Current-Limiting Let-Through and Downstream Withstand

A current-limiting fuse or breaker opens inside the first quarter cycle, before the fault current reaches its prospective peak, and what it lets through is characterized two ways on the manufacturer's curve: PEAK let-through current, which drives the magnetic forces that bend busbars and rip apart terminations, and let-through I-squared-t, which drives the heating that damages insulation. Downstream conductors and equipment have to survive both, so both are compared. The margin is usually enormous, and that is the point -- a current-limiting device converts an unprotectable conductor into a protected one, and the arithmetic shows by how much. A 4 AWG copper conductor at 41,740 circular mils, 75 C initial and a 250 C insulation damage limit, withstands 5,474 A for a half cycle, which is about 299,600 A2s against a device letting through 25,000 -- a margin of twelve to one. The same conductor facing an unrestricted 25 kA fault for that same half cycle would see 6,250,000 A2s, twenty-one times its withstand, and the insulation would be destroyed whether or not the breaker eventually opened. What a let-through curve does NOT do is let a downstream device be applied above its own interrupting rating. That is a series rating, it exists only as a tested combination published by the manufacturer and marked on the equipment, and it cannot be calculated or inferred from a curve. This is the device-curve counterpart to the conductor short-circuit withstand calculation, which works from the raw available fault current instead. A screen; the manufacturer's published let-through curves, the equipment's marked ratings, and the engineer of record govern.

withstand_a = 0.0297 x conductor_cmil x sqrt(log10((damage_temp_c + 234) / (initial_temp_c + 234)) / duration_s); withstand_i2t = withstand_a^2 x duration_s; margin = withstand_i2t / device let-through I2t; and separately, device peak let-through against equipment peak withstand.

Conductor thermal withstand by the public-domain ICEA / Onderdonk adiabatic relation (copper constants 0.0297 and 234), compared against the current-limiting device's published let-through I-squared-t and peak current, by name. The let-through values are read off the manufacturer's curve and entered. A series rating is a TESTED, listed, marked combination per NEC 240.86 and cannot be calculated from a let-through curve. The manufacturer's curves, the equipment's marked ratings, and the engineer of record govern.

The Onderdonk relation is public domain; the let-through values come from the device manufacturer's published curve and are entered by the user.

Estimate. AHJ and licensed professional govern.

Field names used by the API: conductor_cmil, initial_temp_c, damage_temp_c, duration_s, let_through_i2t, let_through_peak_a, equipment_peak_withstand_a, withstand_a, withstand_i2t, margin

• Two characterizations peak let-through drives magnetic force, let-through I2t drives heating; both must be survivedcurrent-limiting device practice
• Onderdonk constants copper 0.0297 and 234 in the adiabatic relationICEA / Onderdonk
• Series rating cannot be calculated it is a tested, listed, marked combination per NEC 240.86, never inferred from a curveNEC 240.86