Arun: When I started dealing with EMC 13 years ago, I always thought ESD is conductive testing but later I found it is not only conductive but also radiative! The way of doing testing looks like conductive but the effect of ESD is beyond that.
Obviously, different dv/dt produce different field distribution - James C. Maxwell told us about 150 years ago - unless you can prove it is wrong. Different field distribution induces different current intensity in different portion of the EUT and results in different sympotum. That why if EUT can pass higher test levels does not mean it can pass lower levels due to the different susceptibility of the circuits exposed to different field distributions. If you only consider the conductive coupling , I guess you can not explain why EUT passed hihger levels but may not pass lower levels, can you? I also would like to share with you my experience when I did a TCF job years ago when I was working in Singapore. We know it is not possible to perform radiated immunity testing at the customer's workshop. It is just simply illegal! But how would you verify the machine's radiated susceptibility? ESD gun can help - think about it. Regards, Leslie --- Arun Kaore <kao...@sg.adi-limited.com.au> wrote: > I differ from this theory of radiated coupling in > air discharges. There will > be air discharge if the creepage/clearance is less > (than say 7mm for 8kV) > such that the disturbance voltage just arcs over, if > there is a path. > > The discharge current flows then via the shortest > path, corrupting > everything in the way, if it is not immune. We note > that the Immunity noise > threshold for most ICs is less than a couple of > volts. > > The easiest way we employ to fix air discharge > problems is by artificially > increasing the creepage by adding plastic foil or > increasing spacing. > > Schaffner has different probes, "loop types", which > are more suited for > radiative coupling (near field types) as you > suggest, but we use them for > pre-compliance purposes only. These are the truly > "radiative coupling > types". They are a poor man's friend for MIL STD > 461/462 CS06 (in air!!), or > CS115/CS116/ RS02 pre compliance. > > I cannot comment on your Fire Alarm system though, > but I would hazard a > guess and attribute it to HCP or capacitive coupling > via unshielded cables > spaced off the metal test sheet (the GRP). > > Regards > > > > Arun Kaore > EMC Engineer > > ADI Limited > Systems Group > Test & Evaluation Centre > Forrester Road, St Marys NSW 2760 > P O Box: 315, St Marys NSW 1790 > > Tel: 61 2 9673 8375 > Fax: 61 2 9673 8321 > Email: kao...@sg.adi-limited.com.au > <mailto:kao...@sg.adi-limited.com.au> > > > --------- > This message is coming from the emc-pstc discussion > list. > To cancel your subscription, send mail to > majord...@ieee.org > with the single line: "unsubscribe emc-pstc" > (without the > quotes). For help, send mail to ed.pr...@cubic.com, > jim_bac...@monarch.com, ri...@sdd.hp.com, or > roger.volgst...@compaq.com (the list > administrators). > > > > Leslie Bai wrote: ESD test is to verify the EUT > immunity due to induced > current (nonsense!). The current can be induced by > conducted coupling > (direct contact) or/and radiated coupling. My > understanding of the test is > that for contact discharge, conduct coupling is > dominate and for > air-discharge, radiated coupling is dominate. > Thus for contactt discharge, if you can pass the > higher level, you may not > have much problem with lower levels, but although > radiated coupling is not > dominate for contact discharge, the effects have to > be verified through > testing. > For air-discharge, ESD test is to verify the effects > of electromagnetic > field on the EUT - i.e. a kind of field immunity > test. Different levels will > have a different field distribution around the EUT > due to the different > dV/dt - Maxwell told us. > Thus, the induced current is (mainly) generated > by the electromagnetic field. > I tested one Fire Alarm system years ago. This > system has 128 ports all > connected with (at least 5m) twisted wires. I > noticed that the cable layout > can affect result (pass or fail) significantly when > doing air-discharge. > However, there is no noticable difference by varying > cable layout when doing > contact discharge. > That was my understanding comes from. > Rgds, > Leslie > __________________________________________________ Do You Yahoo!? Bid and sell for free at http://auctions.yahoo.com --------- This message is coming from the emc-pstc discussion list. To cancel your subscription, send mail to majord...@ieee.org with the single line: "unsubscribe emc-pstc" (without the quotes). For help, send mail to ed.pr...@cubic.com, jim_bac...@monarch.com, ri...@sdd.hp.com, or roger.volgst...@compaq.com (the list administrators).
