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Power Restoration Triage and Delta Systems

Triage is a common tool used to prioritize medical treatment based on urgency of need and severity of the injury or condition. For example, in mass-casualty incidents, victims are tagged using a color-coded system that identifies which individuals should get transported to the hospital first. Colors may vary depending on the triage system you use, but typically there are four colors – red, yellow, green and black – with red indicating that immediate transportation is required while black means that the individual likely will not survive.

Beyond its medical uses, triage is also highly useful for prioritizing power restoration after a storm. For example, if a substation transformer blows up and another one isn’t readily available, the outage may need to be tagged with the color black and abandoned for other work until a replacement transformer is available.

One way to think about power restoration triage is to imagine the human body. The brain is the substation, and the vertebrae are the backbone that represents the primary-voltage, three-phase circuit out of the substation. The body’s arms and legs are single-phase primary laterals off the backbone. The hands and feet are subdivision developments, with fingers and toes being individual customers. In the medical arena, if a doctor is working on a patient’s broken finger, but the patient’s neck is also broken, the finger won’t stand a chance at working until the neck is repaired. The same thought process should logically occur during power restoration, but if you are a lineworker who has worked storm restorations, you may have realized that it’s not always so logical to everyone.

When a line crew plugs in a single-phase lateral fuse and discovers the three-phase backbone is out, they will know appropriate triage has not been done. Yet storm restoration efficiency occurs when the right resources are initially applied in the places that restore power to the greatest number of customers. Triage priorities should be those facilities that address emergencies and critical infrastructure. These include hospitals, fire departments, and water and sewage pumping stations. Next on the priority list should be restoration to those areas with the highest population densities as well as repairing equipment that is the greatest source of power outages.

Delta System Conditions
The bulk of electric grids in the U.S. are wye systems, which means they operate with a phase-to-ground potential to energize transformers. However, delta systems are more prevalent in the rural U.S. and small communities, particularly in the East. They require a phase-to-phase potential to energize transformers. The coil in every delta transformer is fed by two energized phases as opposed to a phase-to-ground condition in the wye transformer.

Why does this matter, particularly in terms of triaging? If one primary fuse blows on a three-phase delta system, the now-dead phase is instantly reenergized through transformers fed by the still-energized phases. This creates a low-voltage situation for customers on that feeder. When reports of brownout conditions come in on a delta circuit, we must assume all customers are experiencing a brownout until proven otherwise. This low-voltage condition can destroy customer equipment and start fires. Brownouts are a true emergency and must be high on the triage list. The remedy is to send out a crew to pull the remaining fuses where the single fuse is blown. The longer the condition exists, the more likely it is that customer equipment will be damaged. Brownouts should be triaged as needing immediate attention (a red tag). Deenergize the other phases as soon as possible.

Another condition unique to delta systems is that one phase of the primary can have a solid ground applied with no change or interruption in service. For a fuse to blow on a delta system, there must be a ground or partial ground on two phases. During storm restoration, a blown fuse may indicate that a second ground has occurred past the open fuse. The only way to determine that is to test the energized portion of the feeder to see if there is an existing ground. Fixing the problem that caused the blown fuse may leave an existing ground still on the circuit. Testing for a ground on the delta circuit will identify if there is a ground on one of the phases. The phase with the ground will read lower than the nominal phase-to-ground voltage or will read zero if there is a solid ground. The phase is energized but at the same potential as the ground when it is zero; however, proper PPE must still be used to work on that phase.

At the end of every storm, delta substations and delta circuits should be phase-checked for a ground on the circuit so that we don’t go into the next storm without assuring a clean circuit. The circuits with a ground on them will be the first ones to go in the dark.

About the Author: Bill Martin, CUSP, NRP, RN, DIMM, is the president and CEO of Think Tank Project LLC ( He has held previous roles as a lineman, line supervisor and safety director.

Editor’s Note: For more about delta and wye systems, read Brian Schneider’s article scheduled for publication in the August-September 2023 issue of Incident Prevention magazine. 

Bill Martin, CUSP, NRP, RN, DIMM

President and CEO of Think Tank Project LLC 30+ Years Electric Utility, 10 years with an Electrical Contractor. Includes Tree Trimmer, Lineman, Line Foreman, Line Supervisor, Project Manager for Cable Make-Ready and Bare Hand/ Live Line, Safety Director, Safety Consultant. Simultaneous Medical Career: Paramedic, Nationally Registered Paramedic, Paramedic Course Instructor Coordinator, Critical Care Paramedic, 21 years Flying on a Critical Care Helicopter, Registered Nurse, Cardiology Nurse, Advanced Wilderness Life Support, Diploma in Mountain Medicine.