GB2318866A

GB2318866A - Electric cooking appliance

Info

Publication number: GB2318866A
Application number: GB9622807A
Authority: GB
Inventors: Gerard Ryder
Original assignee: Ceramaspeed Ltd
Current assignee: Ceramaspeed Ltd
Priority date: 1996-11-01
Filing date: 1996-11-01
Publication date: 1998-05-06
Also published as: GB9622807D0

Abstract

An electric cooking appliance comprises a sheet (1) of glass ceramic material having a radiant electric heater (4) supported in contact with a surface (3) thereof. An electrically conductive screen (9) is provided between the heater (4) and the surface (3) of the sheet (1) of glass ceramic material, the screen (9) being connected to earth (10) in operation of the heater (4). The screen (9) may a wire mesh bonded to the underneath surface (3) of the glass ceramic material. Alternatively, the screen (9) may be deposited on surface (3). The screen may form a translucent layer.

Description

Electric Cooking ADDliance This invention relates to an electric cooking appliance of the type comprising a sheet of glass ceramic material having a radiant electric heater supported in contact with a surface thereof.

Such a cooking appliance is well known in which the sheet of glass ceramic material provides a smooth top cooking surface. Other applications include ovens and grilling appliances.

It is well known that although the glass ceramic materials used in such cooking appliances exhibit high electrical resistivity at normal room temperatures and are essentially electrically insulating at such temperatures, they become increasingly electrically conductive with increase in temperature and this can become a problem, particularly since, during operation of a cooking appliance, the sheet of glass ceramic reaches a high temperature which may be of the order of 600"C.

Electrical appliances of this nature have to meet strict electrical safety standards imposed by official regulatory authorities. In view of the electrically conductive nature of the hot glass ceramic, sufficient clearance must be maintained between this and adjacent electrically live components in the heater, such as a heating element, to satisfy high voltage flash test requirements. In a particular test, a voltage of 3.75 kilovolts applied between the heating element and the hot glass ceramic must not result in electrical breakdown between the element and the glass ceramic.

As a result, practical limitations have been imposed on the minimum achievable depth of the heater, i.e. the distance between the underside of the base of the heater and the surface of the glass ceramic with which the heater is in contact when installed for operation. Contact between the heater and the glass ceramic is usually by way of the top of a peripheral wall of the heater.

Cooking appliance manufacturers, particularly manufacturers of cooking hob units for recessing into work-tops, are desirous of reducing the depth of their products and it is an object of the present invention to provide means by which this can be achieved.

The present invention provides an electric cooking appliance comprising a sheet of glass ceramic material having a radiant electric heater supported in contact with a surface thereof; there being provided between the heater and the sheet of glass ceramic material an electrically conductive screen which is arranged for electrical connection to earth when the heater is operated.

The electrically conductive screen is preferably arranged to be in contact with the surface of the sheet of glass ceramic material.

The screen may comprise a mesh of electrically conductive material, such as a wire mesh.

which may optionally be bonded to the surface of the sheet of glass ceramic material, or may comprise electrically conductive material deposited on the surface of the sheet of glass ceramic material as a continuous layer or in grid or line form.

The electrically conductive screen is suitably arranged over an area of the surface of the sheet of glass ceramic material at least corresponding to that contacted by the heater.

As a result of the provision of the earthed screen, the high voltage flash test, required in respect of appliances of the prior art, does not have to be complied with. The distance between live components in the heater, such as heating elements, and the surface of the sheet of glass ceramic material is therefore able to be reduced, compared with the prior art. As a result, the depth of cooking appliances, such as hob units, is able to be reduced.

The invention is now described by way of example with reference to the accompanying drawing which represents a cross-sectional view of a smooth top glass ceramic cooking appliance.

A smooth top glass ceramic cooking appliance comprises a sheet 1 of glass ceramic material, well known in the art, having an upper surface 2 for supportingly receiving a cooking utensil (not shown).

Supported in contact with an underneath surface 3 of the sheet of glass ceramic material is a radiant electric heater 4 of well-known basic forum. The heater comprises a dish-like support 5, for example of circular, oval or rectangular shape, having the upper surface of a peripheral side wall 6 thereof arranged in contact with the underneath surface of the glass ceramic sheet 1.

One or more radiant electric heating elements 7 of well-known form, such as bare coiled wire form or bare metal ribbon form, is or are provided inside the dish-like support 5 and arranged for electrical connection to an appropriate voltage supply. A temperature limiter 8, of wellknown rod-like form is provided to interrupt the voltage supply to the one or more heating elements 7 to prevent overheating of the sheet 1 of glass ceramic material during operation of the appliance.

Between the heater 4 and the sheet 1 of glass ceramic material an electrically conductive screen 9 is provided which is electrically connected to earth 10. The screen 9 may comprise a mesh of electrically conductive material, such as a wire mesh, which may be bonded to the underneath surface 3 of the sheet 1 of glass ceramic material. The wire mesh suitably comprises a hightemperature-withstanding material, such as an iron-chromium-aluminium alloy, or a nickelchromium alloy. Alternatively, and perhaps preferably, the screen 9 may comprise an electrically conductive material deposited on the underneath surface 3 of the sheet 1 of glass ceramic material. The conductive material may be provided as a continuous layer, or may be provided in grid or line form. The layer may be translucent.

The electrically conductive screen 9 is arranged over an area of the underneath surface 3 of the sheet 1 of glass ceramic material at least corresponding to that contacted by the heater 4. It may be convenient to arrange the screen 9 over substantially the entire area of the underneath surface 3 of the sheet 1 of glass ceramic material, particularly when more than one heater 4 is provided under the sheet of glass ceramic material.

When the heater 4 is in operation, the sheet 1 of glass ceramic material heats up. It is an inherent property of glass ceramic materials, typically used for the sheet 1 of such cooking appliances, that they become electrically conductive when hot. This is disadvantageous, particularly when there are electrically live components, such as bare heating elements 7 in the vicinity thereof. In the absence of the earthed screen 9, sufficient clearance must be maintained between such live components and the sheet 1 of glass ceramic material whereby when a specified high voltage, for example 3.75 kilovolts, is applied as a test between the glass ceramic and the components, electrical breakdown does not occur across the space therebetween.

By means of the provision of the earthed screen 9, according to the invention, the underside of the sheet 1 of glass ceramic material is maintained at earth potential and the high voltage test does not have to be complied with.

As a result, a cooking appliance may be manufactured in which electrically live components, such as the heating elements 7 can be located in closer proximity to the sheet 1 of glass ceramic material than hitherto. The depth D of the heater 4 may consequently be reduced, resulting in a slimmer cooking appliance, which is especially advantageous when the appliance is a cooking hob for recessing into a work-top in a kitchen.

Claims

1. An electric cooking appliance comprising a sheet of glass ceramic material having a radiant electric heater supported in contact with a surface thereof, there being provided between the heater and the sheet of glass ceramic material an electrically conductive screen which is arranged for electrical connection to earth when the heater is operated.

2. An appliance according to claim 1, in which the electrically conductive screen is arranged to be in contact with the surface of the sheet of glass ceramic material.

3. An appliance according to claim 1 or 2, in which the screen comprises a mesh of electrically conductive material.

4. An appliance according to claim 3, in which the mesh comprises a wire mesh.

5. An appliance according to claim 3 or 4, as dependant upon claim 2, in which the mesh is bonded to the surface of the sheet of glass ceramic material.

6. An appliance according to claim 2, in which the screen comprises electrically conductive material deposited on the surface of the sheet of glass ceramic material.

7. An appliance according to claim 7 in which the electrically conductive material is deposited as a continuous layer or in grid or line form.

8. An appliance according to claim 7, in which the layer of electrically conductive material is translucent.

9. An appliance according to any preceding claim, in which the screen is arranged over an area of the surface of the sheet of glass ceramic material at least corresponding to that contacted by the heater.

10. An electric cooking appliance constructed and arranged substantially as hereinbefore described with reference to the accompanying drawing.