Flame supervision device

For gas appliances, a flame supervision device (FSD) – alternative name: flame failure device (FFD) – is a general term for any device designed to stop flammable gas going to the burner of a gas appliance if the flame is extinguished. This is to prevent a dangerous buildup of gas within the appliance, its chimney or the room. Causes of flame failure include chimney downdraught, temporary interruption of the gas supply, gas under-pressure, liquid overspill on cooker hotplates or the draught from an oven door being opened and closed.