Grounding one side of UPS's output

[mikas_m]

Hi,
I'm not sure I used correct term in title, but I want to know the following:
I have 220 VAC that is comming from some kind of UPS (I don't have exact documentation). UPS contains some kind of DC/AC converter and its supply is DC power from batteries. I want to create phase and neutral wire that are outputs from the UPS. Since UPS's output is not grounded, when I measure voltage on both wires to earth (ground), I get about 110 VAC. Now I wonder if I can just connect one wire from UPS to bar in cabinet which is neutral and is grounded and obtain phase 220 VAC and neutral 0 VAC?
Thank you very much

 

[Lancie1]

The DC/AC converter may have a step-up voltage transformer with a center tap. The center tap would be the neutral or 0 point. If you ground one of the other legs, it may create a problem. A schematic of the converter would be necessary to know if one side can be grounded.

 

[10BaseT.]

Hi,
I'm not sure I used correct term in title, but I want to know the following:
I have 220 VAC that is comming from some kind of UPS (I don't have exact documentation). UPS contains some kind of DC/AC converter and its supply is DC power from batteries. I want to create phase and neutral wire that are outputs from the UPS. Since UPS's output is not grounded, when I measure voltage on both wires to earth (ground), I get about 110 VAC. Now I wonder if I can just connect one wire from UPS to bar in cabinet which is neutral and is grounded and obtain phase 220 VAC and neutral 0 VAC?
Thank you very much

Wasn't something like this attempted some time back by if I am not mistaken a Singaporean guy who tried something similar (possibly by mistake)

You have 220V single phase , and I am not sure why you would want to try and pull one side down , since the devices fed from the UPS are always fed CTE ( which is no bad thing for safety - though make fault finding slightly more troublesome)

I would tend to leave very well alone with this unless the UPS supplier will agree it .

 

[Eric Nelson]

Wasn't something like this attempted some time back by if I am not mistaken a Singaporean guy who tried something similar (possibly by mistake)

Yup... and HERE'S the thread...



-Eric

 

[mikas_m]

Hmm,
I'm not sure i really understand why would be a bad thing to do what I suggested.
Let's forget about UPS for one moment, same situation would be if I have simple transformer let's say 220VAC/220VAC (or X/220VAC), then I would need to use two pole circuit breaker since I don't have phase and neutral. If I connect one side to busbar (I definitely know that all neutral wires are connected to it in cabinet) then I'll have one phase and one neutral and I could use one pole circuit breaker.
If this is not a solution please, can you help me understand why, and what danger I can made to myself or others (if there is a danger)

 

[MarkTTU]

With a simple single-phase transformer you are correct, ground one leg and you have a grounded neutral.

With a center-tap transformer typically neither leg is grounded, but the center tap is. This gives you single-phase two-pole where each leg to ground is a known voltage and leg to leg is a known voltage that is twice the leg to ground voltage. This is how the vast majority (all?) of US homes are fed power.

Because you measure 110v leg to ground and 220v leg to leg I think it is likely that the UPS you have is designed with single-phase two-pole operation in mind; so there is already a ground reference, its just not the ground reference you want.

If you can picture what would happen if you grounded one of the legs in the center-tap transformer example you'll get a good idea of what will probably happen to your UPS if you ground one leg. My guess is you'll let all the smoke out of your UPS.

Note that I am making a major assumption based solely on your leg-ground-leg voltage measurements. It would be best to get the make and model of the UPS and then find its documentation.

 

[Lancie1]

Since UPS's output is not grounded, when I measure voltage on both wires to earth (ground), I get about 110 VAC.

Are you absolutely positive that the UPS does not have a connection to Earth somewhere? It could be through the frame or the case, or even though one side of the DC supply to the UPS. If there is no connection to Earth, then there is no reason that you should measure 110 volts from each leg to ground.

Did you measure 110 volts or 120 volts? 110 volts is the usual "minimum voltage at which equipment is guaranteed to operate without exceeding current limit or overheating", while 120 volts is the normal operating voltage. Here in the US, many of the residential power lines are 2300 volts, so the power companies use a 10-to-1 single phase step-down transformer, with a center tap on the secondary side. That would give 230 volts, but most have stepped up their supply voltage to 2400, so that residential voltage runs closer to 240 volts leg-to-leg, or 120 volts leg-to-neutral.

 

[mikas_m]

I'm not sure that the UPS doesn't have connection to the earth. Now I understand why is this fishy. Thanks guys, I'll try to search for the documentation.