Using an EMI Filter on a MSW UPS unit

[KJ-90]

I am wondering if an EMI filter, including those made to European standards whose attenuation starts at 10KHz, can be used to filter the output from a 120 Vac, 60Hz, modified sine wave UPS. On my DSO, the waveform looks almost like a sine wave, except at the very top and bottom peaks. The UPS, which is an industrial rack mount unit, is a FCC Class A device with 44.8% THD and a maximum single harmonic of 33.1%. The concern I have is that if the EMI filter is operated like this, the operating conditions would stress the filter components ( especially the capacitors ), and a rather high noise voltage with respect to ground would be generated. I don't have a spectrum analyzer & attenuators to analyze the UPSs waveform and make the respective calculations. Any ideas?

[floobydust]

An EMI filter does little at mains frequencies. The L-C values in the EMI filter are too small to help. Even 15th harmonic (900Hz) would pass-through.
Higher frequencies (MHz) are shunted to GND through the Y-caps, unless it's a medical-grade filter.

If you want to filter mains and a few related harmonics, you need a large output inductor.

I'm not sure what are you trying to filter, and what load are you powering from the UPS.