Conformity Magazine - March 2008 - (Page 24)

ESD Open Forum Cable Discharges Provided by the ESD Association A cable discharge event (or CDE) occurs when a charged cable is discharged either into a piece of electronic equipment – often, but not limited to computer modems and telecom input ports on products. Manufacturers of telecom equipment have know about this phenomena for some time, but modems and other ports (Ethernet, USB, etc.) are now common in a large variety of consumer products. Some of these manufacturers are just learning. . Q: A: What is meant by a “cable discharge event”? The discharge is composed of a very fast initial transient – much like a system level ESD event – followed by a slower ringing wave which can contain significant energy. The frequency of the ringing wave is determined by the length of the charged cable. Q: A: Q: A: What does the discharge look like? © Photographer: Srdjan Srdjanov | Agency: Dreamstime.com long as the cable is at a different potential than the victim equipment being connected to it, there will be an equalization of charge. Keep in mind that the cable could be charged either positively or negatively with respect to the victim equipment, so current could flow either into or out of the cable. If it’s the “victim” equipment that gets charged, it too could be either positive or negative with respect to the cable, so again the current could flow either in or out of the cable. To take it one step further, it’s possible for the cable to become charged again by the discharge from some ungrounded, charged equipment (a laptop perhaps?). The next laptop that gets connected may NOT be charged, and the result will likely be a cable discharge event. How does the cable get charged up in the first place? Charging typically takes place via triboelectric effects – rubbing two materials together. For example, when an Ethernet cable is installed in an existing facility, it’s likely to be pulled from one point to another – up a wall, across a ceiling, etc. As it is being pulled, the outer layers of the insulation rub against wood, metal, tile or whatever material is in the way, and charging occurs. The charge is then transferred to the metal conductors of the cable capacitively. If the line is long, the charge can get quite high, resulting in voltages of several thousand volts with respect to other objects nearby. Good question – It may or may not be re-charged, but you can still have an ESD event involving the cable. Remember your ESD basics: an electrostatic discharge is simply an equalization of charge between two objects. As 2 Conformity marCh 2008 Q: A: If the cable is already installed and isn’t being moved, how does it get charged up again? The effect is not limited to long cables; however, longer cables can hold more charge. Cable capacitance is dependant on a number of factors but, for all cables, longer ones have more capacitance than shorter ones. Even relatively short cables can be destructive, although it’s widely assumed that ESD at the system level is generally a disruptive event. I managed to permanently disable the audio input of my laptop’s docking station by picking up an Q: A: Is this effect only for long cables? What about USB or other short cables? http://Dreamstime.com

Table of Contents Feed for the Digital Edition of Conformity Magazine - March 2008

Conformity Magazine - March 2008 NewsBreaks Editor's Note Adelstein Renominated to the FCC Industry Canada Publishes FAQs On RF Fields FDA Gives Its Scientific Capabilities Poor Marks FDA Offers Guidance On its Product Review Process FDA Publishes Updates to List of Recognized Standards FDA Warns of Patient Risk From Electric Dental Handpieces Complying with the EMC Directive, Second Edition ESD Open Forum Emerging Issues in Standards - Korean "RoHS": Not Like RoHS at All The iNARTE Informer Challenges in Testing - Field-Induced Charged Device Model: What Really Happens Design Fundamentals - EMI Shielding Technologies for Consumer Electronics Equipment Focus On...Compliance Software Buyer's Guide Additional Standards Published for the EU's ATEX Directive Additional Standards for the EU's Medical Devices Directive Also Published China to Require Bar Codes to Enhance Product Safety CPSC Releases 2007 Self-Assessment CPSC Actions in News IEC Standards Update UL Standards Update Product Reviews Telcordia Standards Update From Our "You Can't Make This Stuff Up" Department Looking Back: Items from Past Issues of Conformity Advertisers

Conformity Magazine - March 2008

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