[
Chip-pan fires can be a frightening phenomenon. Gordon Cooke, who is an international fire safety consultant and visiting professor at the school of Engineering and Mathematical Sciences, City University, London, investigated a way to prevent them using conventional technology in a five-storey hostel in London, with results that could have far reaching benefits.
For many years, chip pan fires have been the cause of deaths, disfiguring injuries and damage to property. There are approximately 4000 accidents and 130 deaths caused by these fires in the UK each year, and these casualty statistics do not include fires which are not attended by the fire service. This frightening phenomenon is recognised by the Government in a current campaign to reduce them.
The options for reducing these fires can be clearly illustrated in a real life situation at a five storey hostel in North London. Electric hobs had been installed near the narrow single doorways in each of the 80 flats in the hostel.
The planning of the individual rooms did not follow the implicit guidance in Approved Document B, or the Home Office guide to Means of Escape and related measures for Houses in Multiple Occupation (HMOs). This recommends that areas of highest fire risk (kitchens are a prime example) are placed as far away as possible from the fire exit door – in this case, this is the flat entrance door in the hostel. It would be difficult – and very dangerous – for occupants to escape past a hob if a chip-pan fire occurred. With this in mind, I was commissioned to consider and mitigate the risk and it was paramount that any work would interfere as little as possible, as all flats were occupied.
The Options
Several options were considered and the preferred one would form part of a package of improvements to the fire precautions systems and management. The options were:
– Leave the planning and installation arrangements as they were. There was a consensus among the approving authorities involved (building control, environmental, housing and fire officers), however, that this was unacceptable.
– Relocate the hob away from the entrance. While this placed a possible chip pan fire away from the entrance, it unfortunately introduced the everyday hazard of having to carry hot cooking utensils over to the work surface and sink that would be some distance away.
– Install an automatic, powder-dump extinguisher immediately above the hob. Arguments against this option included cost and lack of space but, more importantly, a chip pan fire extinguished could spontaneously ignite some time after the powder had done its job.
– Install a portable powder extinguisher and a fire blanket in each room, and provide instructions to tenants what to do in event of a fire. The instructions would include information to extinguish the fire with the powder, immediately cover the pan with the fire blanket and leave covered for several minutes so that the oil drops well below its auto ignition temperature (official advice on chip pan fires is to leave the blanket in place for 30 minutes). But this option was considered unacceptable, as it depends on a high state of consciousness and motivation occupants may not have either due to intake of drugs or alcohol. Also, the use of pressurised extinguishers used too close to a fat fire could aggravate it.
– Leave the hob where it was and provide a portable, fully enclosed chip fryer with instructions to tenants on how to use the fryer. This would not prevent lesser frying operations (such as frying in a shallow frying pan on the hob), but would the chip fryer ever be used?
– Install a low pressure, water mist fire suppression system immediately above the hob connected to the water supply. This would be expensive and the mains pressure would not be high enough (it would probably need to be around 5 bar).
– Install fusible, link actuated glass fibre roller fire curtains to the front and sides of the hob to enclose it. There was no space and, anyway, the curtains would be costly to install and maintain and in descending, they could tip over a pan. They would only provide a flame barrier and have no fire-suppressing function.
– Rely on automatic smoke detection to warn the occupants before a flaming fire could occur. This option could cause many general false alarms due to detector sensitivity, unless a stand-alone smoke detector was fitted in each room. But an effective stand-alone solution relies on a high level of compliance from occupants in terms of testing and not removing the battery as a result of nuisance false alarms or for other purposes. In addition, heat detection could not be relied to give an alarm early enough.
– Leave the hob where it was, and modify the heating controls of both hot plates so that it was impossible to produce a chip-pan fire, but still be able to cook chips and undertake water-based cooking.
This last option was eventually chosen as it was thought technically and commercially feasible. It would be implemented using an arrangement of carefully positioned thermostats and fuses that are frequently used in other cooking appliances.
Thermostatic v Energy Control
It was clearly unacceptable to have a hob which, if left switched on, continued to heat the pan contents, as once the temperature went above the danger temperature fire could occur. The present hob operated in this way. Thermostatic control could, on the other hand, prevent ignition irrespective of time duration. It was very surprising to find that no thermostatically controlled domestic hobs were on the market in the UK, although there were hotplates with thermostatic control available for laboratory use.
Before developing a system of thermostatic control, it was necessary to specify some scenarios. Firstly, the properties of cooking oils need to be considered, so information on their ignition was collected to quantify the auto-ignition points of oils. Generally, it can be said that the maximum temperature to which cooking oils should be raised for proper cooking is around 200-250 degrees C. Differences between the ignition temperatures of various types of cooking oils and fats are trivial, but the current range of products was tested in the hob development work in order to be sure.
Worst case scenarios were identified. It was considered that a fire should not occur from the following actions:
– Leaving a small or large amount of oil in a pan on either hotplate for several hours.
– Dumping wet or dry chips into the oil when it was at its highest temperature, where overflow onto the hotplate occurred.
– Spilling hot oil onto the hotplate, immediately upon removing the pan from the hotplate.
– Any of the above conditions when both hotplates were in use and one hotplate was bare or in use.
– Placing a wad of folded newspaper, dropping a piece of paper towel, a linen tea towel or a plastic cup onto a bare hotplate when at its highest temperature. Although it was accepted that these were less important scenarios – as a smoke detector installed in each room would be expected to warn occupants of a smouldering fire – they were included at the suggestion of the fire officer. Massive flames sufficient to bar the exit could not result from this scenario.
Tests were first carried out on one of the energy-regulated hobs already in use in the hostel. This showed that a chip-pan fire was easily achieved (see photo) as the hob continued to heat the oil well above its ignition temperature. For safety and environmental reasons, the tests were conducted by spilling small amounts of oil onto one of the two hotplates, and this was performed when a maximum setting was selected on the hob controls. Temperatures were measured with digital thermocouples and an infrared digital thermometer.
This showed that thermostatic control was clearly needed and the Italian hob manufacturer was asked, via the importer, to produce a prototype hob with this form of control. Three months later, the new hob was tested. It was then discovered that it simply had hotplates with lower power ratings fitted without thermostatic control. A search was made for a UK specialist design and development company with in-house test facilities to help take the work forward and, as a result; Destech UK became involved in the project.
The strategy was to modify the existing hobs by adding thermostats and fuses, which became active in the required temperatures of the cooking of the oil or water. The hotplates and other parts of the appliance had to enable chips to be cooked, and also allow normal, water-based cooking. The choice of thermostat had to take into account phase lags in heating and cooling processes, and also which thermostats were available on the market – not an easy choice! In addition, they could only be positioned where they would not invalidate warranties, and had to continue to satisfy the relevant British Standards.
Although the heating rate up to the thermostat’s cut off point was under control using the energy regulator fitted (represented by six heat switches), it was not possible to influence the rate of cooling in each on/off cycle. This was due to it being a natural phenomenon, and it depended on what (if anything) was resting on the hotplate. Furthermore, the choice of operating temperature of the thermostats could not be tuned too finely, as there was a lag between the heating of the element embedded in the cast-steel hotplate and the heating of the oil. The heat transfer processes were complicated and of course, there are time-based phenomena such as phase lag. The proving tests were carried out on a spare hob both in the test laboratory and in the hostel.
Conclusions
An electric hob has been successfully modified, using commercially available parts, to produce a fire-safe hob. It proved possible to boil water within a reasonable time on both hotplates, though vigorous boiling was not maintained over the short period when the hotplate was naturally cooling in each cycle. Vitally, it proved possible to cook chips safely and for hot animal or vegetable oil to be spilt onto either hotplate – without causing a fire.
Although a hotplate, after it had reached its maximum temperature, could cause smouldering ignition of a thick bundle of newspaper, it was agreed this was acceptable. Ignition of thin paper or a plastic cup did not occur. All 80 hobs in the hostel have now been replaced with the modified hob.
There is a management issue associated with the hobs in premises such as hostels. It appears that if a tenant uses the hob to complement the conventional room heating, the hob will fuse. This will occur after several hours of continuous operation at maximum setting on both hotplates when left bare. The hob will not fuse when used for normal cooking operations. Such misuse of the hob would shorten the life of the hob and new users need to be warned against doing this.
With the number of deaths and injuries from chip pan fires, it does not need much imagination to see that the use of a fire-safe hob in hostels and other residential buildings could do much to reduce the casualty toll, especially where reliance is on electric cooking devices to avoid gas-related problems, such as explosion.
Figure 1. Test on unmodified hob with both hotplates at maximum power after small amount of oil was poured onto centre of hotplate. A major spillage could produce a chip-pan fire.
Figure 2. Inside of an inverted hob with bottom cover removed. the white energy regulators are at the bottom of the picture.