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The DUT is a relatively high-power LED driver driven by a SELV high-power +24VDC supply. It'll drive LED voltages up to 55V by specification. So everything is considered safe as far voltages are concerned but the power levels are moderate, about 620W actual, 1.2kW design limitation.

Normally I don't thankfully have to deal with UL. But on this project it's a requirement. UL reps told me I need 13mm clearance on mounting points to a non-insulated "track" for ignition hazard reasons. Apparently surface copper in the earth potential is exempt but that further raises the question whether you then have to have 13mm clearance from earth to any other parts on your PCB.

I haven't previously seen this as a reason for clearance/creepage. Could anyone point out to me where is it actually defined? In the eye-wateringly expensive UL 60950-1? IEC 60950 (or 62368) is a bargain by comparison.

Mounting holes are bolted in this case into sheet metal plate that is in contact with device chassis/earth. A device like this certainly can go up in flames albeit the external PSU and on-board fuses should kick in on a catastrophic failure.

Yes, it'd be nice to question UL guys directly but getting answers from them is a long game.

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  • \$\begingroup\$ From what I can tell, the information is muddled here. The mounting hole is not considered an ignition source unless there bolt is used to carry current and this would create an arc PIS. Since the mounting holes are connected to chassis that's at the same potential as the device ground, it can be ignored. However the device itself contains many ignition sources and should comply with the 13mm base 50mm height cone clearance to any combustible (non V-1 or better) material. \$\endgroup\$ – Barleyman Aug 26 '16 at 10:01
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Section 4.7.3 in UL 60950-1 has a bunch of stuff on mechanically separating nonrated or rated less than V1 materials from possible ignition sources to prevent fire propagation. 13mm appears a number of times in this section.

A good engineering library should have a copy of the standard.

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  • \$\begingroup\$ If you're not designing for US market it's fairly redundant. I've got IEC62368-1 and -2 which supersedes IEC60950 and indeed there is a lot of material on PIS under section 6. However in this case the comment was that a mounting hole attached with solid steel bolt to a steel frame should have this clearance. I guess the ignition source would be a short from earth to any circuitry but the mounting hardware can hardly catch fire. \$\endgroup\$ – Barleyman Aug 25 '16 at 15:41
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This was indeed a case of misunderstanding. The mounting hole is not considered an ignition source in any event. 13mm rule only comes into play if you are using non YL94 V-2 rated plastic mounting plugs. Those are considered "fuel" so you have to keep distance from ignition sources. This would be a cone with 13mm base and 50mm height representing "flame".

In case of steel bolts you can completely ignore all that and park your mounting holes as close to high power sources as electrical and mechanical clearances allow.

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