A fireman’s switch makes it possible to disconnect the lines between PV panels and the inverter. They allow firefighters to operate without risk of electrocution from live cables.
It is not unusual that the lines between the PV modules and the inverter can be long and are placed in cable routes together with AC cables. PV modules cannot be turned off by other means than shut out the light, hence the DC cables through the building are live. Live cables increase the hazard for the firefighters during a fire.