Technical guide
Gas Interlock Wiring Diagram Explained
Every interlock follows the same logic regardless of brand: the controller only energises the gas solenoid valve when the emergency stop circuit is healthy and the ventilation proving contact is made.
- Understand the circuit before you troubleshoot a tripping interlock
- Brand-specific terminal notes for Merlin CT1250/CT1650/CT1750 and Banico panels
- Wiring alterations must be carried out by a competent electrical and Gas Safe engineer
System architecture
The standard circuit
- 230V supply into the controller through a dedicated fused, labelled isolator
- Emergency stop wired as a normally closed series loop — break it and the valve drops out immediately
- Air proving device (differential pressure switch, airflow sensor or fan current monitor) into the proving input as a volt-free contact
- Volt-free run/enable output to the ventilation panel or fan starter where the interlock also starts the fans
- Switched live to the gas solenoid valve coil — normally closed, so loss of signal closes the gas
- Optional gas detection or CO/CO₂ alarm input that also drops the valve
Brand notes
| Panel | Wiring notes |
|---|---|
| Merlin CT1250 / CT1650 / CT1750 (S&S Northern) | Separate terminal blocks for e-stop loop, air proving and valve output; valve output is switched live and must not be shared with fan control |
| Banico interlock panels | Air pressure switch across the extract; e-stop in series with the proving line; verify the valve coil voltage (230V) before connecting |
| Medem / Intelligas | Proving input expects a volt-free contact — do not feed a live signal into it |
Common wiring mistakes we find
- Emergency stop wired normally open, so pressing it does nothing
- Air pressure switch piped the wrong way round, so it proves with the fan off
- Valve coil fed from the fan supply rather than the interlock output, defeating the interlock entirely
- Proving input bridged out during a fault and never reinstated — a serious defect
Frequently asked questions
Can you send me a wiring diagram for my panel?
Tell us the make and model and we will point you to the manufacturer's current diagram. We do not publish altered copies, because terminal numbering changes between revisions.
Is bridging out the proving input ever acceptable?
No. Bridging the ventilation proving signal removes the safety function and would fail any inspection. If the kitchen must trade, the gas stays isolated until the fault is fixed.
Does the interlock have to start the fans?
Not necessarily, but it is the usual arrangement in commercial kitchens: one switch starts ventilation and only then releases the gas.
Related guides
Gas Interlock Systems
The pillar guide: how interlocks work, standards and fault-finding.
Gas Safety Control Panels
Interlock and proving panels, indicators and diagnostic codes.
Sensors & Controls
Air pressure switches, airflow sensors, current monitors, e-stops.
Repair & Emergency
Interlock not working or tripping — same-day engineer attendance.
Manufacturer Guides
Merlin/S&S Northern, Medem, Flamefast, Intelligas, Duomo.
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