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?