HIC Testing (UL 489)

[ajb5102]

During interruption testing of a breaker (per UL 489 --> HIC testing?), the Available Fault Current (AFC) is meant to be, let's say, 65kA @ 240VAC... why is the peak let through current of the breaker a lesser value than the available fault current (~34kA)? Why isn't the peak let through current 65kA?

 

[jim dungar]

As the breaker contacts begin to open an arc is drawn between them. This arc has impedance which reduces the amount of current to the fault. The impedance increases as the arc is lengthened in the breaker arc chute, you will often see the term dynamic applied to this effect.

This is why molded case breakers used in 'series rated AIC' applications must be tested rather than applied via charts or graphs.

 

[Jraef]

Besides that, the two values are different things altogether. The interrupt capacity value of the breaker is basically a mechanical rating, saying that at a value of available fault current up to this number, when the breaker tries to open during a fault, it will hold together and not become a bomb. The let-through value is the peak amount if current that will get through the breaker during that process.