19 Jul
2016
19 Jul
'16
12:10 p.m.
On 07/19/2016 01:41 PM, Fred Cisin wrote:
3-phase comes in "delta" or "Wye"("Y")
some installers don't know the difference!
I experienced TWO misdone installations. One was an auto garage, and
resulted in high voltage to the 110 outlets, damaging a bunch of minor
stuff, such as grinder, space heater, clock, etc.
Oh, it gets much worse. In the US, there is also corner-grounded
open-delta, and center-grounded open delta.
These have some advantages in cases with mixed house/office/industrial
use, but you need to mknow what you have. Corner-grounded open delta has
one of the 3-phase wires grounded. This gives you two hot wires. The
giveaway is that two-pole breakers (or fuses) are used. You get two 230
V hot wires that can be used for any single-phase appliance.
Center-grounded open-delta uses one standard residential transformer,
and has a grounded center tap on that transformer. So, the 3-phase hots
all look normal relative to each other, but have strange readings to
ground. You get two 120 V hot wires, and can run standard home or
office equipment from that, with single or 2-pole breakers. And, you
can run 3-phase loads from the 3 hots.
In most of these cases, the pole equipment is two separate single-phase
transformers.
These systems are out of favor, but you still run across them in older
mixed-use buildings.
Jon
The other was was a PDP installation. After excessive
downtime of
third party disk drive, the community college had sold it to a
neighboring school district, and bought a roomful of PCs. Microsoft PC
COBOL and Fortran were crap, but quite adequate for teaching the
languages, and it was great to have dozens of machines for students to
use without fear of downtime. PG&E (our power company) agreed to buy
a new replacement computer, if those involved would go along with the
fiction that it had been a lightning strike (NOT common here). The
bad drive ceased to be a problem. Everybody was happy, and PG&E got
to call it a donation on their taxes.