaway for plans examiners and inspectors: You may not see AIC mismatches documented in every post-incident report, but the physics are proven. If you approve plans or installations without verifying that the available fault current and interrupting ratings match, you are allowing the exact conditions that testing has shown will cause explosive device failure.
Eaton subjected a molded case breaker with a 14 kA interrupting rating to a 50 kA short circuit event at 480 volts. The result was not a simple trip; the breaker ruptured violently, ejecting molten metal and damaging surrounding components. In another test, fuses rated 10 kA under the same fault level suffered catastrophic failures.
NEC 110.9 requires that equipment intended to interrupt current must have an interrupting rating at least equal to the available fault current at its line terminals. NEC 110.10 further requires the overcurrent protective devices, conductors, and other equipment to be coordinated so that a fault is cleared without damage to the electrical system.
These are not hypothetical scenarios. They are physical demonstrations of what the Code exists to prevent. When the available fault current exceeds a device’s interrupting rating, the device itself becomes the hazard.
Key takeaway for plans examiners and inspectors: You may not see AIC mismatches documented in every post-incident report, but the physics are proven. If you approve plans or installations without verifying that the available fault current and interrupting ratings match, you are allowing the exact conditions that testing has shown will cause explosive device failure.