I cannot find clear information on running mains and low voltage on adjacent layers on FR4 PCBs. I understand how much creepage distance to allow, but for example, can I run - 50Vdc, 240Vac, 0Vac, ground, on a 4 layer 1.6mm FR4 PCB. Would it be more appropriate to use a 6layer pcb, leaving blank layers between 50V/240Vac/0Vac?

Could someone point me to a standard on this? I have had not much luck with Kervill's Low Voltage Directive.


2 Answers 2


This is information you get from the datasheet of the prepreg/laminate you use. Example here from Isola: http://www.isola-group.com/wp-content/uploads/2012/09/IS420-Lead%C2%ADfree-Laminate-and-Prepreg-Data-Sheet-Isola.pdf

This specific material datasheet show something like 30 kV isolation per mm material layer to layer.

If you rely on this, make sure the material is fully specified in the PCB fab drawing.

Also note you can pretty much count on the laminate thickness, but the Cu traces will sink into the prepreg effectively reducing the resulting layer to layer distance.

  • \$\begingroup\$ Sounds correct. I'm surprised there isn't a more specific EN standard though "Leave 600V of insulation between 240Vac 0V layers" \$\endgroup\$ Commented Apr 24, 2013 at 14:27
  • \$\begingroup\$ @GarthOates, they may not have even thought of it because the numbers involved. Even a 4 mil (.1 mm) layer of the Isola material can withstand 5 kV. If you're "just" working with 240 or 110 V mains, pretty much any practical layer thickness will withstand those voltages. \$\endgroup\$
    – The Photon
    Commented Apr 24, 2013 at 15:47
  • \$\begingroup\$ AIUI It depends on the type of insulation needed. IIRC it's 1.5kV for basic insulation and 3kV for double/reinforced insulation at normal mains voltages. For functional insulation I don't belive any minimum is specified, it's left up to the designer. \$\endgroup\$ Commented Sep 30, 2021 at 2:57

PCB shall be constructed so that creepage distances are not less than those appropriate for the working voltage, taking into account the material group and the pollution degree. Hence creepage distance depends of the CTI's material (Comparative Tracking Index) and pollution degree. European main standard are for Household Appliance (EN60335) and Information technology (EN 60950). As Reference value, in the worst case (Household appliance), for reinforced insulation between High voltage (220 Vrms) and low voltage (<50Vrms) on the same layer (top or bottom), you should an 8mm creepage distance between tracks (Fig. 1) as required for standard EN 60335-1-2, table 17. These distance are lower with better CTI and Pollution degree. If you can’t maintain these distances, you need a milling (cutting of material large at least 1.5mm) between the two points that do not meet the minimum safety distances (fig.2).

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  • 3
    \$\begingroup\$ Thanks, but i understood the creepage - it was solely between layers I was looking at. \$\endgroup\$ Commented Apr 24, 2013 at 14:27

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