The safety curtain, or 'Iron' drops to become a barrier between the audience and the Stage areas. This allows the audience sufficient time to escape from a venue if fire breaks out on stage.
The curtain is usually not a curtain at all, but a steel screen, with a fire resistant skin. It is very heavy and drops on counterweights, often with a hydraulic or pneumatic braking system. The weight of the 'iron' can be many tons, and most venues ensure that it is able to drop without obstruction by preventing scenery or other items being set beneath it.
There is a long held view that the regulations require a safety curtain to be operated in sight of the audience at least once during a performance, although this is not a legal requirement in every venue (depending on the venue licence issued by the local authority, in most cases). The iron is normally winched back to the raised position by electric motor but some older installations still have a manual winch system.
Operating the safety curtain regularly is vital. The annual inspection sets a prescribed time limit for operation - measured from pulling the release lever, to the moment the seal with the floor is made. This is normally 30 seconds.
Many safety curtains are also fitted with a drencher system. This uses a perforated pipe (much like a garden sprinkler hose) that sprays water onto the safety curtain and/or the house tabs to add another protection 'layer'. The water supply normally comes direct from mains pressure, although some systems use a header tank. It is very difficult to test a drencher due to the mess and damage it can cause. Some systems have a test cock so the flow to the drencher pipe can be closed off, and water diverted to an outlet that can have a drain hose fitted. The seal fitted into older systems is based around a leather disc. Although remaining watertight, this seal is unlikely to seat properly if the system is tested, requiring a new seal - often made of neoprene - to be fitted as a replacement. Remote operation from another location is a common feature. In these cases, a small remote panel is situated at a convenient exit point, often near the stage door. The controls allow the iron to be dropped and the drencher system started. There are obvious dangers in releasing the safety curtain from a position remote from the stage. For this reason, the remote panel should be in a position where accidental release is carefuly controlled, normally with a glass access panel that can be smashed to reach the controls.