Here are some links for those who are interested:
xtheblackknightx gave this basic intro link in another thread: https://electronics.howstuffworks.com/everyday-tech/question110.htm
Clay states: http://www.pinrepair.com/em/index3.htm#shock
" Installing a New Power Cord.
I always replace the power cord on any EM game I fix up. The originals are usually worn, or the insulation is cracking. You can replace the original two prong cord with another two prong power cord - just go to the local Dollar store and buy a 15 foot two prong extension cord for $2. Cut off the recepticle end of the cord, and you have a new power cord for a nice low price. Note the new power cord should have one line with "ribs" in the rubber insulator. This is the white or "neutral" line. The "hot" (black) wire is the new power cord's "smooth" line (which connects to the narrow power prong.) The hot line should connect to the game's power switch (if the game has one) and fuse.
For better safety, it's not a bad idea to replace the game's original two prong power cord with a grounded three prong cord. Again, the same technique can be used - buy an inexpensive three prong 15 foot extension cord at the Dollar store and cut off the recepticle end. Also again the new power cord should have a "ribbed" insulator wire which connects to the larger of the two power prongs. This is the "neutral" (white) wire. The "hot" (black or smooth wire or narrow prong) should connect to the line going to the power switch. The green ground wire should connect to the transformer's metal frame bolt. Optionally, additional grounds can then be run from the transformer's metal frame to other metal objects (like the lockdown bar and metal side rails and metal leg plates).
Flipper Button Shocks.
Just like the coin door button, flipper buttons can often give a nice shock too. This usually happens if touching both flipper buttons. Gottlieb used metal flipper buttons from 1960 (wedgehead cabinets) all the way into the 1970s, so this problem is fairly common. Again just like the coin door replay button, there is fish paper which insulates the the metal activator from the flipper switches (which are 30 volts). If the fish paper wears or breaks, the player can get a shock from the flipper buttons. Replace the fish paper (or use electrical tape) and the problem should go away.
Grounding the Game.
Another good idea is to ground your game. Installing a NEW three prong power cord and plug is a good idea. The original power cord is probably 25+ years old. Remember the black power cord line is "hot", and should run to the game's power switch. On the old power cord often there is a "rib" on the wire which denotes the black (hot) power line. Run the power cord's ground wire to the metal frame of the power transformer. Then run a wire from the transformer's frame to the back end of the metal side rails. Run another wire from the front end of the side rail to the coin door and lock down bar. Lastly, run a wire from the coin door to the other side rail. While you're at it, it may not be a bad idea to add a power switch to your game too (as discussed previously in the Typically What's Wrong section)."
Yet, in another part of his site, Clay says he just replaces 2-prong with 2-prong. "At this point I'm usually replacing the power cord too, using a dollar store 15 foot two prong extension cord" (http://www.pinrepair.com/em/index3.htm#powersw).
Some other links (if you're interested in this topic):