Normal versus abnormal blue sparks under the playfield
With the playfield raised and the room dark, small blue sparks at switches are normal. The biggest should be at the flipper EOS switches, which break the low-ohm (often about 2 ohm) power winding. Any other switch sparking as hard as a flipper EOS has a problem in its circuit.
Systems
Related tips
Source: Repairing Electro-Mechanical (EM) Coin Operated Games to 1978, Part Two, Clay Harrell (pinrepair.com)