We came across this in a Sprinkler report for New Zealand.  It also mentioned OH1 Ordinary Hazard type 1 OH2 Ordinary Hazard type 2 OH3 Ordinary Hazard type 3 EHH Extra High Hazard LH Light Hazard We also came across this article from Australian Building Services which stated: The type of sprinkler system to be installed will depend on the hazard classification of the building according to the NCC (National Construction Code). The 3 main hazard classifications are :  Light Hazard Ordinary (OH1 – OH3) High Hazard The main difference between the 3 categories is the volumes of water and storage required increases with the hazard classifications. i.e High Hazard required more water, in higher quantities in a shorter time.