Shade Tree Notes Blog

By Jim Maloney, National Grid

Are you an Incidental Line Clearance Arborist? Have you discovered the utilities’ long-held secret to determining what voltage you’re working nearby? Is it counting how many discs make up the insulator? Nope. Pssst. Peek at the transformer. You’ll find a label on most of our newer ones stating what voltage its designed for. Can’t find a transformer, the line is crackling overhead and the hair on your arms are standing up? You might want to ask your local utility for I’m positive that you’re not where you should be!

But how about configuration? Do you know how that distribution was designed? Is it Delta or Wye? And why does that matter? Your safety is why.

Delta configured facilities don’t carry a neutral with them, Wye configured does. You actually have Delta and Wye configured extension cords in your home. You know that cord with the 3 prongs? That’s Wye. The little round prong is the neutral. A 3-prong cord is generally considered ‘safer’ as, in the event of a short, the neutral helps trip that breaker faster. It’s the same with our distribution systems.

Delta Config

How that translates into arborist safety is during the worst case scenario when an energized conductor hits the ground. On a Wye system, the neutral is grounded, as in attached to the Earth, which should allow enough load to trip whatever safety device your utility is using, be it a fuse, breaker or recloser. On a Delta system, there isn’t a neutral, so the protective device may not reach its rated capacity and may not trip. Worse yet, Delta requires two primary connections at the transformer, so it is possible to have both halves of the broken conductor remain energized as voltage travels from the unbroken primary conductor, through the transformer, re-energizing the other broken half.

Step potential is a major concern for arborists working on the ground with either configuration, however, Delta has the greatest risk. Step Potential being the voltage gradient that radiates through the ground out from the point of contact. The human nervous system is nothing more than a direct current distribution system, so it offers an excellent, low-resistance path for that big pool of alternating current to flow through with dire consequences.

All this may be good but how can the configuration be identified in the field? I’ll stick to what I know and that’s what Delta and Wye look like on National Grid/ Upstate New York. The two photographs below depict transformer installations that are typical of what you’d find here. The left one is a transformer on a Delta system while the one on the right is Wye. A Delta configured transformer has two primary connections, the Wye has just one.

Power APower B

Care to learn more about electricity? Excellent. Stay tuned as my next blog will get a little deeper into what step potential is and how you might be able to survive it. You can also reach out to any of the Electric Hazard Awareness Program providers that support Shade Tree Notes through their advertising.