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Good day

I am making a pcb layout that is required to comply with the IEC 61010-1. In the standard, there is a table for clearances and creepages, which depends on working voltage, pollution degree and dielectric properties (CTI/comparative tracking index). However, I am a bit confused about the two columns: Printed wiring boards CTI >= 100 VS Material Group III CTI >= 100. Since our pcb uses a standard FR-4 material, I found out that FR-4 typical CTI is 175. And since I am making a pcb (so I am applying the spacing on track-track, track-pad, pad-pad, track-copper pour...etc), I figured that the right column I should be looking is at Printed wiring boards CTI >= 100, which tells me creepage should at least be 1.5mm for 250Vrms. Am I correct that this is the column applicable for my project?

Then where is the other column "Material Group III CTI >=100" used (this column tells me creepage is 3mm instead)? Because now that I think about it, I could also look at this column since FR-4 is under material group IIIA. I simply used the other column because it appears to be more specific (printed wiring board).

And the file provided to me (IEC file) is 2004 enter image description here

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It actually gets even nastier: if you coat the board the clearance become different (and creepage somewhat 'disappears', and pollution level decreases).

The reason for different columns is that you can have exposed terminals on things that are not circuit boards; for example some power relays have quick disconnects on the top and often they have a plastic ridge exactly to meet creepage distance.

Sadly I'm stuck with the 2010 edition (the last one is 2019) and the tables are slightly different. Also your table doesn't mention the overvoltage category which for mains circuit is strange…

I think (the standard doesn't state it explicitly, maybe there is some application guide) boards are treated differently because

  • There are explicit rules for inner and outer layers (mostly for clearance)
  • The microenvironment for the board is better than for the rest of the equipment
  • Board manufacturing tolerances are usually better than switchgear (0.05mm versus 0.2mm)

1.5mm of creepage seems a little low for me in degree 2, mains on dust tracks quite fast in my experience. By the way, my table has the same values with reordered columns (and no CTI reminder for the material groups)

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    \$\begingroup\$ Table 4 in the 2019 version of EN61010-1 explicitly specifies Overvoltage Category II in the table header. Perhaps OP's 61010-1 version is older than 2010? Anyway, Overvoltage Categories III and IV are specified separately in Annex K, so maybe Category II is implied in Table 4. \$\endgroup\$
    – 1N4007
    Jul 27 at 16:14
  • \$\begingroup\$ The one provided to me is 2004. So do you recommend that I instead use 3mm spacing instead (assuming I don't put slots) of 1.5 mm? And yes, our application is overvoltage cat II and max altitude of 2000m \$\endgroup\$
    – user139731
    Jul 28 at 3:10
  • \$\begingroup\$ If you can, the more the better. Slots between primary are not usually useful: since usually the smallest slot is 2mm considering mill to copper clearance you would be 3mm anyway (primary to secondary requires way bigger clearance and slot are useful). Most component for mains tend to have wider pitch anyways (like the 3.96 connector systems). I don't remember a 2.54 pitch component rated for 230VAC \$\endgroup\$ Jul 28 at 7:10
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    \$\begingroup\$ @user139731 I'd like to note that the choice of 3mm would not be because you would be choosing the Material Group III column. Choice of 3mm would either be because (1) you are required to have reinforced insulation in most situations, which means you would need to double the 1.5mm "basic insulation" distance anyway, or (2) as Lorenzo said, the more the better. Even if you were only required to have basic insulation, you would probably be encouraged to increase it in most cases anyway. \$\endgroup\$
    – 1N4007
    Jul 30 at 21:47
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    \$\begingroup\$ Main-to-main usually is often basic (if not functional), main-to-secondary in 99% is at least reinforced; criteria usually failing on all no-brand phone chargers \$\endgroup\$ Aug 2 at 5:54

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