Grounding EM pinballs is an essential safety procedure especially if the
game will be played by children. Often, a machine is located on a concrete
floor in a garage or basement. If a child (or anyone, for that matter)
comes in with bare, wet feet and plays a game located in a basement or
garage with a concrete floor, they risk receiving a bad electric shock.
I once received a bad shock by touching the low voltage terminals on a
garage door opener with wet feet on a concrete floor. By installing a
3-wire cord set, grounding the transformer, and all metal parts, you can
keep your games safe as well as fun to play. However, from a service
standpoint, grounding the side rails on the game can actually be hazardous.
If you are poking around inside the game with the playfield up, you may have
an arm resting on the side rail. If you are perspiring, your bare arm is
completing a path through the grounded side rail, thus subjecting you to a
shock risk. You should install an interlock which will interrupt the ground
to the side rails when the playfield is lifted up. Although most of the
circuits in an older EM pin operate on low voltage, the current is
sufficient to be hazardous or even deadly if it is conducted through your
body to ground.