|> Where in the code is that? |>
|> Not all UPSes have a bypass switch. |>
|>
|> | Even in normal operating mode, it does not qualify. |>
|> I never saw that in the code. |>
| The UPS passes ground straight through. You cannot tie a "line" to ground, | it will create a fault. Therefore it is not separately derived. You can | try, but I would video tape it as it will be fun to watch after.
There is no more of a line tied to ground in the UPS than there is in a transformer producing the same system.
|> |> If the output is a pair of 120 volt inverters syncronized at either 180 |> |> degrees or 120 degrees apart (configurable) to get 240 volts or 208 |> volts, |> |> and these two inverters ARE connected to ground (not neutral as that is |> |> not even present), and the load on the inverter is connected line to |> line, |> |> how is this creating a situation for operating current on the ground |> wire |> |> at either the input or output? |> | |> | It is not a pair of 120V inverters. It is a single 240V inverter that |> can |> | be programmed to output 208 volts instead (pulse width modulated and |> filter |> | usually). |>
|> Then you can have problems with this if the UPS has a bypass switch, since |> it will change ground reference when the switching happens. | | The ground goes straight through the UPS.
As it should. So?
|> Suppose you do have an isolation transformer on the output. Would you |> design |> it so that the secondary winding has a ground tap in the middle, or not? | | No. I don't connect a load to ground so if I had a tranformer on the | output, I would tie one line to ground. This keeps any leakage current from | my load on the output side of the transformer.
And how is a UPS different?
| I think part of your problem is you keep thinking of UPS in terms of | transformers (phase angles between 120 l-g connections to get 208 or 240). | Power electronic devices to not have to work like transformers. Once you | have a DC bus you can do any thing you want.
I'm talking about the "dual conversion continuous online" type of UPS, the only type I ever consider using. It takes the AC input and converts it to DC. That DC is paralleled with the battery with some circuit that manages the usage in some way depending on how sophisticated that UPS design is. Then the DC feeds one or more inverters.
So I have a DC bus. I can do anything I want?
A bypass switch can be present, but some models omit this. I did see a UPS somewhere online once (a rather large one) that had 480V three phase in and 208Y/120V three phase out. I'd say a bypass on that would not be a very good idea. And is it a separately derived system or not?
I see nothing in the NEC that says the inverter is not a separately derived system. A UPS can create a different kind of system than its input. This is easily done when using separate component. It could be done as a whole UPS unit if there was a market demand for it (if it is done anywhere, it is almost certainly a special case). Actually, I did see one once that input three phase and output only single phase (but the loading was balanced on the three input phases, presumably with very near unity power factor).
Looking at just single phase to keep it simple:
A UPS system that has two inverters with synchronized waveforms could be connected as follows:
___Ground / *--[Inv1]--*--[Inv2]--* | | A B
There is no neutral, in or out. The source system may or may not have a neutral. If it has one, it is not used and not connected to.
The bypass switch, if present, is an open transition two pole device.
Where is the short circuit fault? I don't see one. Is there any load current on the ground wire? I don't see where.
You could replace Inv1 and Inv2 with two windings from a single phase transformer. How is this different? Or are you one of those people that always leaves transformer secondaries ungrounded?
But, just for fun, I'll add a transformer to the output:
Ground | *--[Inv1]--*--[Inv2]--* | | \ | | | -------- | ---* A B | | | | *---/\/\/\/\/\/\/\/---* | *---/\/\/\/\/\/\/\/---* | =================== | *--/\/\/\/--*--/\/\/\/--* | | \ | | | --------- | --* | | A B
That's a common 480/240 to 240/120 transformer wired with the 240 windings in parallel and the 120 windings in series so it's a 240 to 240 transformer.
In what way are these two systems different while the bypass switch is in the "not bypassed" position?
Of course if the load needs a neutral or any 120V loading, we can't use this scheme for that load because we can't have load current on the ground wire, on either side. But you could supply a neutral to the output system as a separate wire. That's commonly done on the transformer. I see no reason it can't be done on the UPS other than the fact that you cannot do a bypass on such a UPS because there is no neutral coming in.
For 208V output, the UPS can "bend" the inverters and run them at 120 degree phase angle difference. The inverters will need to handle the effective power factor they will get from that. Add a third inverter and you can do three phase delta in, wye-like out that can serve delta loads (a mix of single phase line to line loads).