EC 135 Master Boxes
I have found the problem lies with the vent above the #2 Master Box. In forward flight the vent extracts air from the area, however in hover this does not occur. Because of rotor direction air and debris / water is blown up the vent. I found # Box with grass and sand inside it. Take the side panel off where the removeable circuit cards are and the top two cards will be contaminated. Fix for land operations is to fix a screen at the top of the vent (inside) plus if desired fix a screen above the vent holes on the Electrical Master Box. Water ops.... Haven't had to address that.
Hope this helps