On 07/19/2016 01:41 PM, Fred Cisin wrote:
Oh, it gets much worse. In the US, there is also corner-grounded open-delta, and center-grounded open delta.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.
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.
