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Old 11th Dec 2018, 9:46 pm   #1
Cathode Ray
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Default UPS as Isolation transformer?

Folks,
Would a computer UPS act as an Isolation transformer?

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Old 11th Dec 2018, 10:02 pm   #2
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Default Re: UPS as Isolation transformer?

I suspect not.
It depends on entirely on the UPS in question, some might be fully floating when used in battery backup mode, some will probably tie the N-E together throughout, besides when running normal bypass mode (mains healthy) they will in effect be direct online.
Also the cheaper end ones run with a pseudo sine wave or more likely a rough approximation of something resembling a stepped square wave even in battery backup mode.
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Old 11th Dec 2018, 10:27 pm   #3
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Default Re: UPS as Isolation transformer?

Thanks, I suspected as much regards to the N and E.
But just wanted to check.
If I ran it in battery mode ( unplugged from mains) I guess it would by default be isolated.

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Old 11th Dec 2018, 10:56 pm   #4
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Default Re: UPS as Isolation transformer?

Yes true, but the N-E could sill be connected together at the output side of the UPS internally even in battery backup mode so do check.

Edit: the N-E won't be connected together on the mains/primary side or else it would trip any RCD connected at source, however the battery backup side might.
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Last edited by Red to black; 11th Dec 2018 at 11:03 pm.
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Old 12th Dec 2018, 12:00 am   #5
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Default Re: UPS as Isolation transformer?

Nice one, I'll check it
Thanks

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Old 12th Dec 2018, 9:25 pm   #6
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Default Re: UPS as Isolation transformer?

Just another thought....
Wouldn't two of those building site 110v isolation transformers placed back to back do the job ?
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Old 12th Dec 2018, 9:34 pm   #7
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Default Re: UPS as Isolation transformer?

You could do, however don't connect the two 110V earths together (safely isolate these) as this is a centre tap as well on these transformers it could cause phasing/shorting problems and it also carries the Earth connection across from the primary side, which you don't want on a fully floating supply.
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Old 12th Dec 2018, 10:09 pm   #8
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Default Re: UPS as Isolation transformer?

Excellent, I just happened to have on of these in the work shed.

Now I also happen to have a 220/110v auto transformer which is not an isolation transformer.
But it stands to reason I could use that combined with the above isolation trafo to create a 220v to 220v isolation system

Your point on the earth's is well made , I should just safely ignore these and leave unconnected.
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