Electricity: electrothermally or thermally actuated switches – Electrothermally actuated switches – Space discharge device
Patent
1986-07-16
1988-01-05
Broome, Harold
Electricity: electrothermally or thermally actuated switches
Electrothermally actuated switches
Space discharge device
337415, 361124, H01H 3900, H02H 724
Patent
active
047179023
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to electrical components incorporating a temperature responsive device and more particularly to such components in which the temperature responsive device is provided to perform a protective function on the component or on an associated component.
Electrical components incorporating temperature responsive devices of various forms are known. Thus, the temperature responsive device may take the form of a bimetallic member which, when heated, is subjected to distortion which in turn can be employed to perform an open-circuiting or short-circuiting function. However such devices are relatively expensive and require careful adjustment.
Another form of temperature responsive device comprises a resilient electrically conductive member which is normally maintained in a stressed condition by means including a heat softenable material, the arrangement being such that when the heat softenable material becomes soft at an elevated temperature, the stress in the resilient electrically conductive member is released to enable it to perform a short-circuiting or open-circuiting function. In such devices the heat softenable material may be a metallic fusible material, for example a relatively low melting point alloy, or a fusible insulating material such as a thermoplastics material which becomes soft at a desired temperature.
Also, where the device employs a fusible insulating material which normally holds the stressed resilient electrically conductive member out of engagement with a cooperating contact, although the heat softening of the insulating material allows the resilient electrically conductive member to engage the contact, there is always the possibility that a film of the softened insulating material remains interposed between the engaging parts thereby preventing the protective short-circuiting function from occuring.
It is an object of the present invention to provide an improved form of temperature responsive device for electrical components.
From one aspect the invention provides an electrical component comprising a temperature responsive device including an electrically conducting member of spring temper in contact with a thin insulating coating of a substance which decomposes on being subjected to an elevated temperature.
From another aspect the invention provides an electrical component having at least two electrodes or regions which become short-circuited when a temperature responsive device associated with the component responds to an increase in temperature and wherein the temperature responsive device comprises an electrically conducting member of spring temper in contact with a thin insulating coating of a substance which decomposes on being subjected to an elevated temperature to thereby effect the short-circuiting operation.
In one embodiment the temperature responsive device comprises a wire or strip of spring temper, for example of phosphor bronze or beryllium copper which is coated with a thin coating of a polyurethane varnish which decomposes at an elvated temperature.
According to a feature of the invention, the thin coating of thermally decomposable varnish is of a thickness not exceeding a few tens of microns.
If desired the thermally decomposable coating may consist of an initial coating of a polyurethane varnish and a subsequent coating of nylon, the overall thickness of the combined coating not exceeding a few tens of microns.
Preferably the thermally decomposable coating has an overall thickness within the range 15 to 30 microns and is such that it decomposes or vapourises at a temperature within an approximate temperature range of 175.degree. to 300.degree. C.
The invention may particuarly be employed as an overheating protection device attached to a component such as an excess voltage arrestor which may take the form of a pair of spaced electrodes defining a spark gap and arranged in a gas filled enclosure. Such arrestors may also take the form of three electrode devices in which case the third electrode is generally located in the region of the spark gap.
REFERENCES:
patent: 2992310 (1961-07-01), Babany
patent: 3046536 (1962-07-01), Sciuto
patent: 4212047 (1980-07-01), Napiorkowski
patent: 4371911 (1983-02-01), Baker
Broome Harold
Dubilier PLC
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