Arc Flash, Coordination and Short Circuit Studies for Four Areas of an Industrial Plant, Grapeland (2019)

In 2019 we prepared four arc flash, coordination and short circuit studies for an industrial plant near Grapeland, in East Texas. Client details are confidential.

Four areas, four utility feeds

The plant has four primary areas: a truck stop, the process facility, a wastewater plant and a fire pump house. Each area has its own utility feed, so they do not share a source and cannot share a model. We built a separate model for each area and issued a separate report for each one. That keeps every report readable on its own, and it means a change in one area only triggers an update to that area.

The process facility

The largest of the four is the process facility. Two utility feeds supply 2,000 kVA transformers, which feed two primary switchgear lineups in the main electrical room, and the same system sub-feeds the maintenance shop. Below the switchboard sit six motor control centers, and the report appendix carries a one-line for each of them, once for arc flash results and once for equipment duty. Low-voltage breaker settings and motor circuit protector settings are listed in their own settings reports, so the plant electricians can check each device against the study.

Normal and emergency power

At the truck stop the study was run twice, once on normal utility power and once on emergency power, with short circuit, duty and arc flash results for both. A source change can move the worst case from one bus to another, and the labels have to reflect whichever case is worse. The wastewater plant and the fire pump house were each studied with their own short circuit, duty and coordination results.

How the studies were done

Our engineers verified the system data on site. Where data could not be read or reached, we made bounding and conservative assumptions. The available short circuit capacity at each service came from the serving utility. Each system was modeled in ETAP^®, with three-phase duty and maximum short circuit cases, and rated short circuit capacities reported to us were checked for their application. The arc flash calculations used the methods of IEEE 1584, the worst-case scenario was used for the labels, and PPE levels follow Annex H.2 of NFPA 70E-2018.

Revisions before the labels went up

The reports require that device settings and field configuration be confirmed against the settings appendix and the one-lines before the labels are attached. The process facility and wastewater reports were reissued as revision 1 with modified trip settings, and a panel that had not been in the first model was added. The labels are only correct if the breakers in the gear match the settings in the study.

Arc flash studies for multi-service sites

Sites with several services are common at plants that grew over time. What each study contains is described on the page for our arc flash study services. Equipment duty is covered on our page for short circuit analysis.