With respect to interconnected electric power production sources (Art. 705), is it required to automatically disconnect a 3-phase primary power source from all ungrounded conductors when one of the phases opens to create a loss of power on one or more phases? For this example, exclude emergency or legally required standby systems — and exclude the exception for this rule.

  1. Yes, as per Sec. 705.42.
  2. No. This applies to single-phase systems only.
  3. No. This applies to 2-phase, 5-wire systems only.
  4. No. There is no such requirement in the 2008 NEC. This is considered a "best practice" suggestion.

Answer: A

Excluding emergency or legally required standby systems — and the exception noted in 705.42 — you are required to design and install the system to automatically disconnect a 3-phase primary power source from all ungrounded conductors when one of the phases of that 3-phase primary source opens to create a loss of power on one or more phases.

Owen is the owner and president of National Code Seminars and the holder of master electrician certifications in 46 states. He can be reached at necexpert@aol.com.