The electrode should be centered in the gas passage between the adjacent collecting plates.  Allowable tolerances vary with a number of factors however, most manufacturers specify 1/4" of center for initial installation of wire electrodes in a new ESP.  Equally important is the position of the electrode relative to the stiffener baffles on the collecting plates.  Electrodes should be centered between stiffener baffles.  The stiffener baffles provide a "sharp" edge which can promote sparking when in close proximity to a discharge electrode.  Minimum acceptable clearances are usually 1-1 ½ times the acceptable cross gas flow tolerance.   Shrouded wire discharge electrodes help to prevent spark-over to the top and bottom collecting plate stiffeners.  In the TOP photo the discharge electrodes are not centered properly in the gas passage and need to be centered.  The barbed wire electrodes in the photo BELOW are in too close proximity to the collecting plate stiffener baffles and need to be repositioned.

Note the erosion of the discharge electrode wire shroud and collecting plate stiffener baffle due to spark-over from reduced clearance.

The high voltage connection is not centered in the bus duct and does not provide sufficient clearance.