On 1/4/22 3:14 PM, Jonathan Chapman wrote: >> High-leg delta is independent of open- or closed-delta. > > That's correct. > >> Open delta uses two single-phase transformers primaries connected to >> phases AB and BC. > > 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.
In fact, in my neighborhood, each house has a big green disconnect box on the street fed by 3-phase 11KV. Every so often, the utility sends out a notice that they'll be "re-balancing" the distribution. I imagine that that involves nothing more than changing one phase wire in the big green box. I recall a winter after an ice storm that resulted in many power outages (the line workers were complaining that with more than an inch of ice on the HV lines, that the weight would break the pole crossarms. Hope that doesn't happen again. As a consequence, the fuse in our home's transformer blew. The service guy opened the transformer case and then grabbed a 10 foot fiberglass pole; unscrewed the bad fuse and then fitted a new fuse to the pole. He advised that I should put some distance between the transformer and myself, as it wasn't unusual for these things to turn into pyrotechnic displays. > 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. When I worked in heavy industry, it was eye-opening to see the range of voltages, phasings and *frequencies* entailed in the operation (we had 25 Hz, 60 Hz and DC). And, of course, the mistakes made by the electricians in installing various equipment. --Chuck