Indeed. This is not corner-grounded delta. Once you figure out open delta, look at dog-leg. Thanks, Jonathan

One of the top Google results: https://www.engineeringradio.us/blog/2012/02/the-open-delta-three-phase-ser… Really really, there's only two pigs on the pole. And yet, delta power, though at a lower rating than transformer nameplate (vs. if you had three). Thanks, Jonathan ??????? Original Message ??????? On Tuesday, January 4th, 2022 at 14:56, Grant Taylor via cctalk <cctalk at classiccmp.org> wrote:

That's correct. A to C is also valid, presumably it's rotated if there's a lot of open delta in an area (again, why?) to balance phases. Right, and you get a mostly unusable "high leg" w.r.t. neutral, usually 208V though I don't know what it ends up actually being in open delta with poor/uneven loading. The 120/240 power available is also supposed to be derated, IIRC it's only supposed to be 5-10% of total service load. High-leg delta exists so you can have 120/240 lighting and appliance loads in a building that consumes mostly 3-phase, like a machine shop with an office. In most areas you aren't allowed to have more than one type of service to a building (not sure if that's true for double-fed sites, never seen one with two kinds though). I've heard the Power Company usually doesn't want to install high-leg delta anymore for a variety of reasons: the load limit, people not understanding they need to skip a breaker, 120/208Y having become the usual form of smaller service three phase, etc. Right, there's one or two of those services in town here for a couple of small commercial buildings. Four wire high leg open delta off the pole, two transformers on the pole. Thanks, Jonathan

On 1/4/22 4:46 PM, Chuck Guzis via cctalk wrote: I'm used to 1-4 houses being on a common transformer where I grew up. They were usually installed on polls / pads at the corner of one of the properties on the back property line and feed the four touching properties. Ya. I've seen some spectacular videos. The higher the voltage, the more spectacular. I knew about 50 and 60 Hz and DC growing up. I learned about 400 (?) Hz being used in military applications about a decade ago. 25 Hz was a surprise to me over the last year or so. I read some interesting things, much of it related to Niagara Falls and other power systems modeled after it. Voltages around the world through the last century was also interesting to read about. As was single, two (90?), and three (120?) phases. -- Grant. . . . unix || die

Well that ruined my evening, Wikipedia says the generating station now uses the power to mine bitcoins, because it's more profitable than selling it to the public grid. Thanks, Jonathan

On 1/4/22 4:04 PM, Grant Taylor via cctalk wrote: 400Hz of course, could be encountered if one shopped war-surplus airborne electronics. I still remember adventures on Chicago's S. Michigan "Surplus Row". Word was that the original plant acquired a generator from a bankrupt carnival operator in the 1910's. But it was used all over the place; my job was with the pyrometer (instrumentation) department. It was incredible seeing the lengths that were taken to avoid using 60Hz power for things like a chart recorder. It was explained that the 25Hz power was generated on-site from coke oven gas and so was essentially free, while the 60Hz was purchased from the local utility. DC, of course, was used for overhead cranes and welding. Every shop had outlets with yellow labels indicating the voltage and frequency. You could plug a coffeepot or hot plate into the 110V (not 120 or 115 or 117) 25Hz outlet, but beware of using it for much of anything else, save lighting. BTW, you have seen no more irritating lighting than mercury-vapor lamps running off of 25Hz. Even in our shop, which used 220V 25Hz incandescent lighting, you could see the pulsation. I did discover that you could take a 110V 25Hz induction motor fan and plug it into 230V 60Hz for a real kick in performance...and probably shorter bearing life. 440V/460V 3-phase in both frequencies were in most shops. Said plant has long since vanished. This was the day of blast furnaces, open hearth furnaces and all the stuff that it fed, like merchant mills, hot- and cold-rolled sheet, tinplate, galvanized...all from a bunch of rocks. --Chuck

I bet you have a fun electrical system :D I love the reaction when a tech or plant electrician encounters corner-grounded delta for the first time. The meter says one phase is missing, yet everything is fine! Often coming off a backfed transformer turning 208Y into 480 delta, which also apparently blows some minds. I did once see what happens when a new guy connects the neutral on the Y primary side. Thanks, Jonathan

It can. That's often how, say, an Edison base decorative luminary is run when all other lighting is 277V (the ratios would of course be different). Otherwise someone invariably screws a 120V bulb into a socket providing 277V. Sure, that's one way of getting split phase 120/240V on a delta system. You've basically made high leg delta the hard way. You do also get to use a completely separate two busbar panel for your 120/240V loads, which prevents the "oops that empty breaker spots give 208V to ground" mistake. A more common method of supplying significant small loads on a delta service is to use a delta:wye transformer and provide 120/208Y. This provides more even loading on the three-phase service, and also provides 120/208Y for, say, computers :P The center/star connection in the secondary is grounded at the transformer. Thanks, Jonathan