Remember that the LISN started out in the pre-semiconductor age, so LISN transients were much less of a danger. My major concern in using LISN's was that I was usually testing large military systems, and the test specimens were often rather buggy when they reached my lab. Unexpected modes and surprise shutdowns could be expected, so sudden 100% drops of current could create a dangerous transient.
OTOH, the LISN should now be standardized with built-in measurement port protection. I would suggest that the design should have a serious high-pass filter (to minimize AC line harmonics from overloading the receiver) and transient limiting. Perhaps it should include a 3 dB attenuator; I think the traditional external 10 dB is a bit of overkill; it's nice to be able to note throw away any more sensitivity than is necessary. This protection might be a problem for anyone still doing injections through the measurement port (does anyone still do that). Ed Price WB6WSN Chula Vista, CA USA From: Doug Powell [mailto:[email protected]] Sent: Tuesday, January 20, 2015 4:59 PM To: [email protected] Subject: Re: [PSES] Design by committee disasters! I would agree as well. However, it is possible to put heavy filtering (capacitive) ahead of the LISN on the utility side to help absorb transients of external origin. If the EUT/DUT generates transients you could be in trouble. A simple 30 amp three phase contactor can cause this very problem. I've known many EMC engineers who have a habit of disconnecting the BNC at the input of the receiver only after energizing the EUT. Another problem with the 50 uH LISN is the high source impedance. When working with large power conversion equipment, it is often the case that the EUT presents a negative impedance load on the LISN. By definition an inductor in series with a negative impedance load is an oscillator at some frequency. Usually this frequency is the unity gain bandwidth of the closed loop control within the EUT. In years past, this was sometimes called a the "input filter oscillation", Dr. David Middlebrook wrote several papers on the subject. I have personally experienced this problem on a 120/208V, 100 Amp three phase feed from a motor gen set. The resonances in the circuit cause the line voltages to Q up over +/- 500 VAC. Mainly because the motor generator added to the total inductance. I ended up putting 3 x 60 uF motor run capacitors in a delta configuration on the three phase lines on the line filters outside the test chamber. Inside we used a 5 uH LISN (CISPR 16) to do the measurements. Since then, I designed and calibrated a tastkopf voltage probe with an inherent 20 dB pad and this seemed to help protect my reciever very well. Thanks ~ doug On Tue, Jan 20, 2015 at 4:40 PM, Ken Javor <[email protected]> wrote: Have to disagree with Doug, and do so with some trepidation. The original LISN was a 5 uH model designed for use on 28 Vdc power modeling an aircraft power-distribution system. Typical application was in a shield room utilizing filtered power, or at least a dedicated 28 Vdc power supply, so I can't see where the original design was fraught with danger. The same LISN was retained when aircraft went to ac power and used transformer-rectifier units to derive 28 Vdc power. Again, the application used filtered power. The same 5 uH LISN was used to evaluate conducted emission limits for office equipment leading up to the imposition of CE/RE requirements on non-antenna connected automatic data processing and office equipment in 47 CFR Part 15 back around 1980. The 50 uH LISN (to which Doug must be referring) was an attempt to keep the impedance closer to 50 Ohms over the range of the requirement, which at the time was 530 kHz to 30 MHz. I don't know if the ten-fold increase in inductance causes the problems to which Doug refers. Just that there wasn't a problem such as he describes in the original application. Ken Javor Phone: (256) 650-5261 <tel:%28256%29%20650-5261> _____ From: Doug Smith <[email protected]> Organization: D. C. Smith Consultants Reply-To: <[email protected]> Date: Tue, 20 Jan 2015 14:35:51 -0800 To: <[email protected]> Subject: [PSES] Design by committee disasters! Hi All, Here are some thoughts of mine on two examples of design-by-committee in the EMC field which ended, in my opinion, a poor outcome: First, is the LISN (line impedance stabilization network), used in conducted emissions testing. I can't believe that a design would be included in standards that can easily source a 1000 Volt transient out of an innocent looking BNC connector intended for connecting to a spectrum analyzer. But that is what happens and many people have burnt out the input of their spectrum analyzer by connecting it to a LISN. The LISN design should not rely on people realizing the BNC output cannot be connected to a spectrum analyzer and putting in various protecting circuits between the spectrum analyzer and LISN. Just on the surface, it seems the original LISN circuit was a concept proposal not a real design, or the designer was completely unfamiliar with the nature of the AC mains the LISN is used with, or both. Second, is the capacitive clamp used with IEC 61000-4-4, Electrical Fast Transients. By the way, EFT bursts as well as inductive kick are what causes the problems above with the LISN. The problem arises in that the capacitive clamp was poorly understood at the time it was included in the standard. It is quite directional and sends much more energy towards the auxiliary equipment than the equipment under test! In turns of peak current, the auxiliary equipment gets 30% to 100% more than the EUT, depending on the nature of how the common mode impedance of the auxiliary equipment interacts with the capacitive clamp. Not a good design! Does anyone else have examples like this? Doug - ---------------------------------------------------------------- This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. 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