Why is the comprehensive short circuit study (DAPPER) used in Arc Flash and not the other short circuit study methodologies (ANSI & IEC)?
The simple answer is that the comprehensive short circuit method provides the most accurate arc flash results. Most other short circuit standards apply various multiplying factors to determine the fault current values. This is adequate for other applications, such as for specifying electrical equipment withstand and interrupting ratings and to selectively coordinate protective devices, but for Arc Flash purposes, it may not be suitable.
In Arc Flash calculations, a conservative fault current doesn’t always result in a conservative incident energy. Generally, protective devices protecting the faulted location will clear much faster when a higher fault current exists, resulting in a lower incident energy and a false sense of security.
In actual sense, a longer clearing time will result in a more conservative incident energy. Arc Flash standards (IEEE 1584 and NFPA 70E) suggest using a Low and High arcing fault tolerance to be used instead. There’s another impedance tolerance option in the software that can be used within Arc Flash as well. Making use of these tolerances and the worst case scenarios will allow the worst case incident energy to be determined.
In Arc Flash calculations, a conservative fault current doesn’t always result in a conservative incident energy. Generally, protective devices protecting the faulted location will clear much faster when a higher fault current exists, resulting in a lower incident energy and a false sense of security.
In actual sense, a longer clearing time will result in a more conservative incident energy. Arc Flash standards (IEEE 1584 and NFPA 70E) suggest using a Low and High arcing fault tolerance to be used instead. There’s another impedance tolerance option in the software that can be used within Arc Flash as well. Making use of these tolerances and the worst case scenarios will allow the worst case incident energy to be determined.