The cable-tied interlock
At a hotel kitchen, a 45 kW six-burner range sits under an extract canopy. The gas supply is interlocked through a solenoid valve and an airflow proving switch so that gas cannot pass unless the extract fan is running. On Friday the extract fan motor failed; a maintenance technician tied the airflow switch in the 'fan running' position with a cable tie so lunch service could continue. By 14:00 staff report headaches and nausea. A handheld CO meter reads 28 ppm at head height in the kitchen and 90 ppm just inside the canopy. The range burners are burning with fairly steady blue flames, but the canopy is visibly failing to capture fumes and the kitchen make-up air door has been propped open onto a corridor. The gas detector in the kitchen is a combustible-gas detector only, mounted at ceiling level.
1. Explain the function of the interlock and why the cable tie creates an immediately dangerous situation.
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The interlock is a safety control: it proves extract airflow before gas can flow, so combustion products and spillage are removed and the kitchen is not filled with CO and heat. Tying the airflow switch closed tells the solenoid 'fan running' when it is not. That defeats the only automatic protection and creates an immediately dangerous situation because a 45 kW range can produce CO and flue products directly into the occupied kitchen.
The technician's action is a deliberate bypass of a safety device and must be treated as a serious safety breach, not a temporary fix. The gas should be shut off at the solenoid or emergency valve and the range isolated. The bypass must be removed and the switch replaced or the fan repaired by a competent person; the system should not be re-energised until the interlock is proven to shut gas off when airflow is lost.
2. Interpret the CO readings and identify the likely combustion/ventilation problem.
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28 ppm at head height is above the normal occupational 8-hour limit of about 20 ppm and indicates sustained exposure; 90 ppm inside the canopy shows concentrated combustion products where capture is failing. The burners are blue, so the CO is probably from spillage or entrainment rather than a badly adjusted burner, but that must not be assumed.
With the extract failed and make-up air compromised, the canopy cannot capture products, and the propped corridor door may be drawing air across the range and pushing fumes into the kitchen. The ceiling-mounted combustible detector is the wrong type for CO; a CO detector and a proper airflow interlock are needed. The combination of headaches, nausea and elevated CO is a medical and evacuation issue.
3. Set out immediate actions and the requirements before the range is returned to service.
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Immediate: stop cooking, shut the gas at the emergency control valve, ventilate, evacuate the kitchen, and seek medical advice for affected staff. Do not restart the fan or range until checked. The maintenance bypass should be removed and documented as an incident.
Before return: repair or replace the fan and airflow switch; prove the interlock shuts gas off on fan failure; restore make-up air; clean and service the range and check combustion; measure CO in the breathing zone with the canopy running; fit a CO detector suitable for the kitchen and test it. Record the incident, the bypass, repairs, interlock test and CO readings. The range should remain isolated until all checks are satisfactory.