Overload current detection in a circuit interrupting device

A circuit interrupting device with overload current detection is provided. It comprises a hot conductor, a main contactor and a first electromagnetic device configured to remove power from an electrical circuit when overload current exceeds a predetermined % of a rated load current. It further comprises a section of conductor that generates heat and a thermal overload current detection mechanism including a temperature sensing switch having contacts. The temperature sensing switch closes the contacts when a temperature reaches a predefined temperature threshold corresponding to an overload current, in which case the temperature sensing switch electrically couples power to a second electromagnet which is disposed across the hot conductor and a connection to a neutral conductor. The energized second electromagnet generates a magnetic force capable of moving an armature that unlatches the latch releasing the spring to open the main contactor removing power from the electrical circuit.

BACKGROUND

Aspects of the present invention generally relate to overload current detection in a circuit interrupting device.

2. Description of the Related Art

Electrical power is distributed to loads throughout buildings using insulated conductors of different sizes appropriate for the amplitude of current being delivered to the load. The amount of current for continuous safe operation for a particular wire gage size is known as a rated current. If the rated current is exceeded, then the conductor will overheat to a point that the insulation melts resulting in hazardous conditions of electrical shock due to exposed voltage potential energy and of flame ignition due to exposed heat energy. Initially fuses were implemented to prevent these hazardous conditions resulting from overloading the electrical circuit. Fuses were eventually replaced by circuit breakers which function as resettable switches. The circuit breaker typically has a robust main contactor that is spring loaded but held in a closed switch position using a latch. For really high overload current situations greater than approximately 800% to 1000% of the rated current on an electrical circuit, the overload current itself is used to generate a magnetic force to unlatch the latch releasing a spring to open the contactor switch removing power from the electrical circuit.

For overload current situations greater than 135% but less than approximately 800% to 1000%, a bimetallic device in series with the electrical current is situated near the latch such that heat generated by the overload current causes the bimetallic device to warp generating a force to unlatch the latch releasing a spring to open the contactor switch removing power from the electrical circuit.

The utilization of a bimetallic device presents several problems. The amount of displacement achieved from warping of the bimetallic device is very small requiring a high degree of precision. The amount of displacement of the bimetallic device varies substantially from part to part requiring calibration to bias the disposition of the bimetallic device. This especially becomes problematic when mass producing circuit breakers that are being sold commercially. After assembly of the circuit breaker, the disposition of the bimetallic device is initially calibrated by adjusting to a typical setting known to produce an optimum first pass test yield percentage of around 70%. It can take up to 1 minute to test the trip time of a circuit breaker. The disposition of the bimetallic device in nonconforming units are readjusted based on the previous test time result. The readjusted units are retested. The nonconforming units are readjusted one more time and retested. Any nonconforming units after a third round of testing is disassembled and the bimetallic device and assembly is scrapped. Typical total yield after three rounds of testing is approximately 85%. This is a very time consuming, inefficient, costly process that still results in approximately 15 percent nonconforming units. And test time and cost really add up when mass producing millions of units per year.

There has not been a simple alternative solution that has high accuracy without adding electronic sensors, amplifiers, and potentially a microprocessor for overcurrent-time calculations, and an electronic ac-dc adaptor power supply to provide power to the electronics.

Therefore, there is a need for providing an alternative method and apparatus to a bimetallic device.

SUMMARY

Briefly described, aspects of the present invention relate to overload current detection in a circuit interrupting device. This invention solves the problem by providing an alternative method and apparatus to the bimetallic device that monitors the temperature of a section of a conductor internal to a circuit breaker and generates a force to unlatch a latch releasing a spring to open a contactor switch removing power from the electrical circuit, and does so at a more precise temperature that is repeatable from unit to unit in mass production. The invention utilizes a new soft magnetic material called Thermorite® developed by Kemet. The magnetic permeability versus temperature characteristic of Thermorite® exhibits a very sharp drop in magnetic permeability at its Curie point. Moreover, Kemet has commercially made available a family of Thermal Reed Switches utilizing Thermorite® technology that have fast switching response and are accurate to within ±2.5° C. The advantage of this invention is that calibration of a thermal overload current detection mechanism of circuit breakers is not necessary. The highly inefficient and costly iterative process of calibrating a bimetallic overload current detection mechanism is eliminated, along with wasted material that is scrapped due to reworking circuit breakers that nonconformed, thus saving precious time and money during mass production of circuit breakers. The cost savings, improvement in test process, efficiency, and factory line throughput more than offset the cost of adding a Thermal Reed Switch and a second electromagnet. In the case of an electronic circuit breaker for ground fault detection and/or arc fault detection, the second electromagnet already exists as a trip mechanism that is activated upon detection of a fault. So, the only additional cost is the Thermal Reed Switch.

In accordance with one illustrative embodiment of the present invention, a circuit interrupting device is provided. It comprises a hot conductor, a main contactor that is spring loaded but held in a closed switch position using a latch and a first electromagnetic device configured to instantaneously generate a magnetic force capable of unlatching the latch releasing a spring to open the main contactor removing power from an electrical circuit when overload current exceeds a predetermined % of a rated load current. It further comprises a section of conductor that generates heat and a thermal overload current detection mechanism including a temperature sensing switch having contacts. The temperature sensing switch is located in close proximity to the section of conductor which closes the contacts when a temperature reaches a predefined temperature threshold corresponding to an overload current, in which case the temperature sensing switch electrically couples power to a second electromagnet which is disposed across the hot conductor and a connection to a neutral conductor. The energized second electromagnet generates a magnetic force capable of moving an armature that unlatches the latch releasing the spring to open the main contactor removing power from the electrical circuit.

In accordance with one illustrative embodiment of the present invention, a method is provided for providing overload current detection in a circuit interrupting device. The method comprises providing a hot conductor, providing a main contactor that is spring loaded but held in a closed switch position using a latch and providing a first electromagnetic device configured to instantaneously generate a magnetic force capable of unlatching the latch releasing a spring to open the main contactor removing power from an electrical circuit when overload current exceeds a predetermined % of a rated load current. The method further comprises providing a section of conductor that generates heat and providing a thermal overload current detection mechanism including a temperature sensing switch having contacts. The temperature sensing switch is located in close proximity to the section of conductor which closes the contacts when a temperature reaches a predefined temperature threshold corresponding to an overload current, in which case the temperature sensing switch electrically couples power to a second electromagnet which is disposed across the hot conductor and a connection to a neutral conductor. The energized second electromagnet generates a magnetic force capable of moving an armature that unlatches the latch releasing the spring to open the main contactor removing power from the electrical circuit.

DETAILED DESCRIPTION

To facilitate an understanding of embodiments, principles, and features of the present invention, they are explained hereinafter with reference to implementation in illustrative embodiments. In particular, they are described in the context of a circuit interrupting device without a bimetallic overload current detection mechanism. A circuit interrupting device comprises a thermal overload current detection mechanism including a temperature sensing switch. For example, a circuit interrupting device includes a Thermal Reed Switch that utilizes a new soft magnetic material called Thermorite®. The Thermal Reed Switch has a fast switching response and is accurate to within ±2.5° C. The highly inefficient and costly iterative process of calibrating a bimetallic overload current detection mechanism is eliminated, along with wasted material. Embodiments of the present invention, however, are not limited to use in the described devices or methods.

These and other embodiments of the circuit interrupting device according to the present disclosure are described below with reference toFIGS. 1-6herein. Like reference numerals used in the drawings identify similar or identical elements throughout the several views. The drawings are not necessarily drawn to scale.

Consistent with one embodiment of the present invention,FIG. 1represents a block diagram of a circuit interrupting device105that provides overload current detection in accordance with an exemplary embodiment of the present invention. The circuit interrupting device105is without a bimetallic overload current detection mechanism. The circuit interrupting device105comprises a hot conductor107, a main contactor110that is spring loaded but held in a closed switch position using a latch112and a first electromagnetic device115(1) configured to instantaneously generate a magnetic force capable of unlatching the latch112releasing a spring117to open the main contactor110removing power from an electrical circuit when overload current exceeds a predetermined %120of a rated load current122. For example, the predetermined %120of the rated load current122may be 800%. The circuit interrupting device105further comprises a section of conductor125that generates heat and a thermal overload current detection mechanism127including a temperature sensing switch130having contacts132. The temperature sensing switch130is located in close proximity to the section of conductor125which closes the contacts132when a temperature135reaches a predefined temperature threshold137corresponding to an overload current140, in which case the temperature sensing switch130electrically couples power to a second electromagnet115(2) which is disposed across the hot conductor107and a connection to a neutral conductor145. When the circuit interrupting device105is an electronic circuit breaker for ground fault detection and/or arc fault detection it has the second electromagnet115(2) built in a trip mechanism which activates upon detection of a fault. The energized second electromagnet115(2) generates a magnetic force capable of moving an armature147that unlatches the latch112releasing the spring117to open the main contactor110removing power from the electrical circuit.

The temperature sensing switch130further comprises a soft magnetic material called Thermorite® that has magnetic permeability versus temperature characteristic which exhibits a very sharp drop in magnetic permeability at its Curie point. For example, the circuit interrupting device105may include a Thermal Reed Switch that utilizes a new soft magnetic material called Thermorite®. The Thermal Reed Switch has a fast switching response and is accurate to within ±2.5° C. The highly inefficient and costly iterative process of calibrating a bimetallic overload current detection mechanism is eliminated, along with wasted material. In the case of an electronic circuit breaker for ground fault detection and/or arc fault detection, the second electromagnet115(2) already exists as a trip mechanism that is activated upon detection of a fault. So, the only additional cost is the Thermal Reed Switch.

Calibration of the thermal overload current detection mechanism127of the circuit interrupting device105is not necessary. However, in a bimetallic device-based design, the amount of displacement of the bimetallic device varies substantially from part to part requiring calibration to bias the disposition of the bimetallic device. This especially becomes problematic when mass producing circuit breakers that are being sold commercially. After assembly of the circuit breaker, the disposition of the bimetallic device is initially calibrated by adjusting to a typical setting known to produce an optimum first pass test yield percentage of around 70%. It can take up to 1 minute to test the trip time of a circuit breaker. The disposition of the bimetallic device in nonconforming units are readjusted based on the previous test time result. The readjusted units are retested. The nonconforming units are readjusted one more time and retested. Any nonconforming units after a third round of testing is disassembled and the bimetallic device and assembly is scrapped. Typical total yield after three rounds of testing is approximately 85%. This is a very time consuming, inefficient, costly process that still results in approximately 15 percent nonconforming units. And test time and cost really add up when mass producing millions of units per year.

The predefined temperature threshold137is selected to ensure compliance of safety standard UL489. For the 200 percent calibration test and the 135 percent calibration test which are performed at 25° C. ambient temperature, a 15 A to 30 A rated circuit breaker must trip within 2 minutes while carrying 200 percent of its rated current, and within 1 hour while carrying 135 percent of its rated current. And for the 100 percent calibration test which is performed at 40° C. ambient temperature, the circuit breaker shall not trip while carrying 100 percent of its rated current until its temperatures have stabilized.

Referring toFIG. 2, it illustrates an empirical model of temperature vs. time characteristic of a heat generating conductor section of a 20 A rated circuit breaker during the various calibration tests in accordance with an exemplary embodiment of the present invention. A predetermined threshold205of the temperature sensing switch130is set to approximately 120° C. to achieve a trip time of approximately 1 minute while carrying 200 percent of its rated current (25° C. ambient), a trip time of approximately 3.5 minutes while carrying 200 percent of its rated current (25° C. ambient), and to not trip while carrying 100 percent of its rated current (40° C. ambient), which are well within the test limits described in UL489.

Turning now toFIG. 3, it illustrates a circuit interrupting device305with a Thermal Reed Switch307in accordance with an exemplary embodiment of the present invention. The Thermal Reed Switch307utilizes Thermorite® such that it has a fast switching response and is accurate to within ±2.5° C. The circuit interrupting device305further comprises an optional Metal Oxide Varistor (MOV)310disposed across a hot conductor312and a connection to a neutral conductor315to protect the Thermal Reed Switch307from high voltage surges.

The circuit interrupting device305further comprises a second electromagnet317. If the current required to energize the second electromagnet317exceeds a current rating of the Thermal Reed Switch307, an off-the-shelf Thermal Reed Switch can be utilized to turn on a solid state switch that has a current rating to safely and repeatedly supply the current required to energize the second electromagnet317.

Suppose the current required to energize the second electromagnet317exceeds the current rating of the Thermal Reed Switch307. Instead of utilizing a custom designed Temperature Sensing Switch or Thermal Reed Switch that has a current rating to safely and repeatedly supply the current required to energize the second electromagnet, an off-the-shelf Thermal Reed Switch can be utilized to turn on a solid state switch that has a current rating to safely and repeatedly supply the current required to energize the second electromagnet317.

FIG. 4illustrates a circuit interrupting device405where the current required to energize a second electromagnet407exceeds the current rating of a Thermal Reed Switch410in accordance with an exemplary embodiment of the present invention. The Thermal Reed Switch410electrically couples power from a hot conductor412to a gate415of a Silicone-Controlled Rectifier (SCR)420through a resistor divider422consisting of a 100K resistor425and 1K resister430which turns on and energizes the second electromagnet407that is disposed across the hot conductor412and a connection to a neutral conductor435through the SCR420. The Thermal Reed Switch410may be a “make” type or a “break” type configured to enable power to be electrically coupled to the gate415of the Silicone-Controlled Rectifier (SCR)420. The “make” type refers to contacts normally open and closes when a predetermined temperature is reached. The “break” type refers to contacts normally closed and opens when the predetermined temperature is reached.

As seen inFIG. 5, it illustrates a circuit interrupting device505that integrates nicely into an electronic ground fault and/or arc fault circuit interrupter to form an additional alternative embodiment. The circuit interrupting device505further comprises an electronic ground fault and/or arc fault detection circuit507where a second electromagnet510and a solid-state switch can be utilized by both a thermal overload current detection mechanism527and a ground and/or arc fault trip mechanism.

In this embodiment, a power supply513disposed across a hot conductor501and a neutral conductor502typically converting 120V AC power to 5V DC power, which is supplied to the electronic ground fault and/or arc fault detection circuit507. Upon detection of a ground fault or arc fault, the electronic ground fault and/or arc fault detection circuit507asserts a trip signal530that is coupled through a 4K resistor535to a gate of a SCR540, which turns on and energizes the same second electromagnet510that it is disposed across a hot conductor501and a connection to the neutral conductor502through the SCR540. The energized second electromagnet510generates magnetic force capable of moving the armature that unlatches the latch releasing the spring to open the contactor switch removing power from the electrical circuit.

As shown inFIGS. 6-8, they illustrate a schematic view of a mechanical form of a circuit interrupting device600in accordance with an exemplary embodiment of the present invention.FIG. 6shows the embodiment in its mechanical form when the circuit interrupting device600is in a “Reset” state where a main contactor603is open and a latch619is latched loading a spring620of the main contactor603. This is accomplished by moving a handle623from a “Tripped” position shown inFIG. 8to the “Reset” position inFIG. 6. A different spring621holds a trip armature622in place.FIG. 7shows the embodiment in its mechanical form when the circuit interrupting device600is in an “On” state where the main contactor603is closed by moving the handle623.FIG. 8shows the embodiment in its mechanical form when the circuit interrupting device600is in a “Tripped” state as a result of a thermal switch inFIG. 1located in close proximity to a heating conductor605that energizes a second electromagnet607upon exceeding a predetermined temperature threshold. The energized electromagnet607exerts force on the trip armature622which unlatches the latch619releasing the spring620. The force of the spring620moves the handle623to the “Tripped” position and opens the main contactor603removing power from the electrical circuit.

InFIG. 9, it illustrates a schematic view of a flow chart of a method900for providing overload current detection in the circuit interrupting device105in accordance with an exemplary embodiment of the present invention. Reference is made to the elements and features described inFIGS. 1-8. It should be appreciated that some steps are not required to be performed in any particular order, and that some steps are optional.

The method900comprises a step905of providing a hot conductor. The method900further comprises a step910of providing a main contactor that is spring loaded but held in a closed switch position using a latch. The method900further comprises a step915of providing a first electromagnetic device configured to instantaneously generate a magnetic force capable of unlatching the latch releasing a spring to open the main contactor removing power from an electrical circuit when overload current exceeds a predetermined % of a rated load current. The method900further comprises a step920of providing a section of conductor that generates heat. The method900further comprises a step925of providing a thermal overload current detection mechanism including a temperature sensing switch having contacts. The temperature sensing switch is located in close proximity to the section of conductor which closes the contacts when a temperature reaches a predefined temperature threshold corresponding to an overload current, in which case the temperature sensing switch electrically couples power to a second electromagnet which is disposed across the hot conductor and a connection to a neutral conductor. The energized second electromagnet generates a magnetic force capable of moving an armature that unlatches the latch releasing the spring to open the main contactor removing power from the electrical circuit.

While a temperature sensing switch based on Thermorite® is described here a range of one or more other types of temperature sensing switch components or other forms of temperature sensing are also contemplated by the present invention. For example, other types of temperature sensing switch components may be implemented based on one or more features presented above without deviating from the spirit of the present invention.

The techniques described herein can be particularly useful for an electronic ground fault and/or arc fault circuit interrupter. While particular embodiments are described in terms of specific ground fault and/or arc fault configuration and specific circuit breakers, the techniques described herein are not limited to such a limited configuration and circuit breakers but can also be used with other configurations and circuit breakers.

Additionally, any examples or illustrations given herein are not to be regarded in any way as restrictions on, limits to, or express definitions of, any term or terms with which they are utilized. Instead, these examples or illustrations are to be regarded as being described with respect to one particular embodiment and as illustrative only. Those of ordinary skill in the art will appreciate that any term or terms with which these examples or illustrations are utilized will encompass other embodiments which may or may not be given therewith or elsewhere in the specification and all such embodiments are intended to be included within the scope of that term or terms.

Respective appearances of the phrases “in one embodiment,” “in an embodiment,” or “in a specific embodiment” or similar terminology in various places throughout this specification are not necessarily referring to the same embodiment. Furthermore, the particular features, structures, or characteristics of any particular embodiment may be combined in any suitable manner with one or more other embodiments. It is to be understood that other variations and modifications of the embodiments described and illustrated herein are possible in light of the teachings herein and are to be considered as part of the spirit and scope of the invention.