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I have a board (Krida electronics, not mine) that uses 2-pole PCB screw terminal blocks with 0.2" (5mm) pin spacing for Line & Neutral input. The conductive parts inside the block are very close together. Worst case scenario, there could be a 0.0275" (0.7mm) air gap between conductive parts. As a hobbyist, this normally freaks me out (I usually leave 5mm air gap for L-N separation).

Is there an equation or UL spec that I can use to calculate if this gap is safe?

Also, before someone suggests this: If this was a permanent installation, I would insulate the terminals with hot melt (assuming the device didn't hear up too much)

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    \$\begingroup\$ Potting it with something is a good idea, but hot glue is a poor choice. It will come off after a few months. Use an epoxy, perhaps Hysol. \$\endgroup\$
    – pericynthion
    Commented Apr 12, 2017 at 3:47
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    \$\begingroup\$ Adding coating will "only" give you polution degree I instead of II and will lower your creepage requirement but not clearance. If it's a UL product, you need to prove to your agency the dielectric withstand of your solid isolation and the flammability of the same. \$\endgroup\$
    – winny
    Commented Apr 12, 2017 at 5:29
  • \$\begingroup\$ Both good pieces of info. In my case, it's a prototype that only needs to last a week or 2. The board is definitely not UL listed. I potted it with hot melt to keep the wires and conductive parts of the terminals in place. It seems like the plastic insulating the terminals would be substantial enough if only it didn't wiggle. \$\endgroup\$
    – Joshua Penn
    Commented Apr 12, 2017 at 5:36

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0.7mm of clearance is pretty small. If functional insulation is all you are really required to have from a primary to a primary, then 1.5 mm is sufficient.

The online calculators at http://www.creepage.com/ are pretty faithful to the IEC (or UL if you prefer) 60950 specification, which dictates safety measures for Information Technology Equipment.

(edited from 1 mm to 1.5 mm. I didn't take into account that the peak working voltage would be as high as mains voltage)

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  • \$\begingroup\$ Thank you, this was the exact kind of thing I was looking for. I was searching for the wrong thing on Google. \$\endgroup\$
    – Joshua Penn
    Commented Apr 12, 2017 at 2:54
  • \$\begingroup\$ I can't think of a single product which is allowed to have just functional isolation between L and N. \$\endgroup\$
    – winny
    Commented Apr 12, 2017 at 5:26
  • \$\begingroup\$ Personally I would want to provide for more clearance than the minimum as well, but best to start with the facts and go from there. Figure 2H of IEC 60950 allows functional insulation between primary circuits. And if the purpose of clearance is mostly to guard the user from mains transients, I suppose an arc between L and N isn't necessarily going to be something the user is likely to be exposed to. \$\endgroup\$
    – 1N4007
    Commented Apr 12, 2017 at 13:21
  • \$\begingroup\$ @1N4007 Start your comments with @ followed by username or he/she will not be notified. I would rather take a flashover from L to N too over primary to secondary but for (a CE) product saftey, you are not allowed to rely on the main breaker to keep you safe. \$\endgroup\$
    – winny
    Commented Apr 12, 2017 at 14:33
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    \$\begingroup\$ @1N4007 They test it against a somewhat controlled impedance source which has "no fuse". If your failure on the mains, except for the CENELEC approved mains cable itself, you are not allowed to blow the test apparatus fuse. I agree this is a fair bit away from risking any end user coming to harm, but that's the standard you test against. \$\endgroup\$
    – winny
    Commented Apr 12, 2017 at 16:33

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