Ian: It sounds like the low-pass powerline filters that are being used with your shielded enclosure are very inadequate for removing low frequency noise. Here is a short description of the filters used on my shielded enclosures (a main EUT chamber and an auxiliary chamber for stimulus and monitoring equipment):
Electrical power (120 VAC 60 Hz Three Phase Wye, 120 VAC 400 Hz Three Phase Wye and 28 VDC) penetrates the RF shielded enclosures through low-pass EMI filters mounted on the enclosure walls. Power is internally distributed, within steel conduit, to convenience outlets and incandescent lighting fixtures. The primary chamber EMI filters for the 60 Hz power lines are 100 Amp rated Rayproof 4X100-60 units. The primary chamber EMI filters for the 400 Hz power lines are 50 Amp rated Corcom CDSRW2050A4-C units. The primary chamber EMI filters for the 28 VDC power lines are 100 Amp rated Rayproof ULW-100-400-4 units. All EMI filters have a rated attenuation of 120 dB or greater from 14 kHz to 10 GHz. The auxiliary enclosure is equipped with AC and DC power line filters with a rated attenuation of 90 dB or greater from 14 kHz to 10 GHz. All of these filters are at least 25 years old now, so the model numbers may not be current. Further, these filters were intended for a full-capability military EMC lab, so you probably will not need the amperage capacity of these filters. However, the attenuations shown are what you should prefer if you want your lab to be well-equipped. It is certainly possible that you could build a pair of heavy current low-pass filters to augument each powerline that penetrates your enclosure. I would be thinking of a double-pi section filter using three 10 uF capacitors and two 100 mH toroidal inductors as a first trial design. However, home-brewing a powerline filter is more complicated (and much more dangerous) than it sounds; it would be much safer for you to just buy some better filters. Take a look at places like eBay. Here's a typical, good quality 2-line EMI powerline filter selling for $600: https://www.ebay.com/itm/RAYPROOF-EMI-FILTER-14KHZ-TO-10GHZ-ULW-2X30-60/250998382668?hash=item3a70ab604c:m:mfuCE0sb33zzXWsoekkNGSA Ed Price WB6WSN Chula Vista, CA USA From: McBurney, Ian [mailto:[email protected]] Sent: Thursday, April 26, 2018 3:33 AM To: [email protected] Subject: [PSES] Mains Conducted rf emissions measurements Dear Colleagues. I am setting up a mains conducted rf emissions measuring system and am experiencing excessive mains noise when connecting the LISN to the 240V wall socket. The noise affects my measurements between 150kHz to 400KHz. Beyond that and up to 30MHz I am satisfied. Is there a way of filtering out this noise. I have tried various EMI mains filter modules but none appear to attenuate enough at that frequency band. They are very good at attenuating frequencies beyond 1MHz. I have tried inserting an isolation transformer before the LISN but this appears to distort the measurements. Can anyone recommend a solution? Many thanks in advance. Ian McBurney Lead Compliance Engineer. Allen & Heath Ltd. Kernick Industrial Estate, Penryn, Cornwall. TR10 9LU. UK T: 01326 372070 E: [email protected] Allen & Heath Ltd is a registered business in England and Wales, Company number: 4163451. Any views expressed in this email are those of the individual and not necessarily those of the company. - ---------------------------------------------------------------- This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. 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