On 4/23/2007, Ernest Rayman wrote:
We are laying out our first product with an on board phone interface. I am curious as to how much spacing is generally needed between traces to pass the part 68 surge and hi-pot test. I found one reference that said 25 mil and that the power and ground should be voided around the phone traces on the network side. It is a 4 layer board. Hi Ernest: In addition to the FCC Part 68 requirements for lightning and hipot, you will likely have to comply with the applicable UL standard, which is typically UL 60950 if your product is an ITE product. The tests in FCC Part 68 are purely functional tests, so no explicit creepage and clearance distances are called out. To meet the functional requirements in FCC Part 68, a conservative value to use for creepage/clearance between tip/ring and ground is 25 mils, and a conservative width to use for traces that will carry the full lightning current is 20 mils (assuming 1 ounce copper). UL standards typically call out specific minimum creepage/clearance distances for safety isolation. In UL 60950, the specified minimum creepage distance between tip/ring and SELV circuits usually works out to 1.6 mm (about 62 mils), although the value that applies to your product may be different depending on several variables that are called out in UL 60950. Some applications experience higher level lightning surges than the ones simulated in FCC Part 68. In actual field installations where the primary protector at the service entrance is not functioning properly, peak lightning levels can reach 5 KV. Depending on your circuit design, it may be easy to extend your immunity to common mode lightning to 5 KV by simply increasing the separation in your isolation barrier to 5 mm or so. Regarding your reference to voiding the power and ground planes under the tip/ring circuits, this is generally a good practice. It helps reduce potential problems with isolation, noise, and balance. There are some limited circumstances where a ground plane can be useful for shielding, but this has to be implemented very carefully to avoid creating other problems. I should mention that for people who are new to phone circuit layout, a common misconception that spacing rules apply to the separation between tip and ring, or between other traces in the tip/ring circuitry. From a regulatory point of view for FCC Part 68 and for UL 60950, the spacing requirements only apply to the isolation barrier between the tip/ring circuits (treated as a group) and other circuits that are either earthed or accessible (treated as a group). The only possible exception is the M1 overvoltage test in UL 60950, where a small portion of the tip/ring spacing needs to stand off 600 VRMS. The remaining spacings within the tip/ring circuitry only need to be large enough to meet your self-imposed goals for proper operation of your circuit. In general, there will be certain areas within your circuit where the voltage between two circuit nodes can get high under fault or surge conditions, but outside of these areas the node-to-node voltages will usually be quite small. Joe Randolph Telecom Design Consultant Randolph Telecom, Inc. 781-721-2848 (USA) [email protected] http://www.randolph-telecom.com <http://www.randolph-telecom.com/> ______________________________________________________________________ This email has been scanned by the MessageLabs Email Security System. For more information please visit http://www.messagelabs.com/email ______________________________________________________________________ - ---------------------------------------------------------------- This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. Website: http://www.ieee-pses.org/ To post a message to the list, send your e-mail to [email protected] 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] All emc-pstc postings are archived and searchable on the web at: http://www.ieeecommunities.org/emc-pstc
