Hi Brian K:
The cross-sectional area of the trace must be capable of carrying the maximum continuous fault current without exceeding the temperature rating of the PWB base material. The key is the "maximum continuous fault current." If the circuit is protected by a UL/CSA fuse, the maximum continuous fault current is 110% of the fuse rating. Every circuit has a current limitation, either the source impedance or an overcurrent protective device. The continuous fault current determines the temperature rise of the conductors. As a general rule, a conductor size based on continuous fault current will also be satisfactory for short-term currents exceeding the continuous fault current. This is because the short-term fault currents usually don't exist long enough for the conductor to heat to a point where the conductor or base material is damaged by the heat. >From your message, I'm not sure your fault currents are maximum continuous fault currents. Put a variable load on the source, and adjust for maximum continuous power (or current). If the fault current pathway is from the board to a metal part as, for example, via the mounting means of the PWB, then the mounting means must not be subject to loosening with time. Because most PWB base material is polymeric, it is subject to cold-flow when under pressure. So, the design of any mechanical connection to the PWB must take cold-flow into account. Best regards, Rich Richard Nute Product Safety Consultant San Diego ----- Original Message ----- From: "Kunde, Brian" <[email protected]> To: "emc-pstc" <[email protected]> Sent: Monday, February 05, 2007 6:48 AM Subject: Protective Earth through PCB traces > Members, > > There was an article in one of the many trade magazines we get a few > years back about running PE or Safety Earth ground traces on a PCB and > how they needed to be designed so not to get damaged during a short or > overcurrent situation. I can't find this article so if anyone help me > out I appreciated it. > > We are using a "Distribution Card" to power AC sub assemblies in our > product. So we want the PE traces in this distribution card to handle > the fault current without damage. The sub assemblies have a fault > current in the 5 to 10 amp range depending on the assembly. > > Our CAD guys just want a current value for the PE so they can plug into > their computer and it will make the trace width wide enough to handle > the current. So is X2 the fault current enough? I thought the article I > mentioned above recommended X3 or X4. > > Appreciate any recommendations. > > The Other Brian. > - 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 ______________________________________________________________________ This email has been scanned by the MessageLabs Email Security System. For more information please visit http://www.messagelabs.com/email ______________________________________________________________________
