A process for polymerizing or copolymerizing ethylenically unsaturated monomers at pressures in the range of from 110 MPa to 500 MPa in a production line comprising a continuously operated polymerization reactor, wherein the process comprises monitoring the surroundings of the production line with respect to an occurrence of a leakage of monomers or of reaction mixture by an IR point detector arrangement of at least three groups of IR point detectors which are capable of detecting hydrocarbons and the groups of IR point detectors are operating according to a voting logic and starting automatically an emergency pressure release program when a group of IR point detectors of the IR point detector arrangement detects the presence of hydrocarbons.