Abus mk4100 door opening detector with external field protection 2m 4 core abus mk4100 p the abus mk4100 door opening detector with external field protection 2m 4 core is a magnetic contact for protecting large garage doors unauthorised opening is detected and reported to the alarm panel p p the door opening detector is mounted chiefly on roll up sliding and up and over doors while the detector consists of a magnetic contact reed and a magnet these are fitted parallel to each ot