Harmonic loading-related performance of power line filters for shielded enclosures

Power line filters (PLFs) are used to maintain integrity of shielded enclosures by preventing unwanted signals from either entering or leaving the enclosure via the power supply conductors. These filters are intended to provide a high insertion loss-typically about 100 dB-above 14 kHz without affecting the quality of the power frequency voltage waveform that is fed to equipment inside the enclosure. Connecting nonlinear loads to these filters will distort the filter input and output voltage waveform, cause increased filter heating, and diminish filter attenuation performance. Most performance specifications require PLFs to have less than 1% fundamental voltage drop under rated load conditions and require a voltage-drop test at rated load. This paper shows that this test is inadequate if the load consists of typical electronic equipment, and proposes a new waveform quality test to augment the test described in military standards commonly used by industry. The new test will identify any degradation in voltage quality caused by the connection of a PLF between a power source and equipment in a shielded enclosure.