A flame supervision device (FSD) — also called a flame failure device — answers one question continuously: "is there still a flame?" If the answer becomes no, it shuts the gas off. On a cooker, that stops raw gas pouring into the kitchen if a hob burner is blown out or boils over, or a pilot fails. This guide explains how it works and how it's tested. It's study material; only a Gas Safe registered engineer may carry out the work.

The job it does

Without supervision, an unlit burner with the gas tap open simply leaks gas. An FSD only holds the gas valve open while it can sense a flame; lose the flame and the valve closes, cutting the supply to that burner. It's a safety interlock between "flame present" and "gas allowed."

Type 1: the thermocouple (thermoelectric)

The most common type on cookers uses a thermocouple. The flame heats the tip of the thermocouple, which generates a small voltage (a few tens of millivolts). That tiny voltage energises an electromagnet holding the gas valve open. While the flame heats it, the valve stays open; when the flame goes out, the thermocouple cools, the voltage falls away, the magnet releases and the valve drops out, shutting off the gas.

Why there's a delay. A thermocouple takes a little time to heat up (so you hold the control in on lighting) and a little time to cool down — so the gas doesn't cut off instantly, but within the appliance's stated drop-out time. That short delay is normal and built in.

Type 2: flame rectification

Electronic appliances often use flame rectification. A flame conducts electricity and acts as a one-way valve (a rectifier) for a small current passed through it via a probe. The control electronics detect that rectified current; if the flame goes out, the current stops and the gas valve is closed. It responds faster than a thermocouple.

Where FSDs are fitted

Modern cookers have flame supervision on the hob burners, the grill and the oven. Older appliances may have it only on the oven, or not at all — which is one of the things you assess. The manufacturer's instructions confirm what's fitted and the expected behaviour.

Testing an FSD

The principle of the test is simple: light the burner, let the FSD heat up and establish, then extinguish the flame (without turning the control off) and confirm the gas shuts off — you should hear/see the supply cut and stop smelling gas — within the manufacturer's stated time. If a burner keeps passing gas after the flame is out, the FSD has failed and the appliance must not be left in that state. Always follow the manufacturer's test method and timing.

A failed FSD is serious. An FSD that doesn't shut off lets unburnt gas escape into the room — a clear unsafe situation, classified and acted on under the GIUSP.
  1. Purpose: shut off the gas to a burner when its flame goes out.
  2. Thermocouple type: flame heats it → millivolts → electromagnet holds the valve open; cools → drops out.
  3. Built-in delay: short heat-up/cool-down, so cut-off isn't instant (normal).
  4. Flame rectification: electronic, uses the flame's one-way current; faster response.
  5. Fitted on hob, grill and oven on modern cookers; older ones vary.
  6. Test: light, establish, extinguish the flame, confirm gas shuts off within the MI time.
  7. Failed FSD = unburnt gas escaping = unsafe situation under the GIUSP.

10-Question Mock Test

Click an option to see whether you got it right. Explanations appear instantly — no submitting at the end.

Your score: 0 / 10
Question 1 of 10
What does a flame supervision device do?
Question 2 of 10
Without an FSD, an unlit burner with the tap open will:
Question 3 of 10
A thermocouple-type FSD works by:
Question 4 of 10
When the flame goes out on a thermocouple FSD, the valve closes because:
Question 5 of 10
Why isn't the gas cut off instantly when the flame fails on a thermocouple FSD?
Question 6 of 10
Flame rectification detects a flame by:
Question 7 of 10
Compared with a thermocouple, flame rectification typically responds:
Question 8 of 10
On a modern cooker, FSDs are typically fitted to:
Question 9 of 10
To test an FSD you light the burner, let it establish, then:
Question 10 of 10
A burner keeps passing gas after its flame is out. This is:

Thermocouple or rectification — know how it fails, and how to prove it works.

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