1967 The Magic of I2t

Evaluation of the effects of a short-time irregular-shape current pulse on the conductors and devices through which such current pulses pass can be a perplexing problem to the industrial power system engineer, as well as to those working in other areas. The adverse effects may range from a nuisance malfunction in the overall circuit to a catastrophic explosive failure in power circuit equipment. Common sources of short-time, nonsinusoidal wave-shaped current pulses in industrial systems include: 1) current-limiting fuse let-through current 2) surge discharge current pulse created by short circuiting a charged capacitor 3) lightning discharge currents 4) controlled current let-through of solid-state switching devices. To a surprising degree, the aggregate effect of a particular short-time current pulse can be evaluated in terms of a single composite quantity ??i2 dt, called I2t, and generally expressed in ampere-square-second units. This paper describes and explains a multiplicity of circumstances in which the simple I2t value can serve to evaluate the severity of the effects as related to conductors, switches, circuit breakers, switching assemblies, and even electric shock effects on human beings.