September 10, 2019
Article co-authored by UL and Eaton's Bussmann division experts.
How does arc flash relate to SCCR?
Equipment SCCR represents the maximum amount of short-circuit (fault) current that the assembly can safely withstand under short-circuit conditions. When the industrial control panel or industrial machinery does not have an adequate SCCR for the available short-circuit (fault) current an electrical hazard exists.
The Institute of Electrical and Electronics Engineers (IEEE) 1584 standard is the method in calculating arcing currents and incident energy level. When the upstream overcurrent protective device (OCPD) is the only device considered when calculating the industrial control panel incident energy and, i.e., the industrial control panel SCCR is not included in the calculation, a major hazard exists. The Occupational Safety and Health Administration (OSHA) section 1910.303(b), requires equipment (new and existing) to be rated to handle the available fault current without causing a hazard. For example; a fuse or circuit breaker is feeding an industrial control panel that has a 5kA SCCR. The available fault current on the line-side of the industrial control panel is calculated to be 15kA. If the industrial control panel SCCR is not considered in the calculation, the incident energy is based only on the magnitude of the arcing fault and the time to clear the fault by the fuse or circuit breaker. A label will be generated based on the incident energy calculated and placed on the inadequate SCCR industrial control panel. In other words, the industrial control panel is not rated for the available fault current, which could result in extensive damage creating a dangerous situation for anybody working on or near the panel.
In the above example, the arc flash warning label should state “DANGER,” no one should be allowed to work on or near the panel when energized until the proper industrial control panel SCCR is achieved.
The National Electrical Code (NEC) and UL recognize the need to provide adequate protection against short-circuit events. This can only be achieved by understanding available fault current, communicating protection requirements to personnel and equipment suppliers, and properly applying components in the industrial control panel. Execution of a proper equipment SCCR plan will help provide adequate protection for employees and equipment.