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PCBD-July2014

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42 The PCB Design Magazine • July 2014 by Amit Bahl siErrA CirCuiTs DESIgN FOR MANUFACTURINg Considering Creepage and Clearance column Several years ago, my company published a guide to the design and manufacture of PCBs that would carry very high voltage, potentials up to 40,000V. Much of the advice in that brief regards a narrow project category, and I have an idea that a review of the basic layout pre- cautions for any board that would carry what is considered hazardous voltage—greater than 30V RMS, or 60V dc—would have a much wider audience. Reiterating such rules now and again could save a lot of headaches and perhaps liabil- ity down the road. If the system or product for which your board is destined must meet IEC or UL standards, the minimum distance permitted between conduc- tors that are subject to hazardous voltage will include a wide margin for safety, which depends on the peak working voltage, laminate material, operating environment (moisture, particulates, altitude), circuit location relative to human ac- cess or proximity, and whether the traces in- volved are on an internal or outer layer. The spacing required on outer layers also depends on whether traces are coated or bare. Determining what spacing to use is simple, if your system or product requires compliance to international safety standards. Find the stan- dard relevant to the product category and ad- here to the tabulated values with respect to the details of your application as noted. For exam- ple, IEC-60950-1 (2nd edition) is the document to consult for most IT products to be sold in- ternationally that are powered from ac mains, or batteries, and applies both to the primary

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