Smart-UPS 1500 (SMT1500RM2U) output reverses polarity when on battery

[ginbot86_apc]

This was originally posted on APC forums on 4/19/2016

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I recently bought a Smart-UPS 1500 2U (SMT1500RM2U), and when I use a 3-prong receptacle tester, I noticed that the output's Hot and Neutral terminals reverse polarity when it switches to battery power. Once it goes back on-line, the polarity is correct again.

Is this typical of the Smart-UPS line-up? It seems like a bit of a design flaw for APC's newest UPSes to have this type of issue.

[vivekjm_apc]

This was originally posted on APC forums on 4/20/2016

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This issue is not unheard of. I can confirm one of my Smart UPS SUA2200I units reverses polarity at the outputs whilst running on battery. But I've not bothered to get it fixed. Maybe it's an internal wiring thing? Must check the other Smart UPS for this issue.

[UnexpectedBill_apc]

This was originally posted on APC forums on 4/20/2016

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The use of a "live neutral" configuration may be confusing your tester, although I'm not sure if the Smart-UPS is designed in that way or not. (I believe but have not proven that it is.)

[ginbot86_apc]

This was originally posted on APC forums on 4/20/2016

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I tried with another non-APC power inverter that I know has a live/floating neutral, and that causes all 3 lights to glow on the tester.

I had a chance today to try the tester on some other rackmount Smart-UPS models. I forgot to write down the model numbers, but I found the 120V models to have either hot-neutral reverse polarity, or a floating ground with all 3 lights lit on the tester.

Strangely enough, the 208V models I tried have proper polarity when on battery.

[BillP · Accepted solution]

This reply was originally posted by Bill on APC forums on 4/26/2016

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Hi,

When the Smart-UPS switch to battery it breaks Neutral (per UL requirements) and the Neutral floats. From H-N you will see 120v, but the split between H and N will fluctuate. I would be willing to bet that polarity tester is actually just looking for voltage on Neutral and that's why it trips. Kbase FA156549 "Why can I measure voltage between the ground and neutral wires when my UPS is on battery?" discusses the voltage when the Smart-UPS is on battery.