Nonwoven cards are extremely expensive. Unfortunately a small piece of metal such as a nut or bolt can destroy the wire on a card and cause many thousands of dollars in damage as well as days of downtime. To prevent metal from being carried to the card with the fiber, the fiber is directed through a metal detector that surrounds the ductwork. The detector is a large electrical coil that can detect even small pieces of metal traveling through it. Once a piece of metal is detected, a valve is quickly opened in the duct and for several seconds the fiber going through the duct is directed into a collection box. If no more metal is detected, the valve is switched back and fiber flow to the card is reestablished. Thus, the metal remains in the collection box.
The whole system can pay for itself if it saves one reclothing of the card. The system needs to be tested frequently with a piece of tin foil to make sure it is working.