Arc Flash

[nvcape]

My understanding is that ground fault protection came about because boards were failing from arc limited faults. The faults did not draw enough to trip the circuit breaker. Ground fault is not required for 208V boards because below about 250V, air is not ionized. The Arc Flash calculators I have tried and the formulas seem to not recognize minimum voltage. A prior forum listed 480V at about 89% of a bolted fault and 120V at about 10%. Has anyone seen more information, possibly a graph with voltages and fault current? Also, in using the formulas, time needs to be set in. If there is an arc limited fault, then a main breaker will probably not trip on over-current, but probably ground fault. Should you use the 1 second level from 230-95?

 

[johnp]

Re: Arc Flash

The current produced by an Arc Flash is substantially less than produced during a bolted fault condition but these currents can be substantial, causing a severe flash, high pressure, and molten copper or aluminum being blown out toward anyone standing nearby. The arc fault is substantially different than a ground fault and is not related to the need for ground fault protection. Many times the arc fault current is sufficient to trip a circuit breaker or blow a fuse. However, often the amount of current is often below the instantaneous portion of the protective device coordination curve.

With respect to the time that is entered into arc fault calculations, the time is the time that is required for the protective device to clear the fault and can only be determined by looking at the protective device coordination curve. Your first determine the amount of fault current and the look up the clearing time on the coordination curve.

The calculation for arc faults determine the total amount of incident energy produced over the time period that protective device is still closed. This is anywhere from 1/4 cycle to minutes. See www.pfeiffereng.com/arc_flash_article.pdf for examples.