Electric heating – Heating devices – Combined with container – enclosure – or support for material...
Reexamination Certificate
1998-11-24
2001-01-30
Walberg, Teresa (Department: 3742)
Electric heating
Heating devices
Combined with container, enclosure, or support for material...
C219S438000, C219S540000, C219S548000
Reexamination Certificate
active
06180925
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to electric heating elements, for example for use in liquid heating vessels, such as kettles, rice cookers, coffee makers, etc. The invention relates particularly to heating elements which comprise an electrically heated conducting track provided over a substantially planar metal substrate.
This type of heating element is increasingly being used in electric kettles, where it provides the advantage that cleaning the inside of the kettle is easier, and it may be possible to boil a small quantity of water, since a smaller quantity of water is required to cover the heating element than is required for conventional immersion elements. Safety requirements dictate that electric kettles require two protection devices to ensure that the electrical supply to the heating element is broken in the event of overheating of the electric kettle (for example if the steam sensitive cut-off switch for boiling fails, or if the kettle is turned on with no water in it). Conventionally, the two overheat protection devices have been integrated into a control unit of the electric kettle, and one or both of these overheat protection devices may comprise a bimetallic switch which switches off when a bimetallic strip reaches a predetermined temperature. Additionally, or alternatively, portions of the control housing may be formed from a plastic which melts at a predetermined temperature so that in the event of failure of all other overheat protection devices, the body of the control housing melts resulting in movement of components causing disconnection of the electrical supply to the heating element. If this melt-down protection is employed, only one thermal protection device in the form of a bimetallic switch may be required.
EP 0 715 483 which corresponds substantially to U.S. Pat. No. 5,793,929, discloses an electric heating element comprising a conductive heating track provided over a substrate, and extending between two contact terminals. The track comprises a first portion which extends around the circumference of the heating element and is an unheated portion of the track. This first portion is connected in series to a second, inner, heating portion of the track. The unheated portion of the track acts as a thermal fuse which breaks the connection between the two contact pads in the event of overheating of the heating element.
SUMMARY OF THE INVENTION
The present invention is based on the realization that the heating track itself may act as a thermal fuse, thereby avoiding the need for additional unheated track portions to be provided to act as a thermal fuse.
According to a first aspect of the present invention, there is provided a heating element for a liquid heating vessel, comprising a metal substrate, an insulating layer provided over the substrate, and an electrically conductive heating track provided over the insulating layer, the heating track comprising a path extending between two contact pads, the heating track defining regions of relatively high density of track portions and regions of relatively low density of track portions, the contact pads being located in regions of relatively low density, and wherein the layout of the heating track is designed such that in the event of thermal overheating of the element, the heating track ruptures at one of a predetermined set of locations in high density regions of the heating track.
In the heating element of the invention, the track layout is designed with regions of high density and regions of low density of track portions, and this gives rise to local hot spots caused by the heating track. Appropriate design of these hot spots enables the position of track rupture in the event of overheating to be selected, so that the heating track can act as a reliable fuse. The position where rupture takes place is important, because this enables the risk of arcing to be minimized, as well as the risk of high current surges during track rupture.
Preferably, the predetermined locations are remote from the contact pads, so that when there is track rupture at the selected location, arcing does not occur from the point of rupture to the contact pads, which could potentially lead to a fire hazard.
In order to limit the level of the current surge which occurs during track rupture, it is desirable for the track rupture to occur towards the middle of the heating track, so that there is a resistive portion of the heating track between the rupture point and each of the contact pads. Thus, irrespectively of the polarity of the voltage applied to the contact pads, there is some resistance in the path from the high voltage (live) contact pad to the point of rupture, and this limits the current surge which occurs during track rupture.
Thus, according to a second aspect of the present invention there is provided a heating element for a liquid heating vessel, comprising a metal substrate, an insulating layer provided over the substrate, and an electrically conductive heating track provided over the insulating layer, the heating track comprising a path extending between two contact pads, the heating track defining regions of relatively high density of track portions and regions of relatively low density of track portions, wherein the contact pads are positioned in a low density region in an inner portion of the heating element, and the portions of the heating track leading directly from the two contact pads each extend radially outwardly through a low density region to an outer portion of the heating element, and then follow a path which progresses towards the center of the element.
It has been found that the point of rupture of the heating track occurs at a point of the heating track in which local hot spot conditions are present as well as a high voltage. Consequently, when the heating track portion extends from the contact pads to a periphery of the heating element through a low density region of the element, rupturing of the heating track in portions of the heating track adjacent the contact pads is avoided. Thus, the maximum current surge upon rupturing can be reduced. Furthermore, when the heating track follows a path which progresses towards the center of the elements, multiple ruptures which can occur will progress outwardly towards a cooler portion of the heating element and thus die out.
A region of relatively high density of track portions preferably comprises a region where there are more than two heating track portions in close proximity to and substantially parallel to each other.
The invention also provides an electric kettle including a heating element of the invention. The electric kettle may comprise a single overheat control device so that the heating track itself and the overheat control device together provide two levels of overheat protection.
REFERENCES:
patent: 5140134 (1992-08-01), Reusche et al.
patent: 5693244 (1997-12-01), Pragt et al.
patent: 5774627 (1998-06-01), Jackson
patent: 5793929 (1998-08-01), Taylor
patent: 0715483A2 (1996-06-01), None
patent: 94/18807 (1994-08-01), None
Moore Robin K.
Slegt Sander
Bartlett Ernestine C.
Fuqua Shawntina
U.S. Philips Corporation
Walberg Teresa
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