Impact of Arc Flash Hazards on Medium Voltage Switchgear

This paper discusses the arc flash ramifications associated with the design and selection of medium voltage switchgear. Arc flash hazard/risk categories are determined on the basis of the available bolted fault current at a particular location as well as the protective settings of upstream protective devices. In some instances, the NFPA 70E hazard/risk category (HRQ at medium voltage switchgear can easily exceed Category 4, for which no safe PPE exists. Proper design considerations, which include an arc flash analysis at the time of design, can provide a switchgear application that can reduce the HRC below category 4. NEC article 110.16 presently lacks a provision for such a requirement as a design constraint on new equipment and further lacks a requirement for placement of arc flash warning labels on switchgear. Accounting for arc flash hazards at the design stage can enable the design of a medium voltage switchgear application that is within category 4 or less. Placement of warning labels on equipment can warn qualified persons of potential arc flash hazards. Large industrial, commercial, and governmental facilities typically receive power from non-dedicated medium voltage distribution feeders. Since such feeders serve multiple customers, the corresponding Utility protective devices may not provide sufficient protection against arc flash hazards on medium voltage switchgear at a customer´s point of common coupling (PCQ or service entry point. In the event of a change in Utility fault duty or a change in Utility protective device settings, the arc flash hazard/risk category may increase at the customer´s incoming medium voltage service-entry switchgear without the customer´s knowledge. In order to ascertain accurate arc flash hazard assessments at such switchgear, it is imperative that the Utility promptly notifies the customer of a change in the electrical system parameters. It is unrealistic to assume all PCC´s will be hazard/risk Category 4 or less. It is ho- - ped that this paper will prompt the NEC to require (i) that an arc flash analysis be performed at the time of equipment design so that efforts can be made to limit the hazard/risk category to Category 4 or less and (ii) placement of warning labels on switchgear to warn qualified persons of potential arc flash hazards. It is also hoped that this paper will call to attention the need for enacting regulations requiring Utility companies to notify their customers (serviced at medium voltage or above) in the event that the Utility company changes its electrical system parameters. These actions are required to provide and to maintain safe working conditions throughout an electrical distribution system within a plant/facility.