Short answer
A gas interlock links the kitchen gas supply to the extraction system so a solenoid valve stays closed unless the canopy fans are proven running. It prevents carbon monoxide and unburnt gas building up in the kitchen, and is expected on new and substantially altered commercial kitchens under IGEM/UP/19 and BS 6173.
How it works
Fan proving by air pressure switch or current sensing feeds a control panel that energises the gas solenoid. Loss of proving closes the valve, and a manual reset is needed to restore gas once appliances are confirmed off.
Testing
The engineer proves the gas supply shuts when each fan stops, that the system needs a manual reset, and that the emergency stop works.
Figures are typical for UK natural gas. The current edition of the relevant IGEM standard and the manufacturer's instructions always take precedence on site.
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