Yes. It's very effective for high frequency stuff like ESD.
The problem is that the HF stuff is just as happy to flow across the moisture 
on the surface of the PCB. It sees the opto as a bit of an impedance and 
takes the easier route - around it.
However, since its' ultimate goal is to get to the common earth point, giving it 
a track to ground is just too attractive and it will head down there, rather than 
across the opto and into your sensitive parts.

Steve.

On 20 Sep 2007 at 22:53, Xiaofan Chen wrote:

> Yesterday an engineer showed me a strange thing. It is an isolated DC
> input I/O with relay output. The isolation specification is low. So
> every input is going through an opto-coupler and the circuits are
> quite simple. Strange thing is that there is a long trace underneath
> all the optos and it is connecting to the common shield connection.
> Apparently this is not for safety reasons as it actually reduce
> clearance/creepage distance (still enough). So I can only guess it is
> for EMI reasons.
> 
> Any clue?
> 
> Regards,
> Xiaofan
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