3-phase power; was: cctalk Digest, Vol 88, Issue 2

[Chuck Guzis]

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