About the protective role of optocouplers... One of their counter-measures is that when some really bad spike or short circuit or a strong static electricity discharge happens, the optocoupler gets fried but the protected part of the circuit remains untouched (cheap part dies to protect many expensive ones). Some people mount the optocouplers on DIP sockets so they were easy to replace in such case. What are your opinions about this?

BTW, what good protection does the optocoupler provide if the static electricity spark will jump from traces on one side to the other? 2-3mm gap is a good idea. Whole optocoupler width gap is even better. Just keep the two sections away, just like on separate boards, with optocouplers where the cables would go. This applies to all, ground plane, voltage plane, tracks, parts, radiators, mounting pins, screws, signs (in copper), metallic stickers, water cooling pipes and generally anything that -could- conduct electricity.

+--------+      +--------+
|        |      | unsafe |
| Safe   |      |circuit |
|circuit |=[oc]=|        |
|        |=[oc]=|        |
|        |=[oc]=|        |
+--------+      +--------+

           ^^^^^
        safety zone