Almost all modern gas ovens and hobs now have built-in 'flame supervision' devices. This means that if the flame unexpectedly goes out because of a draft, spillage, or break in the gas supply, the flame supervision device will stop the gas. This prevents gas from building up in the oven, and in the kitchen – keeping you safe.
Hobs and ovens fitted with flame supervision devices are controlled by something called a 'thermal couple'.
When the thermal couple is warm, it keeps the gas valve open to let gas flow to the burner. If the flame goes out, the thermal couple cools – and stops the flow of gas. With these devices, it's normal for the flame to burn on a high setting for a few seconds when you turn the hob on. One the thermal couple has reached the right temperature, the flame will reduce to the correct setting.
Some cookers with fully automatic timers have 'flame failure' devices fitted instead of normal thermal couples. These work in the opposite way to a thermal couple: the flame will start to burn at a low temperature until it's warmed up, then go up to the correct setting.
When lighting the hob or oven, the knob on the control panel needs to be held in while it warms up. The flame height will rise and fall as the thermostat opens and closes to find the correct temperature.