A shredder is disclosed having a jam proof system. In one embodiment, the jam proof system provides a thickness detector having a contact member which displaces as an article is inserted into a throat of the shredder and a resistance generating mechanism configured to provide a resistance force to the contact member, in response to displacement of the contact member. The greater the thickness of the material the greater the resistance force that will be realized. When the material reaches a predetermined thickness, there will be a significant change in the resistance force. The resistance generating mechanism may include at least two spring mechanisms serially arranged, such as, a first spring mechanism and a second spring mechanism. This feature provides immediate and direct feedback to the user that the article inserted into the shredder may be too thick. In addition, the thickness detector may include a sensor, and in particular, a Hall effect sensor assembly configured to measure the thickness of the article inserted into the throat. The sensor may communicate with a controller that is configured to alert the user, and/or alter the operation of the shredder, in response to the thickness of the material. For example, the controller may visually and/or audibly alert the user, or control the shredder motor response.