National Electrical Installation Standards

Standards as High as Your Own

Thursday, January 17, 2019


How is it possible to have a L-G fault on an ungrounded delta system? The delta is ungrounded (no neutral or ground), so physically speaking, how can we have a L-G fault? Thank you

Fas Tef



Hey Fas thanks for your question. There are a few ways to think about this. You can't have a single line to ground fault because the system is ungrounded. If that situation happens the system becomes a grounded system by way of that connection to a metal object that is connected to ground. If a second fault occurs, from a different line (phase), we now have a short circuit condition, using the metal object(s), that is actually a 2-fault situation. Ground detectors are required for ungrounded systems under certain conditions as stated in 250.21(B). This is so when the first fault (ground fault) occurs an indication is provided so that hopefully a qualified person locates and corrects the fault before a second one occurs.

ABOUT CQD: NECA’s Code Question of the Day (CQD) is a leading National Electrical Code® forum for NECA and the industry. The CQD generates a lively dialogue and relative practical and Code-based responses to an ever-increasing and interactive audience.

IMPORTANT NOTICE: All answers are based on the latest edition of the National Electrical Code®, unless the question requests a response based on a specific edition. This correspondence is not a formal interpretation of the NEC®. Any responses expressed to the questions are opinions and do not necessarily represent the official position of NECA, NFPA, or any technical committee. In addition, this correspondence is neither intended, nor should it be relied upon, to provide professional consultation or services.

ABOUT CHARLIE: Charles M. Trout, better known as Charlie, was a nationally known NEC® expert and author. He served on several NEC® technical committees and is past chairman of CMP-12. In 2006 Charlie was awarded the prestigious Coggeshall Award for outstanding contributions to the electrical contracting industry, codes and standards development, and technical training. Charlie was also a member of NECA’s Academy of Electrical Contracting. Charlie’s experienced team of industry experts keep the CQD dialogue and discussions active and informative in the spirit of the man himself, as he wanted.

NECA STANDARDS: NECA publishes the National Electrical Installation Standards™ (NEIS™), a series of ANSI-approved performance and quality standards for electrical construction. Visit for more information. NEISÔ can be purchased in three formats: as paper books, on CD, or as electronic downloads.

NECA SAFETY PRODUCTS: NECA publishes valuable electrical safety books and CDs for the industry. Visit to view or purchase NECA safety products.

 CONTACT US: To submit a Code question, subscribe, or unsubscribe from this list, please e-mail: