MEP EP#288: We have clearance, Clarence

PCB design rules for clearances

• Stephen's favorite calculator
• Clearance and creepage
• According to the IEC (international electrotechnical commission) a global standards organization
  • Clearance - The shortest path between two conductive parts, or between a conductive part and the bounding surface of the equipment, measured through air
  • Creepage - The shortest path between two conductive parts, or between a conductive part and the bounding surface of the equipment, measured along the surface of the insulation
• IEC/UL 60950-1 Safety standards - has more in depth information on spacing and how to handle mains
• IPC2221 generic pcb design rules - Has simple information on spacing vs voltage
• The creepage requirements depend on
  • insulation material group
    • Comparative Tracking Index (CTI) is used to measure the electrical breakdown (tracking) properties of an insulating material. Tracking is an electrical breakdown on the surface of an insulating material wherein an initial exposure to electrical arcing heat carbonizes the material. The carbonized areas are more conductive than the pristine insulator, increasing current flow, resulting in increased heat generation, and eventually the insulation becomes completely conductive.
      • I 600V
      • II 400 to 600V
      • IIIa 175 to 400V
      • IIIb 100 to 175V
  • pollution degree
    • Degree 1 - no pollution
    • Degree 2 - non-conductive contamination that might temporarily become conductive due to condensation
    • Degree 3 applies to conductive pollution, or to dry non-conductive one which could become conductive due to condensation
  • working voltage
  • Altitude and elevation - Multiply by 1.48
  • Appliance insulation class
    • Functional
    • Basic
    • Supplementary
    • Double Insulation - Double the creepage
    • Reinforced - Double the creepage
• IPC2221 requirements - Actually tend to be larger
  • External
  • Internal
  • Coated

Stephen’s thoughts on the AND!XOR hardware puzzle