1-150 Log #2969 NEC-P01
(110-9)
Final Action: Reject
Submitter:
David Soffrin, American Petroleum Institute
Recommendation:
Add the following after the first sentence:
During the momentary paralleling of an automatically controlled power transfer between two sources, the single-source condition
interrupting rating shall be considered adequate provided there is no intentional time delay and the parallel condition cannot be
maintained. The initiation of the automatically-controlled transfer shall be permitted to be manual or automatic.
Substantiation:
The condition identified in the proposal is common in double ended substation arrangements where maintaining power to the loads is
critical for operation or system safety. It is also a common practice in generating stations where continuity of power flow may be
requried for public safety. These systems are designed for only a momentary parallel condition, typically only long enough for the
closure of the paralleling breaker to initialize the opening of the designated breaker returning the system to single source configuration.
During the brief parallel time (often only a few cycles), the short circuit rating of the switchgear feeder breakers may be exceeded. This
added provision in 110.9 would recognize this arrangement and establish limitations under which it can be applied. IEEE 666, Design
Guide for Electric Power Service Systems for Generating Stations, section 4.6.1 specifically allows this arrangement and equpiment
rating.
Panel Meeting Action: Reject
Panel Statement:
The operational practice described is only one of a number of methods that may be permitted under 90.4. This issue is best addressed
through evaluation of specific installations.
Number Eligible to Vote: 12
Ballot Results: Affirmative: 12