If the generation of explosive atmospheres cannot be safely ruled out, the effectiveness of primary explosion protection measures has to be verified (prevention of the emergence of explosive atmospheres) by means of measurements. If it can be ascertained that a sufficient degree of safety is achieved, measurements are not necessary: This is the case when atmospheres (in the form of gas and vapours) have been shown to be 10 per cent below the lower explosion limit (LEL). Measurements are not required either if zones have been delineated and measurements of these are not needed.
