|> | Powerware 9125, neutral straight through. I have a lab full of european |> | spec UPS that pass the neutral straight through. I have them from |> several |> | manufacturers and in sizes from 2kVA up to 50kVA, single phase and three |> | phase. Some do have a contactor to open the neutral when the UPS is |> turned |> | off. |>
|> Is that a 120 volt or 240 volt model? |>
|> So _why_ does the neutral need to be passed through on models that are |> dual conversion continuous online? Since not all models do this, there |> must be some reason that varies depending on targeted uses for various |> models. |>
| | It makes the bypass simpler since it only has to switch the line conductors.
You used plural. Are you assuming TWO line conductors?
In some locations, 220 to 240 volts is carried with two conductors where each is about equal distant from ground potential, sometimes at 180 degrees phase vector, sometimes at 120 degrees (and possibly even at 60 degrees in an uncommon setup). In other locations, 220 to 240 volts is carried with two conductors where one of the is grounded. Some of those locations may have a correctly wired polarized outlet and plug which identifies which of the two conductors is grounded. Others may have unpolarized plugs making it possible that either is grounded.
If the UPS assumes one of the input lines is grounded, and makes a new hot voltage relative to it, it could result in as much as 360 volts to ground on the output in the case of a split 120/240 volt system coming in, and it could result in as much as 480 volts to ground on the output in the case of straight 240 volts in where it assumed the neutral on wrong conductor, depending on the phase angle of input and produced voltages.
I talked to a support engineer at one of the UPS companies, which I beleive was Powerware, who told me that indeed most, but not all, 120 volt models did indeed pass the neutral through, and the 220-240 volt models specifically did not due to the variety of electrical systems around the world. Much of the world that normally utilizes 220 volts still does so with a L-L connection, either from a single phase 110/220 volt system, or from a three phase 127/220 volt system where usually just two of the phases are used. The only way to make a 220-240 volt UPS work in all locations, even if North America is not considered, is to NOT pass the neutral through, and switch all conductors in a bypass switch.