Gas Interlock Systems for Commercial Kitchens: MLG and MES Specification, Commissioning and BS EN 203

Quick Answer: A commercial kitchen gas interlock proves the extract (and where required the make-up air supply) ventilation is running before it allows gas to flow to the catering appliances, shutting the gas off if airflow fails. The two common build types are a Manual Latching Gas valve system (MLG) — manual reset, simplest and most robust — and a Monitored/Electronic System (MES/automated) with current or air-pressure proving and electronic control. Specification follows BS 6173 (commercial catering gas installation), the appliances follow BS EN 203, and ventilation interlock is guided by IGEM ventilation guidance (IGEM/UP/19). All work must be done by a Gas Safe registered engineer on the commercial catering scope (COMCAT).

Summary

This article is the specification and commissioning companion to the interlock systems reference. Where that article explains the solenoid valve, airflow proving and BS 6173 basics, this one takes the commercial-kitchen specifier's and commissioning engineer's angle: how to choose between an MLG and an MES build, how the air-proving is achieved on extract and supply fans, where BS EN 203 sits relative to BS 6173, and exactly what a Gas Safe engineer checks at commissioning and at the annual test.

The principle behind every interlock is simple: a gas catering range or oven dumps combustion products and heat into the kitchen, and if the extract is not running those products — including carbon monoxide — accumulate. The interlock makes the gas supply conditional on proven ventilation. No proven airflow, no gas. It is, in effect, an engineered control that removes the human error of staff lighting the range with the extract switched off.

The common confusion is treating "interlock" as one product. In practice there is a spectrum from a basic manual-latching valve that you physically push to open (and that drops shut on power loss) through to fully monitored electronic systems that prove both extract and supply fans by current sensing or air-pressure switch and integrate CO detection. Choosing the right level for the kitchen — and commissioning it so the proving genuinely reflects real airflow, not a fan merely spinning — is the engineer's job. This article assumes commercial catering competence (COMCAT) and cross-references commercial gas safety.

Key Facts

• Purpose — prevents gas appliances operating without proven ventilation, controlling the build-up of combustion products (including CO), heat and unburnt gas in the kitchen
• BS 6173 — UK standard for the installation of gas-fired catering appliances in commercial premises; sets the interlock requirement and extract proving expectation
• BS EN 203 — European standard series for gas-heated commercial catering appliances themselves (safety and performance of the ranges, ovens, fryers, etc.) — distinct from the installation standard
• IGEM ventilation interlock guidance — IGEM/UP/19 covers design and installation of ventilation systems with gas interlocks for commercial kitchens
• MLG (Manual Latching Gas valve system) — a normally-closed gas valve that is held open by power and manually reset (pushed/latched) after any trip; simplest, fail-safe, no automatic restart
• MES / monitored electronic system — electronic controller with current/air-pressure proving on extract and supply fans, fault indication, and (often) CO integration; more capability, more to commission and maintain
• Air-proving (extract) — confirmed by a differential air pressure switch across the extract duct/canopy, or by a current sensor on the extract fan motor proving it is drawing running current
• Air-proving (supply/make-up) — where mechanical make-up air is fitted, the supply fan must also be proven so gas only flows when both extract AND supply are running
• Current sensing vs pressure switch — a current sensor proves the motor is running but not that air is actually moving (a snapped belt/blocked filter can fool it); a pressure switch proves airflow directly — best practice combines or uses pressure proving
• Normally-closed (NC) gas valve — fail-safe; loss of power or proving signal closes the valve and cuts gas
• Manual reset is mandatory — after any trip the system must require a deliberate manual reset; automatic restart is not permitted (defeats the safety logic)
• CO detection integration — in enclosed or higher-risk kitchens a CO alarm (BS EN 50291 type) can be wired to trip the interlock and require manual reset above a threshold
• ECV (Emergency Control Valve) — separate manually-operated valve in a prominent, accessible position upstream of the interlock; required in commercial gas premises (IGEM/UP/1B)
• Solenoid sizing — the gas valve must be sized for the diversified gas demand of all controlled appliances; undersizing causes pressure drop and poor appliance performance
• Annual test — the complete interlock (proving, trip, manual reset, CO if fitted) must be tested at least annually as part of the commercial gas safety regime; results recorded
• Competence — installation, commissioning and testing require a Gas Safe engineer on the relevant commercial catering (COMCAT) categories

Quick Reference Table

Detailed Guidance

MLG vs MES — choosing the build

The decision is about kitchen scale, ventilation complexity and the level of monitoring the operator needs:

• MLG (Manual Latching Gas valve system) is the simplest, most robust approach. A normally-closed latching gas valve is held open electrically while the proving condition is satisfied; on any trip — fan failure, power loss, blocked extract — it drops closed and stays closed until someone physically presses the reset/latch. There is no automatic restart, which is exactly what you want safety-wise. MLG suits small, single-zone kitchens — one extract canopy, one bank of gas appliances. Fewer electronics means fewer nuisance faults and easier annual testing.
• MES (monitored electronic system) uses a programmable controller with current and air-pressure proving on both extract and supply fans, fault indication, optional CO integration, and sometimes BMS connectivity and event logging. It suits larger kitchens, multi-zone extract, mechanical make-up air, and operators who need diagnostics. The trade-off is more components to commission correctly and more to go wrong — and more nuisance trips if proving setpoints are poorly commissioned.

A useful rule: specify the simplest system that genuinely proves the ventilation serving the gas appliances. Over-specifying a small kitchen with a complex MES often just creates maintenance and nuisance-trip problems.

Air-proving on extract and supply

The whole safety case rests on the proving genuinely reflecting real airflow:

• Extract proving confirms the canopy is actually pulling air. A differential pressure switch across the extract duct senses the pressure drop when the fan moves air — this proves airflow, not just that the motor is energised. A current sensor (CT) on the extract fan motor proves the motor is drawing running current. The weakness of current sensing alone is that a motor can run with a snapped fan belt, a blocked filter, or a slipping coupling — the motor draws current, but no air moves. For that reason airflow (pressure) proving is the more rigorous method, and many systems combine both.
• Supply (make-up air) proving matters where the kitchen has mechanical make-up air. A large extract canopy pulling air with no make-up air starves the kitchen of replacement air, depressurises the space and can pull combustion products back down flues elsewhere in the building. Where a supply fan is fitted, the interlock should prove both extract and supply running before allowing gas — extract alone is not sufficient.

   Commercial kitchen interlock logic
   ----------------------------------
   Power on
     |
   Extract fan proven? (pressure switch / CT)
     no  -> gas valve CLOSED, fault indicated
     yes -> continue
     |
   Supply fan fitted? 
     yes -> supply proven?
              no  -> gas valve CLOSED, fault indicated
              yes -> continue
     no  -> continue
     |
   CO detector fitted and below threshold?
     no  -> gas valve CLOSED, requires manual reset
     yes -> GAS VALVE OPEN (latched)
     |
   Any proving lost OR power lost OR CO high
     -> gas valve CLOSES (fail-safe)
     -> MANUAL RESET required to restore

Where BS EN 203 fits versus BS 6173

These two are frequently confused:

• BS EN 203 is the appliance standard — it covers the design, safety and performance of the gas catering appliances themselves (ranges, ovens, griddles, fryers, boiling tops). A compliant appliance carries the relevant approval. It is what the manufacturer builds to.
• BS 6173 is the installation standard — it governs how those appliances are installed in a commercial catering establishment, including ventilation and the gas interlock requirement.

As the installing engineer you verify the appliances are suitable (BS EN 203 approved) and install them to BS 6173, with the interlock and ventilation designed per the IGEM ventilation guidance (IGEM/UP/19). Don't cite BS EN 203 as the source of the interlock requirement — that comes from BS 6173 and the IGEM ventilation guidance.

Commissioning sequence

Commissioning is where an interlock is proven to actually work, not just be installed. A Gas Safe COMCAT engineer should:

1. Confirm the install — correct NC gas valve sized to the diversified demand, ECV in place and accessible upstream, proving devices fitted on the correct fans, controller and reset accessible.
2. Set the extract proving — with the extract running normally, set the air-pressure switch so it proves at normal airflow and drops out when airflow falls below safe (test by partially blocking the canopy/filters).
3. Set the supply proving (if fitted) — confirm gas is only enabled with both extract and supply proven.
4. Test the trip — with gas enabled, stop the extract fan; confirm the gas valve closes promptly.
5. Test manual reset — confirm the system requires a deliberate manual reset and does not auto-restart.
6. Test CO integration (if fitted) — apply CO test gas; confirm the interlock trips above threshold.
7. Verify appliance performance — check working pressures and that the solenoid is not throttling supply (gas rate / pressure with all appliances firing).
8. Document — record the commissioning, settings, and test results; the interlock test forms part of the commercial gas safety record. Submit the Gas Safe notification within the required period.

Annual test and maintenance

The interlock is a safety-critical control and must be re-tested at least annually alongside the commercial gas safety inspection:

• Re-confirm extract (and supply) proving trips when airflow is reduced (partial canopy block / fan off).
• Confirm the gas valve closes on trip and requires manual reset.
• Test CO integration with test gas where fitted.
• Inspect proving tappings/sensors for grease and blockage (kitchen extract gets greasy — pressure tappings clog).
• Record results on the commercial gas safety record. Never bypass a tripping interlock to keep the kitchen running — investigate and rectify; bypassing is a serious safety violation. See commercial gas safety and carbon monoxide.

Frequently Asked Questions

What is the difference between an MLG and an MES interlock?

An MLG (Manual Latching Gas valve system) is the simplest build: a normally-closed gas valve held open while ventilation is proven, with a manual reset after any trip and no automatic restart. An MES (monitored electronic system) adds an electronic controller, current and air-pressure proving on both extract and supply fans, fault indication, optional CO integration and logging. MLG suits small single-zone kitchens; MES suits larger, multi-zone or higher-monitoring installations. Both must use manual reset.

Is current sensing on the fan enough to prove airflow?

Not on its own. A current sensor confirms the fan motor is drawing running current, but a motor can run with a snapped belt, a blocked grease filter or a slipping coupling — current flows, but no air moves. A differential air-pressure switch proves air is actually moving and is the more rigorous method. Best practice is to prove airflow directly (pressure switch), or to combine current sensing with pressure proving.

Do I need to prove the supply (make-up air) fan as well as extract?

Where the kitchen has mechanical make-up air, yes. A large extract running without make-up air depressurises the kitchen, can pull combustion products back down other flues, and starves the space of replacement air. The interlock should prove both extract and supply running before allowing gas. If make-up air is purely natural (grilles/openings), only the extract proving applies — but verify the design with the IGEM ventilation guidance.

Does BS EN 203 require the interlock?

No. BS EN 203 is the appliance standard — it covers the safety and performance of the gas catering appliances themselves. The requirement for a ventilation interlock comes from the installation standard BS 6173 and the IGEM ventilation interlock guidance (IGEM/UP/19). You install BS EN 203-approved appliances to BS 6173 with the interlock specified per the IGEM guidance.

How often must a commercial kitchen interlock be tested?

At least annually, as part of the commercial gas safety inspection, by a Gas Safe engineer with the relevant commercial catering competence. The test covers proving, the gas-valve trip, the manual reset, and CO integration if fitted, and the results are recorded. The interlock must never be bypassed to keep the kitchen operating if it trips — the fault must be investigated and rectified.

Regulations & Standards

• Gas Safety (Installation and Use) Regulations 1998 (GSIUR) — statutory framework for gas work, including commercial; safe installation and use duty

• BS 6173 — Specification for installation and maintenance of gas-fired catering appliances in commercial catering establishments; interlock and ventilation requirement

• BS EN 203 (series) — gas-heated commercial catering appliances: safety and performance (the appliances themselves)

• IGEM/UP/19 — design and installation of ventilation systems with gas safety interlocks for commercial kitchens

• IGEM/UP/1B — emergency control valves and aspects of commercial gas installation

• BS EN 50291 — carbon monoxide detection apparatus (where CO integration is used)

• COSHH Regulations 2002 — control of combustion products as substances hazardous to health in commercial kitchens

• Gas Safe Register — Commercial catering — competence requirements and commercial catering guidance

• IGEM — Utilization Procedures (UP series) — UP/19 ventilation interlock and UP/1B commercial installation guidance

• HSE — Catering and hospitality — gas safety and ventilation in commercial kitchens

• BSI — BS 6173 and BS EN 203 — installation and appliance standards for commercial catering gas

• interlock systems — companion reference on solenoid valves, extract proving and BS 6173 basics

• commercial gas safety — commercial gas safety inspection regime and engineer competence

• carbon monoxide — CO risk, detection and alarm integration

• grease traps — associated commercial kitchen compliance (FOG management)