GB1566231A - Overcurrent cutout - Google Patents

Description

(54) OVERCURRENT CUTOUT (71) We, STANDARD TELEPHONES AND CABLES LIMITED, a British Company of 190 Strand, London, W.C.2, England, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to an electrical over-current cut-out.

According to the present invention there is provided an electrical over-current cut out which includes a movable element which is subjected to the influence of a first spring made from a normal metallic alloy and a second spring made from a memory metal alloy, wherein the two springs urge the movable element in opposite directions, wherein below a threshold temperature the urgence of the first spring is predominant whilst above the threshold temperature the urgence of the second spring is predominant, wherein an electrical current to be monitored, or an electrical current derived from the current to be monitored, flows in said second spring so that the latter is subjected to heating due to said current flow, wherein when the current to be monitored exceeds a defined value the second spring is subjected to a temperature at least equal to the threshold temperature, and wherein when the second spring predominates the movable element moves to cause an electrical contact operation to cut off the current being monitored.

A memory metal alloy is one which has a first condition above a transition or threshold temperature and a second and different condition below that temperature. In the case of a coil spring made from wire of a memory metal, the spring is extended and stiff above that temperature, i.e. it is an effective spring, while below that temperature it is relatively soft. As the temperature rises and falls above and below the threshold temperature the spring switches between its two conditions.

Details of the memory metal process will be found in a paper entitled "55 Nitinol, Unique Wire Alloy with Memory" by W. J.

Buehler and W. B. Cross, in the Wire Journal for June 1969 at pp. 41A9. This discusses the memory metal phenomenon, and how to make one particular memory metal alloy. However, it must be appreciated that since 1969 a range of other memory metals have been studied, and that the choice of which such alloy to use depends on the application contemplated.

An embodiment of the invention will now be described with reference to the accompanying drawing, which shows an over-current cut out in section.

The cut out has a sliding plunger 1 slidably mounted in a casing 2, the plunger 1 having a contact-operating extension 3, the tip of which is of an electrically-insulating material. This plunger is subjected to the ,influence of two springs, a spring 4 of a conventional alloy such as phosphor-bronze and a spring 5 of a memory alloy. From the positioning of the springs 4 and 5 with respect to the casing 2 and the plunger 1, it will be seen that they urge the plunger 1 in opposite directions. One end of the spring 5 is electrically connected to the metallic body of the plunger 1 and therefrom via a wire 6 to a terminal 7. The other end of the spring 5 is connected via a spring washer 8 to a terminal 9. Note that the connection from the left hand end of the spring 5 to the plunger body also uses a spring washer.

The contact set to be controlled by the device includes a movable contact arm 10 switchable between engagement with two stationary contacts 11 and 12, the changeovers when they occur toggling due to a compression spring 13 on the end of which there is a steel ball 14 engaged by the arm 10. Tn the normal condition the arm 10 en the X3 contact 11.

'Jb prOrties of the memory metal spring s a,re Ugh that the plunger 1 has two stable posItions a left-ward one when the tem perature of the spring is above the threshold tinparalure of the memory metal, and a ght-ward one where T < Ttr. In the first of the positions the memory metal spring is stiff and extended, and its urgence dominates that of the spring 4, which in the second state the memory metal spring 5 is collapsed by the urgence of the spring 4.

The memory metal spring 5 is heated by current passing through it, this current being either the current to be monitored or a current whose value is proportional to the current to be monitored. The spring is also anchored by the normal thermal loss mechanisms, so that for any value of current in the spring 5 there is an equilibrium temperature, and the condition of the device depends on that temperature.

The memory metal alloy used to make the spiting $ is so chosen that the spring retches Ttr, when the current being monitored reaches its intended threshold. When this occurs the spring 5 stiffens and its urgence dominates that of the spring 4.

Hence the device changes state and the plunger 1 moves left-wards.

When the plunger 1 thus moves leftwards from its low temperature position to its high temperature position, the plunger's extension 3 flips the movable contact arm 10 from one side of its pivot to the other so that the 10-11 connection is broken and the 10--12 connection made.

Movement in the reverse direction to reset the device is caused by pushing the reset button 16. which, if the spring 5 has cooled t9 below its Tt, and has thus contraeted, wiU reSet the device.

WHAT WE CLAIM IS : - 1. An electrical over-current cut out which includes a movable element which is subjected to the influence of a first spring made from a normal metallic alloy and a second spring made from a memory metal alloy, wherein the two springs urge the movable element in opposite directions, wherein below a threshold temperature the urgence of the first spring is predominant whilst above the threshold temperature the urgence of the second spring is predominant, wherein an electrical current to be monitored, or an electrical current derived from the current to be monitored, flows in said second spring so that the latter is subjected to heating due to said current flow, wherein when the current to be monitored exceeds a defined value the second spring is subjected to a temperature at least equal to the threshold temperature, and wherein when the second spring predominates the movable element moves to cause an electrical contact operation to cut off the current being monitored.

2. A cut-out as claimed in claim 1, wherein said movable element is a generally cylindrical plunger having a basal flange, wherein said second spring is inside the plunger and acts between the plunger and a stationary surface near the plunger's flange, and wherein the first spring is outside the plunger and acts between the flange and another stationary surface adjacent to the opposite end of the plunger from said flange.

3. A cut-out as claimed in claim 2, and wherein said plunger has a central rod-like projection which, when the second spring predominates, causes a contact operation.

4. A cut-out as claimed in claim 3, wherein the contact operation is caused by a snap-action-type switch, and wherein a reset button is provided therefor.

An electrical over-current cut-out substantially as described with reference to the accompanying drawing,

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (4)

**WARNING** start of CLMS field may overlap end of DESC **. 10. Tn the normal condition the arm 10 en the X3 contact 11. 'Jb prOrties of the memory metal spring s a,re Ugh that the plunger 1 has two stable posItions a left-ward one when the tem perature of the spring is above the threshold tinparalure of the memory metal, and a ght-ward one where T < Ttr. In the first of the positions the memory metal spring is stiff and extended, and its urgence dominates that of the spring 4, which in the second state the memory metal spring 5 is collapsed by the urgence of the spring 4. The memory metal spring 5 is heated by current passing through it, this current being either the current to be monitored or a current whose value is proportional to the current to be monitored. The spring is also anchored by the normal thermal loss mechanisms, so that for any value of current in the spring 5 there is an equilibrium temperature, and the condition of the device depends on that temperature. The memory metal alloy used to make the spiting $ is so chosen that the spring retches Ttr, when the current being monitored reaches its intended threshold. When this occurs the spring 5 stiffens and its urgence dominates that of the spring 4. Hence the device changes state and the plunger 1 moves left-wards. When the plunger 1 thus moves leftwards from its low temperature position to its high temperature position, the plunger's extension 3 flips the movable contact arm 10 from one side of its pivot to the other so that the 10-11 connection is broken and the 10--12 connection made. Movement in the reverse direction to reset the device is caused by pushing the reset button 16. which, if the spring 5 has cooled t9 below its Tt, and has thus contraeted, wiU reSet the device. WHAT WE CLAIM IS : -

1. An electrical over-current cut out which includes a movable element which is subjected to the influence of a first spring made from a normal metallic alloy and a second spring made from a memory metal alloy, wherein the two springs urge the movable element in opposite directions, wherein below a threshold temperature the urgence of the first spring is predominant whilst above the threshold temperature the urgence of the second spring is predominant, wherein an electrical current to be monitored, or an electrical current derived from the current to be monitored, flows in said second spring so that the latter is subjected to heating due to said current flow, wherein when the current to be monitored exceeds a defined value the second spring is subjected to a temperature at least equal to the threshold temperature, and wherein when the second spring predominates the movable element moves to cause an electrical contact operation to cut off the current being monitored.

2. A cut-out as claimed in claim 1, wherein said movable element is a generally cylindrical plunger having a basal flange, wherein said second spring is inside the plunger and acts between the plunger and a stationary surface near the plunger's flange, and wherein the first spring is outside the plunger and acts between the flange and another stationary surface adjacent to the opposite end of the plunger from said flange.

3. A cut-out as claimed in claim 2, and wherein said plunger has a central rod-like projection which, when the second spring predominates, causes a contact operation.

4. A cut-out as claimed in claim 3, wherein the contact operation is caused by a snap-action-type switch, and wherein a reset button is provided therefor.

An electrical over-current cut-out substantially as described with reference to the accompanying drawing,