The mesh DOES NOT prevent vapors from igniting inside the alternator
, which is where the ignition source (the sparks) are. As I said before, the mesh absorbs so much heat form the flame front, that it quenches the fire before it can pass through to ignite additional vapors surrounding the alternator
The mesh does make the metal body of the alternator, which already functions as a Faraday Cage, more functional by closing off some of the holes in the cage. A Faraday Cage can be made of mesh, or bars, and the size of the holes in only relevant in relation to the wavelength of any electrical
charge hitting it, i.e. whether the charge is something like radio
waves, which penetrate differently depending on frequency, or something like a mass charge from a lightning
strike. A "perfect" Faraday Cage will be made of metal sheeting, with welded, soldered, or lapped seams, and a layer of mumetal or other material to absorb ay magnetic eddy currents that would penetrate the skin and cause additional damage.
The publicly accessible military papers discussing how to secure critical buildings against the EMP from nuclear blasts go into a great deal of detail as to just how cpmplex it can be unless you have been secured in a solid tin can. (Like a submarine undersea.)