Codes & Standards
Converting an existing ungrounded delta supply to a grounded wye • Nansy Hanna
By Nansy Hanna
September 12, 2016 – In 2013 I published a column very similar to this one to raise awareness about the hazards with delta (ungrounded) to wye (grounded) conversions. Delta systems have become increasingly less common—many of the experts on these systems have retired and lessons learned have been forgotten.
Earlier this year, it was discovered that a delta transformer was changed to a solidly grounded transformer, and the supply to the customer remained 3-wire ungrounded service. This, combined with a fault in the customer’s electrical equipment, could have been the cause of an electrical fire. An investigation revealed that as part of a voltage conversion by the local utility, multiple transformers had been changed to solidly grounded—without the installation of a grounded or neutral conductor. In some cases, there were customers with both grounded and ungrounded services being supplied from the same transformer.
Remember, it is never safe to install a solidly grounded transformer and supply a customer with 3-wire ungrounded service.
Incidents like the one I describe on page 30 of Electrical Business July 2013 remind us that we always need to be aware of the basic engineering concepts for electricity distribution. It illustrates how any changes in the system need to be carefully reviewed, and that required changes be identified and completed.
Caution should be exercised when local utilities decide to convert existing ungrounded delta system to wye grounded. By design, a ground fault condition on a delta system will not operate an overcurrent device, but will only indicate a grounded phase conductor. When the supply transformers are changed to a wye grounded secondary system, and a neutral conductor is not brought to the existing service, a ground fault with high impedance at the customer side will allow fault current to flow, without the overcurrent device operating, eventually leading to fire.
Where the utility and customer are agreeable, conversions should be preceded by a general inspection to check the condition of equipment, grounding and overcurrent protection. In particular, any pre-existing phase-to-ground faults need to be rectified.
• All existing ground fault indicators are to be removed and all openings filled.
• The main overcurrent protection must be adequate for the available fault current it must interrupt (Rule 14-012).
• A new grounded (neutral) conductor must be installed, even if line-to-neutral loads are not added.
• Where the existing service equipment grounding conductor meets the requirements of Rule 10-812, it can be re-used and shall be terminated so as to ground the new system grounded conductor (Rule 10-204[b]). Adequate provisions shall be made to ensure that the service box enclosure is bonded to ground.
• There must be space and provision in the service box (main disconnect switch) for the termination and grounding of the new grounded (neutral) conductor (Rules 4-026 and 10-204). A new bonding jumper shall be installed to bond the service box enclosure to the new neutral block installed inside (Rule 10-624).
• The grounded conductor shall not be smaller than that permitted by Rule 10-204(2) and Table 16A or 16B, and shall comply with Rule 4-022(3). The grounded conductor shall be installed in the same manner as the ungrounded service conductors.
• For services operating at more than 150V to ground and 1000A or more, or less than 150V to ground and 2000A or more, ground fault protection shall be provided as required by Rule 14-102. Converting a delta service to wye without the inclusion of ground fault or similar protection exposes the service equipment to significant risk in case of a fault.
The changes outlined above need to be completed for converting existing ungrounded delta systems to wye grounded systems for safe, compliant installations. CE Code Rule 10-204 requires that 3-phase, 4-wire systems be connected to a grounding conductor at each individual service and have a grounded conductor installed and connected to the service box enclosure.
Nansy Hanna is the director for Engineering & Program Development at Electrical Safety Authority (ESA) where, among other things, she is responsible for product safety, code development, improving harmonization and alternative compliance, worker safety, and aging infrastructure programs. She is a LEED-Accredited Professional and a member of CSA CE Code-Part I, Sections 24, 32, 46, 50 and 64. Nansy can be reached at firstname.lastname@example.org.* This article also appears in the September 2016 edition of Electrical Business Magazine. Check out our ARCHIVE page for back issues.