Kitchen interlocks

The importance of kitchen interlocking

Power monitoring is an effective way of interlocking the gas supply to the ventilation system in a commercial kitchen, writes Trent Products.

Why is interlocking important?

Workers in commercial kitchens need to be provided with a safe and comfortable working environment. A well-designed and correctly working ventilation system will remove the fumes, vapours and heat generated by the appliances and cooking processes effectively – and deliver fresh air.

The vast majority of commercial catering establishments use gas appliances. This means that an interlock of the correct type, as specified in BS:61731 and IGEM/UP/192 will be required.

For Type A appliances, which are designed to operate without a flue, this could be by air flow, air pressure, power monitoring or air quality. Type B appliances designed to operate with a flue and which must be under a canopy, must be interlocked by air flow, air pressure or power monitoring only.

The ventilation system should maintain CO2 levels below 2,800ppm, and it is important that the interlock is set up correctly

at the minimum air flow rate and maximum cooking load to ensure this. Air quality testing in accordance with IGEM/UP/19 will confirm this at annual servicing and following any work on the ventilation system.

Air pressure and air flow switches mounted on or directly in a grease-laden duct are prone to becoming stuck or blocked. Power monitoring interlock sensors have the advantage of having no moving parts in the airstream, reducing servicing requirements and improving reliability.

How does power monitoring work?

The fan wires are taken through sensors that convert the alternating current to a control signal. The interlock controller uses this signal to determine whether the ventilation is working correctly.

Some interlocks are set up manually, either by turning potentiometers on a PCB or setting current switches mounted remotely. Others calibrate automatically and add suitable pre-determined margins. When setting up a system manually, it is important to set a sensible margin that will allow for slight fluctuations but allow for appropriate interlocking.

How does the current change?

Simply, when an electrical motor spins, it acts like a generator. A voltage known as a back EMF (electro-motive force) is generated by the spinning motor, this has the opposite polarity to (and tries

to resist the voltage from) the power supply.

Interlocks with both undercurrent and over-current monitoring can detect defective motors and low air-flow conditions. A blocked filter or closed damper may cause the fan speed to increase and the current to fall. Damaged or worn bearings will cause the fan speed to decrease and an over-current fault to be detected.

Fans do not always draw more current at higher speeds, particularly those controlled with electronic speed controllers. The minimum speeds should be dialled into the speed controllers.

Commissioning and servicing

Most interlock controllers have interfaces for external emergency circuits and ancillaries. Emergency stop buttons at appropriate points should disable the gas and/or cookline, and a fire alarm should additionally stop the ventilation.

Many systems are fitted with a CO2 sensor. These are usually self-calibrating but should be checked and maintained annually in accordance with the manufacturer’s instructions and replaced at their end of life.

To correctly commission and service an interlock, the manufacturer’s instructions should be followed by a competent person. As a minimum, the work will usually involve: • Atmospheric testing: not all flue gas analysers read CO2 directly, in which case a direct reading meter will be required • Setting of fan power values or flow or pressure switches to shut off gas at required flow rate • Sensor gas test, breathing directly on to the sensor will achieve the alarm or relay point • Emergency circuit check, location and effectiveness of

any emergency stop buttons • Fire alarm circuit test: the interlock may be standalone and not control the fans – in this case, the fans should be interfaced directly to the fire alarm • Simulated test for fan failure, ie, isolate fans electrically to check that the gas goes off.

Some aspects of commissioning may be carried out by other trades, particularly setting of ventilation flow rates. The methodology for determining flow rates is published in BESA document DW1723 .

New interlocks should not contain any form of override. Existing interlocks with an override should be dealt with in accordance with the Gas Industry Unsafe Situations Procedure (IGEM/G/11). ■ • For a free CIBSE CPD presentation, email:

info@trentproducts.com

“Air pressure and air flow switches mounted on or directly in a grease-laden duct are prone to becoming stuck or blocked.”

1 BS 6173:2020 – Installation and maintenance of gas fired catering appliances for use in all types of catering establishments (2nd and 3rd family gases). Specification 2 IGEM/UP/19 – Design and application of interlock devices and associated systems used with gas appliance installations in commercial catering establishments 3 DW 172: Specification for Kitchen Ventilation Systems