IMC-2018 Learning Session 38:35

by Tommy Northcott, Jacobs

The NFPA 70E provides the requirements for protecting employees when exposed to electrical hazards suck as shock and arc flash. However, is your company's maintenance practice actually elevating the severity of a potential arc flash event? The details of any maintenance program can have significant impact on the severity of an arc flash event. Three primary variables are required for the calculations associated with an arc flash analysis: available fault current, distance from the fault, and duration of the fault. The available fault current is a property of the electrical distribution system being evaluated and the distance from the fault is determined by the IEEE 1584 standard. Fault duration is the variable that can be directly impacted by your company's electrical maintenance strategy. Fault duration is the characteristic of the arc flash event. The duration of the fault is determined by the upstream over current protective device's (OCPD) ability to interrupt the fault. Circuit breakers are one of the most common OCPDs and are considered as time limiting equipment in the Arc Flash Analysis. For the purpose of an arc flash incident energy analysis, it is assumed that the OCPD will interrupt the fault in the manner and speed at which it was designed. However, the time required for a circuit breaker to interrupt a fault can be dramatically impacted by the lack of proper maintenance. If an Effective Electrical Equipment Maintenance Program (E3MP) is not utilized on a breaker, then the operating times provided by the manufacturer become irrelevant. Circuit breaker mechanisms can start to malfunction (contacts can start to stick and grease can begin to build up), and even worse, the breaker might not open at all. A delay in response time might not seem like much but it only takes milliseconds for the incident energy on a typical 480-volt motor starter to go from negligible to deadly.