>In other words, components in the secondary side must
>be 8mm away from anything on the primary side. I've
>never heard of this. Can anyone shed some light on this?

This is the reason many opto isolators for mains use come in a o.4 inch pin
space package instead of a 0.3 inch pin space. It is possible to meet this
requirement by spreading the pins of a o.3 inch package to a 0.4 inch pin
spacing on the PCB, but the arc tracking distance across the chip package
surface is only just meeting the spec with nothing to spare.

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