On Mon, 04 Jun 2007 20:49:37 -0700 essenz wrote: | I almost always work with pure 115V AC equipment, but now with the new | Cisco 6500 equipment I am getting, I am little confused about | something. | | In a house, you have two 115V supply lines of the same phase. In your | breaker panel, you can install a breaker which grabs voltage from each | bus, and you wire a standard 230V device - like a dryer or AC unit, | etc.,. | | Here is where I get confused. Because I have a triple phase UPS, I | only have two types of output power - 115V or 208V. I have three 115V | phases, and none of them are of the same phase. So its impossible for | me to wire the 230V equipment like I would at home. However, I have | the 208V power.
The three phase UPS will not operate correctly if supplied with single phase power. I hope you are not trying to do that.
| Is it safe to assume that any 230V appliance can run off 208V? Cisco's
No. I've seen motors burn up because they got 208 volts but were designed for 230 volts or so. Due to the low voltage, they pulled extra amps and may have even stalled due to reduced torque.
| power spec for the 6500 just says anything between 100V and 250V. Can | someone clarify this for me. I just want to be sure I am correct in | assuming that 208V and 230V are the same in when it comes to powering | a devices. Obviously the wiring is different. 230V is 4 wires (115V | supply, 115V supply, return, and ground), and 208V has 3 wires (208V | supply, return, and ground)....
Computer switching power supplies can easily be made to handle dual voltages in two ranges, or with just a bit more cost (usually offset by the savings of producing and stocking a single part instead of two) can handle the full range from 100 volts (as available in Japan) to
250 volts (as available in China). The PSU that handle just two separate ranges (they have a "115/230" switch) may have some trouble with 208 volts. The ones listed as "100-250" or "100-240" should be just fine on 208.
In the USA, the 240 volts connection involves two hot wires, each being
120 volts relative to ground or neutral. This may pose some risk to appliances expecting one of the power supply wires that provide power to be grounded.
Not all circuits have the neutral. You will see that in appliances that need to have both voltages, such as a stove with a 120 volt light bulb in the oven. The typical stove and clothes dryer circuits have the extra neutral wire for the extra voltage tap. Other uses don't need it, such as electric water heater, or air conditioners (central or window type).
A computer power supply designed for all countries will have to deal with that fact that in many countries, there is no specific polarization that defines which wire is grounded. This is the case with the classic Europlug and continues with the Schuko (larger round grounded version of Europlug). These can be reversed and are expected (and now required) to be equally as safe. That means neither wire is allowed to be touchable. Neither can be connected to a metal case. And in many countries such as Germany, it is not allowed to use the large screw in bulb bases on plug connected lamps (because the rim of the socket is touchable and could present a 230 volt shock if plugged in a certain way). The computer power supply must be safe in dealing with this reversable plug. Rather than have extra cost circuit to detect a reversed plug and shut down, it's cheaper to just support either wire being hot by keeping them both isolated. That works out to an advantage for the USA style 240 volt circuit. The rocker style power switch controls both wires (not just one). So it should be just as safe on USA 240 volt power as it is on German 230 volt power.
208 volts, if that is what you have due to a two wire tap on three phase, would just be a "slight brownout" to something expecting 230 volts. It would be nominal for something expecting 100 to 250 volts.