The tramp metal detector prevents metal objects such as nuts and bolts from getting into the card. It does this by putting a small electric voltage on the feed rolls so that when a piece of metal goes between the feed rolls it causes an electric current to flow between the rolls. The detector picks up on this and stops the feed rolls and the card and sounds an alarm so you can pull out the fiber and find the metal. Even a small piece of metal can cause great and expensive damage to a card. Notice the flat strips of metal with screws that touch the ends of the feed roll shafts in the picture. These carry the electricity to the rolls. The rolls must be insulated from the card, and yes the rolls can shock you.