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First off, I am aware that plugging an UPS into an inverter is a bit redundant. The only situation I can think of it being useful is in a car when you want full amps for cranking over the motor but still want to run equipment. Anyhow, besides the silliness of why would you plug a UPS into an Inverter, my question is why it will not work. I have been using a motor master 3000 watt modified sine wave inverter with an APC Back-UPS RS 1500VA. When it is plugged in the UPS gives a wiring fault error. I was thinking it may be that the Inverter is not grounded properly so I tied the earth ground pin from the UPS to the buildings ground and still the UPS say’s there is a wiring fault. I heard people say that UPS’s do not like inverters in the past, but the troubling part that I have is that I had the ups working off a Robin Subaru R1700i inverter series generator without any issues. Anyone out there have any thoughts on this matter?

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I ended up trying a true sine inverter, and that seemed to do the trick. I guess the ups did not like the square wave from the inverter.

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The wiring fault indicator usually means that there is either a bad ground or the pins have a reversed parity. Since you have checked the ground the pins are probably reversed.

So, this means that the two straight pins on the power jack need to be switched to make the UPC happy. With out knowing more about UPSs myself, I am not sure if this would cause any issues with how the UPS works.

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  • \$\begingroup\$ The pins are not reversed parity. I have checked all silly connections. \$\endgroup\$ – Rich May 27 '10 at 11:31
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Could be that high frequency stuff from the inverter gets through into the UPS and confuses it. You could test that theory by putting an EMI filter in line between the two and see what happens.

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  • \$\begingroup\$ I am thinking you are right, the UPS does not like the high frequency square wave. I am really wishing that I had looked at the generators output on a scope... but its since be brought back. \$\endgroup\$ – Rich May 27 '10 at 11:34

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