Arc Flash, Coordination and Short Circuit Study of a Multi-Building Campus, Built Up in Three Phases, Galveston (2021-2022)

Some sites are too large to study in one pass. For a multi-building campus in Galveston, we built the arc flash model in three phases as the field data came in, with reports issued in September 2021, December 2021 and March 2022.

One service, many buildings

The campus takes a single utility feed at 12.47 kV into its main switchgear, stepped down by transformers to the buildings, and is protected mostly by fuses and circuit breakers. One-line diagrams, utility fault data and walkdowns came from the electrical contractor. We did not verify them on site, so the reports list the assumptions made where data was missing and use the typical enclosure and bus gap dimensions of IEEE 1584-2018. Each phase was modeled in ETAP^® with its cables, transformers, panels, transfer switches and generators, and run for two cases: normal utility supply, and the standby generator carrying the load. Arcing time was capped at 2 seconds.

How the model grew

The first phase covered 212 buses: five at 12.47 kV, 163 at 480 V and 44 at 208 V. Eight came out above 40 cal/cm2, the highest at about 116 cal/cm2 on a 480 V main switchboard with roughly 59 kA of bolted fault current. The second phase added 35 buses, including a later building addition, and only its 12.47 kV feeder points exceeded 40 cal/cm2. The final model had 389 buses, three of them at 4.16 kV. On utility power, 53 locations were above 40 cal/cm2, with a peak near 139 cal/cm2; on the generator, 43 were, peaking at about 126 cal/cm2.

No setting changes were required in any phase. Because the labels assume the settings in the study, the reports ask that every breaker and relay be checked against the settings document, and that the study be updated within five years or whenever the system changes.

Above 40 cal/cm2

Where incident energy could not be brought below 40 cal/cm2, the reports are direct about it. De-energize first. Establish an electrically safe work condition before entering the arc flash boundary, test for absence of voltage with hot sticks and non-contact sensors from as far away as possible, and wear arc flash suits rated for the hazard. The labels give the arc flash boundary, the incident energy at the working distance, the shock hazard, the glove class, and the limited and restricted approach boundaries.

Related services

Studies like this one are explained on our page for arc flash study services. The breaker settings come out of our work on protection coordination.