The available short-circuit current from the electric utility is a vital part of an arc flash study. Large short-circuit currents can result in an increase in the incident energy. However, lower short circuit currents can lead to protective devices taking longer to operate, which also results in a large incident energy. Quite often, very large incident energy values are due to these long device operating times. The degree to which the utility short-circuit data affects the incident energy is also a function of the equipment’s location. Equipment located farther from the source is generally affected less than equipment closer to the source, because of transformer and conductor impedance.
Question: What data does the utility provide for arc flash studies (assuming data is provided)?
Normal conditions: 18%
Maximum fault current: 28%
Minimum fault current (line out/contingency, etc.): 14%
Infinite bus: 12%
Not sure: 0%
Depends on the utility: 28%
(Respondents were instructed to select all that applied.)
The electric utility’s protective device that serves a facility is likely to be the device that defines the arc flash duration at the service equipment. To determine the arc flash duration, data such as fuse size, type of circuit breaker and relay setting information must be known.
Question: Are you able to “easily” obtain protective-device information from the electric utility?
We are the electric utility: 5%
We don’t perform studies: 2%
The 2002 edition of IEEE 1584 states, “Equipment below 240 V need not be considered unless it involves at least one 125 kVA or larger low impedance transformer in its immediate power supply.” This language, sometimes referred to as the “125-kVA exception,” was intended to permit excluding some circuits from the arc flash study. Some use 125 kVA as the upper limit of the cutoff, while others have reduced the limit to even smaller transformers.
Question: What transformer size do you use for the IEEE “125-kVA exception”?
125 kVA (112.5): 67%
75 kVA: 12%
45 kVA: 6%
30 kVA: 0%
We don’t use the exception: 15%Click here for upcoming scheduled Arc Flash Safety Training and arc flash certification. All Scheduled Electrical Safety Training https://brainfiller.com/scheduled-events/ Request an Onsite Training: https://brainfiller.com/on-site-training/