We are now required to perform IEC 61000-4-16 (Conducted Common Mode Disturbances), 10 kHz to 150 kHz, on our products (protective relays for electrical system control). One of the specifications for the test is that the total harmonic distortion (THD) of the applied disturbance signal needs to be below 1%. My question is what definition of THD did the committee have in mind when they put together the standard?
When I look around on IEC's web site http://www.electropedia.org/ I find this: IEV ref 551-17-06: the ratio of the rms value of the harmonic content of an alternating quantity to the rms value of the fundamental component of the quantity IEV ref 702-07-62: the total harmonic factor at the output of a transmission channel or two port device producing harmonic distortion, when the input signal is a sinusoidal oscillation of specified frequency and amplitude IEV ref 103-07-32: ratio of the rms value of the harmonic content of an alternating quantity to the rms value of the quantity What this all comes down to for IEC 61000-4-16 is that the rms value of the harmonics must be below 0.01 (-40 dB) of the fundamental (give or take a smidge, depending on whether the harmonics are compared to the fundamental alone or the total signal. In practical terms, the ratios are the same at this distortion level). My comment (and thus question): 1. This low level of harmonics is unnecessarily strict. 2. The effect on level of the disturbance signal is minimal even with total harmonic power at -20 dB relative to the fundamental. 3. It can be difficult to achieve -40 dB with broadband RF signal generators and power amplifiers. 4. Harmonics as high as -6 dBc are allowed for IEC 61000-4-3 radiated immunity. 5. Harmonics as high as -15 dBc are allowed for IEC 61000-3-6 conducted immunity (which starts at 150 kHz, exactly where IEC 61000-4-16 stops). I am wondering if the IEC committee really intended this, or if they were thinking of total harmonic distortion as a ratio of powers, as some definitions hold it to be. This would put the total harmonic power at -20 dB relative to the fundamental, which is within the performance specifications of standard broadband RF equipment. Donald Borowski EMC Engineer Schweitzer Engineering Labs Pullman, WA, USA - ---------------------------------------------------------------- This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. To post a message to the list, send your e-mail to <[email protected]> All emc-pstc postings are archived and searchable on the web at: http://www.ieee-pses.org/emc-pstc.html Attachments are not permitted but the IEEE PSES Online Communities site at http://product-compliance.oc.ieee.org/ can be used for graphics (in well-used formats), large files, etc. Website: http://www.ieee-pses.org/ Instructions: http://listserv.ieee.org/request/user-guide.html List rules: http://www.ieee-pses.org/listrules.html For help, send mail to the list administrators: Scott Douglas <[email protected]> Mike Cantwell <[email protected]> For policy questions, send mail to: Jim Bacher: <[email protected]> David Heald: <[email protected]>
