I am trying to find any documentation on what the purpose is of a Zero-Crossing Circuit TRIAC Optoisolator. The datasheets don't explain the concept well enough. If you do answer, please include references or explain how you found out. Thank you!
Zero-crossing is typically used for incandescent bulbs. You may have noticed that when incandescent bulbs fail they always fail when they're switched on. That's because the mains phase can be near its maximum when switching on. Combined with the low resistance of a cold bulb this results in a high current peak, which may burn the filament. When you switch on a zero crossing you avoid these peaks.
How I found out? I've known this since my time in college. It simply makes sense.
Zero Crossing refers to the load voltage at which the triac will switch.
Wikipedia has a little information on the theory of Zero Crossing.
Basically, the triac will hold off its switching until the alternating waveform of the load signal crosses the 'zero' or midpoint of the waveform. This helps alleviate sudden voltage spikes when the switched load jumps from 0V to say 100V in one instant. By ensuring that the load is only switched when it's waveform is crossing the mid-point, the voltage increase will be a smooth rise from 0 to maximum.
According to my research, a zero crossing detector is used specifically for protecting a connected load. It does not exactly matter whether it's with a triac or directly with mains. As mentioned in one of the replies above, a load could most vulnerable at the introduction of a sudden peak level of the AC phase during a power switch ON, a zero crossing sensor ensures that the load is always switched ON at the first zero crossing of the applied AC phase, thus safeguarding the load from the dangerous peak levels.
With triacs a zero crossing could be useful for protecting the loads as well as for reducing RF interference.
I have tried to explain the concept comprehensively in the following article