NEMA ABP 7:2015

Introduction The 2005 National Electrical Code® (NEC) introduced a change regarding series ratings for circuit breakers that deserves careful review before applying an engineered series rating. The additional section 240.86(A) reads: (A) Selected under Engineering Supervision in Existing Installations. The series rated combination devices shall be selected by a licensed professional engineer engaged primarily in the design or maintenance of electrical installations. The selection shall be documented and stamped by the professional engineer. This documentation shall be available to those authorized to design, install, inspect, maintain and operate the system. This series combination rating, including identification of the upstream device, shall be field marked on the end-use equipment. Notice that this new paragraph deals exclusively with existing installations where an increase in the available fault current (due to factors such as increases in transformer size, lowering of transformer impedances, and changes in utility distribution systems) puts the existing circuit overcurrent equipment at peril in regard to interrupting fault currents and provides for a licensed professional engineer to determine the series rating in these installations. In the 2008 NEC, a sentence was added to section 240.86(A) that reads: For calculated applications, the engineer shall ensure that the downstream circuit breaker(s) that are part of the series combination remain passive during the interruption period of the line side fully rated, current-limiting device. Several issues must be understood related to the new paragraph (A): a) Why is this provision for only existing installations? b) Is an added series rating a solution for the situation for which this is intended? c) What information does a licensed professional engineer need, and what information does he/she have, that will permit him/her to determine an effective series rating? d) Are there any downstream circuit breakers that are known to be passive, or any suspected to not be passive? NOTE -A circuit breaker is considered to be 'passive' if its contacts remain closed during the interruption of the fault, such that it provides no additional arcing impedance to aid in the fault interruption. This additional impedance is often called 'dynamic impedance.'