Thermal protective switch

Details Thermal protective switch Thermal protective switch

1984-06-06

1986-04-01

Broome, Harold

Electricity: electrothermally or thermally actuated switches

Electrothermally actuated switches

With bimetallic elements

337102, H01H 6102, H01H 7116

Patent

active

045801231

DESCRIPTION:

BRIEF SUMMARY
The invention relates to a thermal protective switch having a contact assembly made up of moving contact with a counter-contact, a bimetallic sensing element for moving the moving contact clear of the counter-contact, and a resistor.
When a fault develops, the first action of known thermal switches is to interrupt the load circuit, for example the circuit of a radiator. After the bimetallic sensing element has cooled down the contacts are moved together and the radiator load circuit is closed again. If the fault or other cause for the thermal protective switch turning off is not taken care of, the switch will open again. Because this may take place at an ever increasing rate, so that the switched-on intervals become shorter and shorter, the bimetallic strip and the contact faces are likely to be damaged by the heating effect of the electric current, the contact assembly no longer being able to take up and dissipate such heat in the desired way. Frequent switching at excessive temperatures may cause the contacts to weld together. Such undesired switching of a thermal protective switch so that it is repeatedly switched on and off is referred to as "hunting". If the contacts are welded together temperature control by thermal protective switch will no longer be possible, because the radiator will be permanently connected with the electricity supply and it will not be switched off by the said thermal protective switch. The load and any other devices in circuit will be irreparably damaged. There will furthermore be a danger of fire because of the failure of the controller.
A further form of known thermal protective switch has so-called thick film resistors in parallel with the contact assembly for keeping the switch from hunting. These thick film resistors are purely ohmic resistors which are produced by screen printing resistance ink on a thermally stable material and baking. The thermal isolation of such resistors and the putting of the metal parts in position is a very complex operation and is frequently not possible. Further shortcomings are that because the resistor is not fully thermally separated or isolated from the contact assembly the resistor has to be run at higher temperatures, which are likely to be responsible for thermal overloading of the resistors themselves and they may then be damaged by the heat generated in them, while further resin components as frequently used in protective switches will become brittle and develop cracks so that they may fracture. The first-noted form of danger is even likely in a design in which an ohmic resistor, which heats itself up, is connected by leads between the two sides of the contact assembly.
Generally speaking, and without paying particular attention to these shortcomings, the use of PTC resistor elements is quite well known. The German unexamined (Offenlegungsschrift) specification No. 2,927,475 is with respect to a controller housed in a glass bulb with a PTC resistor element responding to heat. The resistor element has holes through which electrode leads may be threaded and it is kept in place by such leads. There is no possible chance of being able to dismount the element for replacement. Because of the glass bulb, use for the purpose noted above is not possible. More specially, use in a forced air flow heater would not be possible, because the glass bulb would cut down the heat transfer excessively. The German unexamined specification No. 2,606,201 is with respect to an actuating member with a bimetallic element and a PTC element such that mechanical holding functions such as locking functions are to be possible. Here as well the heat sensitive resistance element may not be replaced. Moreover the flow of heat and the thermal coupling of the PTC element is badly designed in respect of the lever motion. The German unexamined specification No. 2,907,763 is with respect to timer switch for the delayed switching on and off of electrical devices and may be compared with the function of a two pole, two throw switch, it being fitted with a ceramic resistance el

REFERENCES:
patent: 3265839 (1966-08-01), Johnson
patent: 4177443 (1979-12-01), Willeke
patent: 4287500 (1981-09-01), Ueda

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