The controls upstream would depend on the product and process. We have a dry product, and ferrous metal contamination is our major hazard. We have sifter screens and magnets upstream, but the CCP is the magnet at the final pack-out station.
All of the magnets are inspected each shift, but they are not all CCPs and therefore most are not important to HACCP. HACCP is only interested in the CRITICAL control point that prevents any metal from entering the product. Since there are moving parts right before the final pack-out station, the magnets that precede the last one don't prevent metal from entering the final product. Only that final one makes sure it's all out.
For glass and brittle plastic foreign materials, we have a prerequisite program that makes sure that those items do not enter the product. So that hazard is prevented by that program.
That is how we handle foreign materials, hope that helps you consider how to analyze your systems.
Martha
"...everything can be taken from a man but one thing: the last of the human freedoms--to choose one's attitude in any given set of circumstances, to choose one's own way." Viktor E. Frankl
"Life's like a movie, write your own ending." The Muppets