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It is a snapshot from an AC/DC converter. do I need to consider "dist. 1" or "dist. 2 at" the below image to measure the correct clearance space.The caps are both Electrolytic that have an aluminum body.

enter image description here

If I need to refer the dist. 2, isn't there an issue on the below reference design on the marked part?

enter image description here

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  • \$\begingroup\$ creepage.com although solder mask is a good vertical insulator it does not help with surface creepage and pollution. So I suspect the referenced blue part is non-compliant. \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Mar 17 '17 at 16:19
  • \$\begingroup\$ dist2 is to the silk screen, that's a don't care. dist1 solder mask don't care. Clearance is relevant to copper, and board edge cuts. \$\endgroup\$ – Misunderstood Mar 17 '17 at 18:24
  • \$\begingroup\$ @Misunderstood dist1 is not solder mask, it's between copper fills \$\endgroup\$ – pipe Mar 17 '17 at 18:59
  • \$\begingroup\$ @pipe Opps, that would then make it a valid clearance issue. Thanks. \$\endgroup\$ – Misunderstood Mar 17 '17 at 19:08
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The can of the capacitor should only be assumed to have functional insulation between itself and its pins. Therefore in this case the clearance should be between the bounding surfaces of the electrolytic capacitors on both sides. That could be dist2, although @Misunderstood pointed out that that marks the silkscreen, not necessarily the outside of the cap.

In your first picture it actually looks like there is less clearance between the top of that same capacitor and the copper adjacent to it. I would think that would be the shortest distance and therefore the clearance.

As for your second question, the amount of clearance required depends on a few factors which are not clear based on what is known about the circuit. For example, mains transient voltage, and the pollution degree of the board. See also Section 2.10.3.3 of IEC 60950-1.

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From IEC 60335:

clearance shortest distance in air between two conductive parts or between a conductive part and the accessible surface

I wouldn't consider the plastic wrapping of the capacitor to be a "conductive part". However, any exposed case at its top or bottom certainly is. Also consider that the capacitors could be assembled not perfectly vertical, possibly reducing the clearance at the top if they are nearly the same height and leaning toward each other.

Err on the side of caution.

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  • \$\begingroup\$ To your point of erring on the side of caution, even if the whole can were wrapped in the plastic, it's not known whether there are pinholes in the plastic wrap of the can. Unless that plastic wrap passes tests appropriate for solid insulation, it should be assumed to not be there for the purposes of clearance. \$\endgroup\$ – 1N4007 Mar 17 '17 at 21:10

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