Industrial Automation

Current-limiting fuses often do provide additional protection and they are very good devices but they must be applied properly. In this example, the circuit breaker provides the best protection.

a relatively small change in calculated bolted fault current has a major effect on the calculated arc fault current.

as the calculated arc fault current is reduced, the clearing time increases, resulting in the incident energy level increasing and thus, the PPE requirement increases.

2. A reduction in bolted fault current and thus a reduction in arc fault current can actually result in a worse situation. In the motor example above an arc fault current reduction from 4000 amps to 1800 amps resulted in an increase in arc fault energy from 0.6 cal/cm2 to 78.8 cal/cm2. Exactly the opposite one would expect before doing the math. In terms of the above example coordination curves, this occurs because the arc fault current moves from the instantaneous portion at the bottom of the coordination curve to a point higher up, incurring a the time delay before the device trips.

Categories of PPE as described in NFPA 70E are: Category Cal/cm2 Clothing 0 1.2 Untreated Cotton 1 5 Flame retardant (FR) shirt and FR pants 2 8 Cotton underwear FR shirt and FR pants 3 25 Cotton underwear FR shirt, FR pants and FR coveralls 4 40 Cotton underwear FR shirt, FR pants and double layer switching coat and pants