Sound Reinforcement - Forums for Live Sound Professionals - Your Displayed Name Must Be Your Real Full Name To Post In The Live Sound Forums > AC Power and Grounding

Open-Leg Delta

(1/3) > >>

Jonathan Johnson:
Found this in the wild the other day -- see attached photos. It's a transformer bank supplying 240V three-phase (delta) service using only two transformers from only two primary phases. The larger transformer is center-tapped to provide 120/240V split single phase service. I don't know the nature of the three-phase load in the building, but I suspect it's for the booster heater for the commercial dishwasher.

Sorry for my poor drawing skills and the hideous green paper, but I've also included a diagram.

Note that while all three primary phases are available on the pole, only two are fed to the transformers. The second lead of each transformer primary is bonded to neutral/ground. The "high leg" is 208V to neutral/ground.

There are other possible ways to configure an open delta connection. For example, L2 could be bonded to neutral/ground, and the one transformer wouldn't be center tapped.

I'm posting this as a matter of curiosity; it's something that you could run into, and maybe this will help you understand "what is going on?" when you see this unusual configuration.

Brian Jojade:
Uncommon? Yes. Unusual, not really.

Wild leg delta is used when there is need for both single phase AND 3 phase connections in the same system.  Common when you have 240v devices alongside 3 phase devices.

If you look at a breaker panel and see a blank every third spot, or you don't seen any single pole breakers every 3rd slot, that's a pretty good indication that you're dealing with this type of system.

The high leg is typically indicated by orange, but code lets orange be used for non-high legs if there isn't one. (stupid)

Anyway, I've certainly run into situations where idiots have seen open breaker slots and decided to use that as an extra circuit, assuming that it would be a 120v connection.  They can use it for a lot of devices that have auto ranging power supplies, so it could go years before it's noticed that something is wrong.

ALWAYS measure voltages before turning stuff on.  ALWAYS.

Frank Koenig:
Theyíre common as dirt around the San Francisco Bay Area which is mostly served by Pacific Gas and Electric (there are a few local utilities that buy power from PG&E such as the City of Palo Alto). You see them on most light industrial buildings. Easy to recognize as thereís one big pot and one little pot on the pole, as in your picture.

--Frank

Frank Koenig:
Just noticed that in your circuit diagram youíve got one phase of the input grounded. Iím pretty sure this is incorrect. The input to the two transformers uses two of three symmetrical phases and the POCO rotates which two of the three they use from transformer-pair  to transformer-pair so as to balance the load across the phases. There is no hard reason one of the phases couldnít be grounded but generally isnít at the distribution level. (All three insulators are the same size.)

--Frank

Jonathan Johnson:

--- Quote from: Frank Koenig on August 21, 2023, 03:15:10 PM ---Just noticed that in your circuit diagram youíve got one phase of the input grounded. Iím pretty sure this is incorrect. The input to the two transformers uses two of three symmetrical phases and the POCO rotates which two of the three they use from transformer-pair  to transformer-pair so as to balance the load across the phases. There is no hard reason one of the phases couldnít be grounded but generally isnít at the distribution level. (All three insulators are the same size.)

--Frank

--- End quote ---

Well, that was an assumption on my part. But for the transformers, there appeared to be only one insulator on the top, so I made the assumption that the other lead of the primary was grounded.

Navigation

[0] Message Index

[#] Next page

Go to full version