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
Leslie: 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).
